arXiv:1510.00872v2 [math.NT] 9 Dec 2016

Compactifications of S-arithmetic quotients for the
projective general linear group
Takako Fukaya, Kazuya Kato, Romyar Sharifi
December 12, 2016

Dedicated to Professor John Coates on the occasion of his 70th birthday
Abstract
Let F be a global field, let S be a nonempty finite set of places of F which contains
Q
the archimedean places of F , let d ≥ 1, and let X = v ∈S X v where X v is the symmetric

space (resp., Bruhat-Tits building) associated to PGLd (Fv ) if v is archimedean (resp., nonarchimedean). In this paper, we construct compactifications Γ\X̄ of the quotient spaces
Γ\X for S-arithmetic subgroups Γ of PGLd (F ). The constructions make delicate use of the
maximal Satake compactification of X v (resp., the polyhedral compactification of X v of
Gérardin and Landvogt) for v archimedean (resp., non-archimedean). We also consider a
variant of X̄ in which we use the standard Satake compactification of X v (resp., the compactification of X v due to Werner).

1

Introduction

1.1. Let d ≥ 1, and let X = PGLd (R)/ POd (R) ∼
= SLd (R)/ SOd (R). The Borel-Serre space (resp.,

reductive Borel-Serre space) X̄ contains X as a dense open subspace [3] (resp., [26]). If Γ is a
subgroup of PGLd (Z) of finite index, this gives rise to a compactification Γ\X̄ of Γ\X .
1.2. Let F be a global field, which is to say either a number field or a function field in one
variable over a finite field. For a place v of F , let Fv be the local field of F at v . Fix d ≥ 1.

In this paper, we will consider the space X v of all homothety classes of norms on Fvd and a

certain space X̄ F,v which contains X v as a dense open subset. For F = Q and v the real place,
X v is identified with PGLd (R)/ POd (R), and X̄ F,v is identified with the reductive Borel-Serre
space associated to PGLd (Fv ). We have the following analogue of 1.1.
1

Theorem 1.3. Let F be a function field in one variable over a finite field, let v be a place of F ,
and let O be the subring of F consisting of all elements which are integral outside v . Then for
any subgroup Γ of PGLd (O) of finite index, the quotient Γ\X̄ F,v is a compact Hausdorff space
which contains Γ\X v as a dense open subset.
1.4. Our space X̄ F,v is not a very new object. In the case that v is non-archimedean, X v is
identified as a topological space with the Bruhat-Tits building of PGLd (Fv ). In this case, X̄ F,v is
similar to the polyhedral compactification of X v of Gérardin [7] and Landvogt [19], which we
denote by X̄ v . To each element of X̄ v is associated a parabolic subgroup of PGLd ,Fv . We define
X̄ F,v as the subset of X̄ v consisting of all elements for which the associated parabolic subgroup
is F -rational. We endow X̄ F,v with a topology which is different from its topology as a subspace
of X̄ v .
In the case d = 2, the boundary X̄ v \ X v of X̄ v is P1 (Fv ), whereas the boundary X̄ F,v \ X v of

X̄ F,v is P1 (F ). Unlike X̄ v , the space X̄ F,v is not compact, but the arithmetic quotient as in 1.1
and 1.3 is compact (see 1.6).
1.5. In §4, we derive the following generalization of 1.1 and 1.3.

Let F be a global field. For a nonempty finite set S of places of F , let X̄ F,S be the subspace
Q
of v ∈S X̄ F,v consisting of all elements (x v )v ∈S such that the F -parabolic subgroup associated
Q
to x v is independent of v . Let X S denote the subspace v ∈S X v of X̄ F,S .
Let S 1 be a nonempty finite set of places of F containing all archimedean places of F , let S 2

be a finite set of places of F which is disjoint from S 1 , and let S = S 1 ∪ S 2 . Let OS be the subring
of F consisting of all elements which are integral outside S.

Our main result is the following theorem (see Theorem 4.1.4).
Theorem 1.6. Let Γ be a subgroup of PGLd (OS ) of finite index. Then the quotient Γ\(X̄ F,S 1 × X S 2 )
is a compact Hausdorff space which contains Γ\X S as a dense open subset.

1.7. If F is a number field and S 1 coincides with the set of archimedean places of F , then the
space X̄ F,S 1 is the maximal Satake space of the Weil restriction of PGLd ,F from F to Q. In this
case, the theorem is known for S = S 1 through the work of Satake [23] and in general through
the work of Ji, Murty, Saper, and Scherk [14, 4.4].
♭
♭
1.8. We also consider a variant X̄ F,v
of X̄ F,v and a variant X̄ F,S
of X̄ F,S with continuous surjections
♭
X̄ F,v → X̄ F,v
,

♭
X̄ F,S → X̄ F,S
.

♭
In the case v is non-archimedean (resp., archimedean), X̄ F,v
is the part with “F -rational bound-

ary” in Werner’s compactification (resp., the standard Satake compactification) X̄ v♭ of X v [24,
2

25] (resp., [22]), endowed with a new topology. We will obtain an analogue of 1.6 for this variant.
To grasp the relationship with the Borel-Serre compactification [3], we also consider a vari♯

♯

ant X̄ F,v of X̄ F,v which has a continuous surjection X̄ F,v → X̄ F,v , and we show that in the case
♯

that F = Q and v is the real place, X̄ Q,v coincides with the Borel-Serre space associated to
♯

PGLd ,Q (3.7.4). If v is non-archimedean, the space X̄ F,v is not Hausdorff (3.7.6) and does not
seem useful.
1.9. What we do in this paper is closely related to what Satake did in [22] and [23]. In [22], he
defined a compactification of a symmetric Riemannian space. In [23], he took the part of this
compactification with “rational boundary” and endowed it with the Satake topology. Then he
showed that the quotient of this part by an arithmetic group is compact. We take the part X̄ F,v
of X̄ v with “F -rational boundary” to have a compact quotient by an arithmetic group. So, the
main results and their proofs in this paper might be evident to the experts in the theory of
Bruhat-Tits buildings, but we have not found them in the literature.
1.10. We intend to apply the compactification 1.3 to the construction of toroidal compactifications of the moduli space of Drinfeld modules of rank d in a forthcoming paper. In Section
4.7, we give a short explanation of this plan, along with two other potential applications, to
asymptotic behavior of heights of motives and to modular symbols over function fields.
1.11. We plan to generalize the results of this paper from PGLd to general reductive groups in
another forthcoming paper. The reason why we separate the PGLd -case from the general case
is as follows. For PGLd , we can describe the space X̄ F,v via norms on finite-dimensional vector
spaces over Fv (this method is not used for general reductive groups), and these norms play
an important role in the analytic theory of toroidal compactifications.
1.12. In §2, we review the compactifications of Bruhat-Tits buildings in the non-archimedean
setting and symmetric spaces in the archimedean setting. In §3 and §4, we discuss our compactifications.
1.13. We plan to apply the results of this paper to the study of Iwasawa theory over a function
field F . We dedicate this paper to John Coates, who has played a leading role in the development of Iwasawa theory.
Acknowledgments. The work of the first two authors was supported in part by the National
Science Foundation under Grant No. 1001729. The work of the third author was partially supported by the National Science Foundation under Grant Nos. 1401122/1661568 and 1360583,
and by a grant from the Simons Foundation (304824 to R.S.).
3

2

Spaces associated to local fields

In this section, we briefly review the compactification of the symmetric space (resp., of the
Bruhat-Tits building) associated to PGLd of an archimedean (resp., non-archimedean) local
field. See the papers of Satake [22] and Borel-Serre [3] (resp., Gérardin [7], Landvogt [19], and
Werner [24, 25]) for details.
Let E be a local field. This means that E is a locally compact topological field with a nondiscrete topology. That is, E is isomorphic to R, C, a finite extension of Qp for some prime
number p , or Fq ((T )) for a finite field Fq .
Let | |: E → R≥0 be the normalized absolute value. If E ∼
= R, this is the usual absolute value.
∼
If E = C, this is the square of the usual absolute value. If E is non-archimedean, this is the
unique multiplicative map E → R≥0 such that |a | = ♯(OE /aOE )−1 if a is a nonzero element of
the valuation ring OE of E .

Fix a positive integer d and a d -dimensional E -vector space V .

2.1 Norms
2.1.1. We recall the definitions of norms and semi-norms on V .
A norm (resp., semi-norm) on V is a map µ: V → R≥0 for which there exist an E -basis

(e i )1≤i ≤d of V and an element (ri )1≤i ≤d of Rd>0 (resp., Rd≥0 ) such that

(r 2 |a 1 |2 · · · + rd2 |a d |2 )1/2 if E ∼
= R,

 1
µ(a 1 e 1 + · · · + a d e d ) = r1 |a 1 | + · · · + rd |a d |
if E ∼
= C,


max(r1 |a 1 |, . . . , rd |a d |) otherwise.
for all a 1 , . . . , a d ∈ E .

2.1.2. We will call the norm (resp., semi-norm) µ in the above, the norm (resp., semi-norm)
given by the basis (e i )i and by (ri )i .
2.1.3. We have the following characterizations of norms and semi-norms.
(1) If E ∼
= C), then there is a one-to-one correspondence between semi-norms
= R (resp., E ∼
on V and symmetric bilinear (resp., Hermitian) forms ( , ) on V such that (x ,x ) ≥ 0 for

all x ∈ V . The semi-norm µ corresponding to ( , ) is given by µ(x ) = (x ,x )1/2 (resp.,

µ(x ) = (x ,x )). This restricts to a correspondence between norms and forms that are
positive definite.

(2) If E is non-archimedean, then (as in [9]) a map µ: V → R≥0 is a norm (resp., semi-norm)
if and only if µ satisfies the following (i)–(iii) (resp., (i) and (ii)):
4

(i) µ(a x ) = |a |µ(x ) for all a ∈ E and x ∈ V ,

(ii) µ(x + y ) ≤ max(µ(x ), µ(y )) for all x , y ∈ V , and

(iii) µ(x ) > 0 if x ∈ V \ {0}.

These well-known facts imply that if µ is a norm (resp., semi-norm) on V and V ′ is an E subspace of V , then the restriction of µ to V ′ is a norm (resp., semi-norm) on V ′ .
2.1.4. We say that two norms (resp., semi-norms) µ and µ′ on V are equivalent if µ′ = c µ for
some c ∈ R>0 .
2.1.5. The group GLV (E ) acts on the set of all norms (resp., semi-norms) on V : for g ∈ GLV (E )

and a norm (resp., semi-norm) µ on V , g µ is defined as µ ◦ g −1 . This action preserves the
equivalence in 2.1.4.

2.1.6. Let V ∗ be the dual space of V . Then there is a bijection between the set of norms on V
and the set of norms on V ∗ . That is, for a norm µ on V , the corresponding norm µ∗ on V ∗ is
given by


|ϕ(x )|
µ∗ (ϕ) = sup
| x ∈ V \ {0}
µ(x )


for ϕ ∈ V ∗ .

For a norm µ on V associated to a basis (e i )i of V and (ri )i ∈ Rd>0 , the norm µ∗ on V ∗ is associated to the dual basis (e i∗ )i of V ∗ and (ri−1 )i . This proves the bijectivity.

2.1.7. For a norm µ on V and for g ∈ GLV (E ), we have
(µ ◦ g )∗ = µ∗ ◦ (g ∗ )−1 ,
which is to say (g µ)∗ = (g ∗ )−1 µ∗ , where g ∗ ∈ GLV ∗ (E ) is the transpose of g .

2.2 Definitions of the spaces
2.2.1. Let X V denote the set of all equivalence classes of norms on V (as in 2.1.4). We endow
X V with the quotient topology of the subspace topology on the set of all norms on V inside
RV .
2.2.2. In the case that E is archimedean, we have

PGLd (R)/ POd (R) ∼
= SLd (R)/ SOd (R) if E
XV ∼
=
PGL (C)/ PU(d ) ∼
if E
= SLd (C)/ SU(d )
d

∼
=R
∼
= C.

In the case E is non-archimedean, X V is identified with (a geometric realization of) the BruhatTits building associated to PGLV [4] (see also [5, Section 2]).
5

2.2.3. Recall that for a finite-dimensional vector space H 6= 0 over a field I , the following four
objects are in one-to-one correspondence:

(i) a parabolic subgroup of the algebraic group GLH over I ,
(ii) a parabolic subgroup of the algebraic group PGLH over I ,
(iii) a parabolic subgroup of the algebraic group SLH over I , and
(iv) a flag of I -subspaces of H (i.e., a set of subspaces containing {0} and H and totally ordered under inclusion).

The bijections (ii) 7→ (i) and (i) 7→ (iii) are the taking of inverse images. The bijection (i) 7→ (iv)

sends a parabolic subgroup P to the set of all P-stable I -subspaces of H , and the converse
map takes a flag to its isotropy subgroup in GLH .
2.2.4. Let X̄ V be the set of all pairs (P, µ), where P is a parabolic subgroup of the algebraic
group PGLV over E and, if
0 = V−1 ( V0 ( · · · ( Vm = V
denotes the flag corresponding to P (2.2.3), then µ is a family (µi )0≤i ≤m , where µi is an equivalence class of norms on Vi /Vi −1 .

We have an embedding X V ,→ X̄ V which sends µ to (PGLV , µ).

2.2.5. Let X̄ V♭ be the set of all equivalence classes of nonzero semi-norms on the dual space V ∗
of V (2.1.4). We have an embedding X V ,→ X̄ V♭ which sends µ to µ∗ (2.1.6).
This set X̄ V♭ is also identified with the set of pairs (W, µ) with W a nonzero E -subspace of V
and µ an equivalence class of a norm on W . In fact, µ corresponds to an equivalence class µ∗
of a norm on the dual space W ∗ of W (2.1.6), and µ∗ is identified via the projection V ∗ → W ∗
with an equivalence class of semi-norms on V ∗ .

We call the understanding of X̄ V♭ as the set of such pairs (W, µ) the definition of X̄ V♭ in the
second style. In this interpretation of X̄ V♭ , the above embedding X V → X̄ V♭ is written as µ 7→

(V, µ).

2.2.6. In the case that E is non-archimedean, X̄ V is the polyhedral compactification of the
Bruhat-Tits building X V by Gérardin [7] and Landvogt [19] (see also [11, Proposition 19]), and
X̄ V♭ is the compactification of X V by Werner [24, 25]. In the case that E is archimedean, X̄ V is
the maximal Satake compactification, and X̄ V♭ is the minimal Satake compactification for the
standard projective representation of PGLV (E ), as constructed by Satake in [22] (see also [2,
1.4]). The topologies of X̄ V and X̄ V♭ are reviewed in Section 2.3 below.

6

2.2.7. We have a canonical surjection X̄ V → X̄ V♭ which sends (P, µ) to (V0 , µ0 ), where V0 is as
in 2.2.4, and where we use the definition of X̄ V♭ of the second style in 2.2.5. This surjection is
compatible with the inclusion maps from X V to these spaces.
2.2.8. We have the natural actions of PGLV (E ) on X V , X̄ V and X̄ V♭ by 2.1.5. These actions are
compatible with the canonical maps between these spaces.

2.3 Topologies
2.3.1. We define a topology on X̄ V .
Take a basis (e i )i of V . We have a commutative diagram
PGLV (E ) × Rd>0−1

XV

PGLV (E ) × Rd≥0−1

X̄ V .

Here the upper arrow is (g , t ) 7→ g µ, where µ is the class of the norm on V associated to
Q
((e i )i , (ri )i ) with ri = 1≤j <i t j−1 , and where g µ is defined by the action of PGLV (E ) on X V
(2.2.8). The lower arrow is (g , t ) 7→ g (P, µ), where (P, µ) ∈ X̄ V is defined as follows, and g (P, µ) is
then defined by the action of PGLV (E ) on X̄ V (2.2.8). Let

I = {j | t j = 0} ⊂ {1, . . . , d − 1},
and write
I = {c (i ) | 0 ≤ i ≤ m − 1},
where m = ♯I and 1 ≤ c (0) < · · · < c (m − 1) ≤ d − 1. If we also let c (−1) = 0 and c (m ) = d , then

the set of

c (i )
X

Vi =

Fej
j =1

with −1 ≤ i ≤ m forms a flag in V , and P is defined to be the corresponding parabolic sub-

group of PGLV (2.2.3). For 0 ≤ i ≤ m , we take µi to be the equivalence class of the norm on
Q
Vi /Vi −1 given by the basis (e j )c (i −1)<j ≤c (i ) and the sequence (r j )c (i −1)<j ≤c (i ) with r j = c (i −1)<k <j t k−1 .
Both the upper and the lower horizontal arrows in the diagram are surjective, and the

topology on X V coincides with the topology as a quotient space of PGLV (E ) × Rd>0−1 via the

upper horizontal arrow. The topology on X̄ V is defined as the quotient topology of the topol-

ogy on PGLV (E ) × Rd≥0−1 via the lower horizontal arrow. It is easily seen that this topology is

independent of the choice of the basis (e i )i .

7

2.3.2. The space X̄ V♭ has the following topology: the space of all nonzero semi-norms on V ∗
∗

has a topology as a subspace of the product RV , and X̄ V♭ has a topology as a quotient of it.
2.3.3. Both X̄ V and X̄ V♭ are compact Hausdorff spaces containing X V as a dense open subset.
This is proved in [7, 19, 24, 25] in the case that E is non-archimedean and in [22, 2] in the
archimedean case.
2.3.4. The topology on X̄ V♭ coincides with the image of the topology on X̄ V . In fact, it is easily
seen that the canonical map X̄ V → X̄ V♭ is continuous (using, for instance, [25, Theorem 5.1]).

Since both spaces are compact Hausdorff and this continuous map is surjective, the topology
on X̄ V♭ is the image of that of X̄ V .

3

Spaces associated to global fields

Let F be a global field, which is to say, either a number field or a function field in one variable
over a finite field. We fix a finite-dimensional F -vector space V of dimension d ≥ 1. For a place
v of F , let Vv = Fv ⊗F V . We set X v = X Vv and X v♭ = X V♭ v for brevity. If v is non-archimedean, we

let Ov , k v , qv , ̟v denote the valuation ring of Fv , the residue field of Ov , the order of k v , and a
fixed uniformizer in Ov , respectively.
⋆
In this section, we define sets X̄ F,v
containing X v for ⋆ ∈ {♯, , ♭}, which serve as our rational

♭
partial compactifications. Here, X̄ F,v (resp., X̄ F,v
) is defined as a subset of X v (resp., X̄ v♭ ), and
♯

X̄ F,v has X̄ F,v as a quotient. In §3.2, by way of example, we describe these sets and various
topologies on them in the case that d = 2, F = Q, and v is the real place. For ⋆ 6= ♯, we construct

⋆
⋆
more generally sets X̄ F,S
for a nonempty finite set S of places of F . In §3.1, we describe X̄ F,S
as
Q
⋆
a subset of v ∈S X̄ F,v .

In §3.3 and §3.4, we define topologies on these sets. That is, in §3.3, we define the “Borel-

Serre topology”, while in §3.4, we define the “Satake topology” on X̄ F,v and, assuming S con♭
tains all archimedean places of F , on X̄ F,S
. In §3.5, we prove results on X̄ F,v . In §3.6, we com♭
pare the following topologies on X̄ F,v (resp., X̄ F,v
): the Borel-Serre topology, the Satake topol-

ogy, and the topology as a subspace of X̄ v (resp., X̄ v♭ ). In §3.7, we describe the relationship
between these spaces and Borel-Serre and reductive Borel-Serre spaces.

3.1 Definitions of the spaces
3.1.1. Let X̄ F,v = X̄ V,F,v be the subset of X̄ v consisting of all elements (P, µ) such that P is F rational. If P comes from a parabolic subgroup P ′ of PGLV over F , we also denote (P, µ) by
(P ′ , µ).
8

♭
3.1.2. Let X̄ F,v
be the subset of X̄ v♭ consisting of all elements (W, µ) such that W is F -rational

(using the definition of X̄ v♭ in the second style in 2.2.5). If W comes from an F -subspace W ′ of
V , we also denote (W, µ) by (W ′ , µ).
♯

3.1.3. Let X̄ F,v be the set of all triples (P, µ, s ) such that (P, µ) ∈ X̄ F,v and s is a splitting
s:

m
M
i =0

∼

(Vi /Vi −1 )v −
→ Vv

over Fv of the filtration (Vi )−1≤i ≤m of V corresponding to P.
♯

We have an embedding X v ,→ X̄ F,v that sends µ to (PGLV , µ, s ), where s is the identity map

of Vv .
3.1.4. We have a diagram with a commutative square
♯

X̄ F,v

X̄ F,v

♭
X̄ F,v

X̄ v

X̄ v♭ .

Here, the first arrow in the upper row forgets the splitting s , and the second arrow in the upper
row is (P, µ) 7→ (V0 , µ0 ), as is the lower arrow (2.2.7).
♯

♭
3.1.5. The group PGLV (F ) acts on the sets X̄ F,v , X̄ F,v
and X̄ F,v in the canonical manner.

3.1.6. Now let S be a nonempty finite set of places of F .
Q
• Let X̄ F,S be the subset of v ∈S X̄ F,v consisting of all elements (x v )v ∈S such that the parabolic
subgroup of G = PGLV associated to x v is independent of v .
Q
♭
♭
• Let X̄ F,S
be the subset of v ∈S X̄ F,v
consisting of all elements (x v )v ∈S such that the F subspace of V associated to x v is independent of v .

We will denote an element of X̄ F,S as (P, µ), where P is a parabolic subgroup of G and µ ∈
Q
♭
v ∈S,0≤i ≤m X (Vi /Vi −1 )v with (Vi )i the flag corresponding to P. We will denote an element of X̄ F,S
Q
as (W, µ), where W is a nonzero F -subspace of V and µ ∈ v ∈S X Wv . We have a canonical

surjective map

♭
X̄ F,S → X̄ F,S

which commutes with the inclusion maps from X S to these spaces.

9

3.2 Example: Upper half-plane
3.2.1. Suppose that F = Q, v is the real place, and d = 2.
♯

♭
, and X̄ Q,v are described by using the upper
In this case, the sets X v , X̄ v = X̄ v♭ , X̄ Q,v = X̄ Q,v

half-plane. In §2, we discussed topologies on the first two spaces. The remaining spaces also
♯

have natural topologies, as will be discussed in §3.3 and §3.4: the space X̄ Q,v is endowed with
the Borel-Serre topology, and X̄ Q,v has two topologies, the Borel-Serre topology and Satake
topology, which are both different from its topology as a subspace of X̄ v . In this section, as a
prelude to §3.3 and §3.4, we describe what the Borel-Serre and Satake topologies look like in
this special case.
3.2.2. Let H = {x + y i | x , y ∈ R, y > 0} be the upper half-plane. Fix a basis (e i )i =1,2 of V . For

z ∈ H, let µz denote the class of the norm on V corresponding to the class of the norm on

V ∗ given by a e 1∗ + b e 2∗ 7→ |a z + b | for a ,b ∈ R. Here (e i∗ )1≤i ≤d is the dual basis of (e i )i , and

| | denotes the usual absolute value (not the normalized absolute value) on C. We have a
homeomorphism

∼

H −→ X v ,

z 7→ µz

which is compatible with the actions of SL2 (R).
For the square root i ∈ H of −1, the norm a e 1 +b e 2 7→ (a 2 +b 2)1/2 has class µi . For z = x +y i ,

we have

µz =

The action of

!
−1 0
0

1

y

x

0

1

!
µi .

∈ GL2 (R) on X v corresponds to x + y i 7→ −x + y i on H.

3.2.3. The inclusions
X v ⊂ X̄ Q,v ⊂ X̄ v
can be identified with

H ⊂ H ∪ P1(Q) ⊂ H ∪ P1 (R).
Here z ∈ P1 (R) = R ∪ {∞} corresponds to the class in X̄ v♭ = X̄ v of the semi-norm a e 1∗ + b e 2∗ 7→

|a z +b | (resp., a e 1∗ +b e 2∗ 7→ |a |) on V ∗ if z ∈ R (resp., z = ∞). These identifications are compatible with the actions of PGLV (Q).

The topology on X̄ v of 2.3.1 is the topology as a subspace of P1 (C).

10

3.2.4. Let B be the Borel subgroup of PGLV consisting of all upper triangular matrices for the
basis (e i )i , and let 0 = V−1 ( V0 = Qe 1 ( V1 = V be the corresponding flag. Then ∞ ∈ P1 (Q) is

understood as the point (B, µ) of X̄ Q,v , where µ is the unique element of X (V0 )v × X (V /V0 )v .
♯

♯

Let X̄ Q,v (B ) = H ∪ {∞} ⊂ X̄ Q,v and let X̄ Q,v (B ) be the inverse image of X̄ Q,v (B ) in X̄ Q,v . Then

for the Borel-Serre topology defined in §3.3, we have a homeomorphism
♯
X̄ Q,v (B ) ∼
= {x + y i | x ∈ R, 0 < y ≤ ∞} ⊃ H.

Here x + ∞i corresponds to (B, µ, s ) where s is the splitting of the filtration (Vi ,v )i given by the

embedding (V /V0 )v → Vv that sends the class of e 2 to x e 1 + e 2 .
♯

The Borel-Serre topology on X̄ Q,v is characterized by the properties that
♯

(i) the action of the discrete group GLV (Q) on X̄ Q,v is continuous,
♯

(ii) the subset X̄ Q,v (B ) is open, and
♯

(iii) as a subspace, X̄ Q,v (B ) is homeomorphic to {x + y i | x ∈ R, 0 < y ≤ ∞} as above.
3.2.5. The Borel-Serre and Satake topologies on X̄ Q,v (defined in §3.3 and §3.4) are characterized by the following properties:
(i) The subspace topology on X v ⊂ X̄ Q,v coincides with the topology on H.
(ii) The action of the discrete group GLV (Q) on X̄ Q,v is continuous.
(iii) The following sets (a) (resp., (b)) form a base of neighborhoods of ∞ for the Borel-Serre
(resp., Satake) topology:

(a) the sets U f = {x + y i ∈ H | y ≥ f (x )} ∪ {∞} for continuous f : R → R,

(b) the sets Uc = {x + y i ∈ H | y ≥ c } ∪ {∞} with c ∈ R>0 .

♯

The Borel-Serre topology on X̄ Q,v is the image of the Borel-Serre topology on X̄ Q,v .
3.2.6. For example, the set {x +y i ∈ H | y > x }∪{∞} is a neighborhood of ∞ for the Borel-Serre

topology, but it is not a neighborhood of ∞ for the Satake topology.

3.2.7. For any subgroup Γ of PGL2 (Z) of finite index, the Borel-Serre and Satake topologies
induce the same topology on the quotient space X (Γ) = Γ\X̄ Q,v . Under this quotient topology,
X (Γ) is compact Hausdorff. If Γ is the image of a congruence subgroup of SL2 (Z), then this is
the usual topology on the modular curve X (Γ).

11

3.3 Borel-Serre topology
♯

3.3.1. For a parabolic subgroup P of PGLV , let X̄ F,v (P) (resp., X̄ F,v (P)) be the subset of X̄ F,v
♯

(resp., X̄ F,v ) consisting of all elements (Q, µ) (resp., (Q, µ, s )) such that Q ⊃ P.

The action of PGLV (Fv ) on X̄ v induces an action of P(Fv ) on X̄ F,v (P). We have also an action
♯

of P(Fv ) on X̄ F,v (P) given by
g (α, s ) = (g α, g ◦ s ◦ g −1 )
for g ∈ P(Fv ), α ∈ X̄ F,v (P), and s a splitting of the filtration.
3.3.2. Fix a basis (e i )i of V . Let P be a parabolic subgroup of PGLV such that
• if 0 = V−1 ( V0 ( · · · ( Vm = V denotes the flag of F -subspaces corresponding to P, then
each Vi is generated by the e j with 1 ≤ j ≤ c (i ), where c (i ) = dim(Vi ).

This condition on P is equivalent to the condition that P contains the Borel subgroup B of
PGLV consisting of all upper triangular matrices with respect to (e i )i . Where useful, we will
identify PGLV over F with PGLd over F via the basis (e i )i .
Let
∆(P) = {dim(Vj ) | 0 ≤ j ≤ m − 1} ⊂ {1, . . . , d − 1},
and let ∆′ (P) be the complement of ∆(P) in {1, . . . , d − 1}. Let Rd≥0−1 (P) be the open subset of
Rd≥0−1 given by

Rd≥0−1 (P) = {(t i )1≤i ≤d −1 ∈ Rd≥0−1 | t i > 0 for all i ∈ ∆′ (P)}.

In particular, we have
∆′ (P)

Rd≥0−1 (P) ∼
= R>0 × R≥0 .
∆(P)

3.3.3. With P as in 3.3.2, the map PGLV (Fv ) × Rd≥0−1 → X̄ v in 2.3.1 induces a map
π̄P,v : P(Fv ) × Rd≥0−1 (P) → X̄ F,v (P),
which restricts to a map
πP,v : P(Fv ) × Rd>0−1 → X F,v .
The map π̄P,v is induced by a map
♯

♯

π̄P,v : P(Fv ) × Rd≥0−1 (P) → X̄ F,v (P)
defined as (g , t ) 7→ g (P, µ, s ) where (P, µ) is as in 2.3.1 and s is the splitting of the filtration

(Vi )−1≤i ≤m defined by the basis (e i )i . For this splitting s , we set
X
V (i ) = s (Vi /Vi −1 ) =
Fej
c (i −1)<j ≤c (i )

12

for 0 ≤ i ≤ m so that Vi = Vi −1 ⊕ V (i ) and V =

Lm
i =0

V (i ) . If P = B , then we will often omit the

subscript B from our notation for these maps.
3.3.4. We review the Iwasawa decomposition. For v archimedean (resp., non-archimedean),
let A v ≤ PGLd (Fv ) be the subgroup of elements of that lift to diagonal matrices in GLd (Fv ) with

positive real entries (resp., with entries that are powers of ̟v ). Let K v denote the standard
maximal compact subgroup of PGLd (Fv ) given by


POd (R)


K v = PUd


PGLd (Ov )

if v is real,
if v is complex,
otherwise.

Let B u denote the upper-triangular unipotent matrices in the standard Borel B . The Iwasawa
decomposition is given by the equality
PGLd (Fv ) = B u (Fv )A v K v .
3.3.5. If v is archimedean, then the expression of a matrix in PGLd (Fv ) as a product in the
Iwasawa decomposition is unique.
3.3.6. If v is non-archimedean, then the Bruhat decomposition is PGLd (k v ) = B (k v )S d B (k v ),
where the symmetric group S d of degree d is viewed as a subgroup of PGLd over any field via
the permutation representation on the standard basis. This implies that PGLd (Ov ) = B (Ov )S d Iw(Ov ),
where Iw(Ov ) is the Iwahori subgroup consisting of those matrices in with upper triangular image in PGLd (k v ). Combining this with the Iwasawa decomposition (in the notation of 3.3.4),
we have
PGLd (Fv ) = B u (Fv )A v S d Iw(Ov ).
This decomposition is not unique, since B u (Fv ) ∩ Iw(Ov ) = B u (Ov ).
∼

3.3.7. If v is archimedean, then there is a bijection Rd>0−1 −
→ A v given by

t = (t k )1≤k ≤d −1 7→ a =

where ri =

Qi −1

t −1
k =1 k

diag(r1 , . . . , rd )−1

if v is real,

diag(r 1/2 , . . . , r 1/2 )−1

if v is complex,

1

as in 2.3.1.

Proposition 3.3.8.

13

d

(1) Let P be a parabolic subgroup of PGLV as in 3.3.2. Then the maps
♯

♯

π̄P,v : P(Fv ) × Rd≥0−1 (P) → X̄ F,v (P) and π̄P,v : P(Fv ) × Rd≥0−1 (P) → X̄ F,v (P)
of 3.3.3 are surjective.
(2) For the Borel subgroup B of 3.3.2, the maps
πv : B u (Fv ) × Rd>0−1 → X v ,

π̄v : B u (Fv ) × Rd≥0−1 → X̄ F,v (B ),

♯

♯

and π̄v : B u (Fv ) × Rd≥0−1 → X̄ F,v (B ).
of 3.3.3 are all surjective.
♯

(3) If v is archimedean, then πv and π̄v are bijective.
(4) If v is non-archimedean, then π̄v induces a bijection
(B u (Fv ) × Rd≥0−1 )/∼ → X̄ F,v (B )
where (g , (t i )i ) ∼ (g ′ , (t i′ )i ) if and only if
(i) t i = t i′ for all i and
Q
(ii) |(g −1 g ′ )i j | ≤ ( i ≤k <j t k )−1 for all 1 ≤ i < j ≤ d , considering any c ∈ R to be less than
0−1 = ∞.

♯

♯

Proof. If π̄v is surjective, then for any parabolic P containing B , the restriction of π̄v to B u (Fv )×
♯

♯

Rd≥0−1 (P) has image X̄ F,v (P). Since B u (Fv ) ⊂ P(Fv ), this forces the surjectivity of π̄P,v , hence of
π̄P,v as well. So, we turn our attention to (2)–(4). If r ∈ Rd>0 , we let µ(r ) ∈ X v denote the class of

the norm attached to the basis (e i )i and r .

Suppose first that v is archimedean. By the Iwasawa decomposition 3.3.4, and noting 3.3.5
and 2.2.2, we see that B u (Fv )A v → X v given by g 7→ g µ(1) is bijective, where µ(1) denotes the

class of the norm attached to (e i )i and 1 = (1, . . . , 1) ∈ Rd>0 . For t ∈ Rd>0−1 , let a ∈ A v be its image

under the bijection in 3.3.7. Since p a µ(1) = p µ(r ) , for p ∈ B u (Fv ) and r as in 2.3.1, we have the
bijectivity of πv .

Consider the third map in (2). For t ∈ Rd≥0−1 , let P be the parabolic that contains B and is

determined by the set ∆(P) of k ∈ {1, . . . , d − 1} for which t k = 0. Let (Vi )−1≤i ≤m be the correQm
sponding flag. Let M denote the Levi subgroup of P. (It is the quotient of i =0 GLV (i ) < GLV
Lm
by scalars, where V = i =0 V (i ) as in 3.3.3, and M ∩ B u is isomorphic to the product of the
upper-triangular unipotent matrices in each PGLV (i ) .) The product of the first maps in (2) for
the blocks of M is a bijection
∆′ (P) ∼

(M ∩ B u )(Fv ) × R>0

−
→

14

m
Y
i =0

X Vv(i ) ⊂ X̄ F,v (P),

such that (g , t ′ ) is sent to (P, g µ) in X̄ F,v , where µ is the sequence of classes of norms determined by t ′ and the standard basis. The stabilizer of µ in B u is the unipotent radical Pu of P,
and this Pu acts simply transitively on the set of splittings for the graded quotients (Vi /Vi −1 )v .

Since B u = (M ∩ B u )Pu , and this decomposition is unique, we have the desired bijectivity of
♯

π̄v , proving (3).

Suppose next that v is non-archimedean. We prove the surjectivity of the first map in (2).
Using the natural actions of A v and the symmetric group S d on Rd>0 , we see that any norm on
Vv can be written as g µ(r ) , where g ∈ PGLd (Fv ) and r = (ri )i ∈ Rd>0 , with r satisfying
r1 ≤ r2 ≤ · · · ≤ rd ≤ qv r1 .
For such an r , the class µ(r ) is invariant under the action of Iw(Ov ). Hence for such an r , any
′

element of S d Iw(Ov )µ(r ) = S d µ(r ) is of the form µ(r ) , where r ′ = (rσ(i ) )i for some σ ∈ S d . Hence,
′

any element of A v S d Iw(Ov )µ(r ) for such an r is of the form µ(r ) for some r ′ = (ri′ )i ∈ Rd>0 . This

proves the surjecivity of the first map of (2). The surjectivity of the other maps in (2) is then
shown using this, similarly to the archimedean case.
Finally, we prove (4). It is easy to see that the map π̄v factors through the quotient by
the equivalence relation. We can deduce the bijectivity in question from the bijectivity of
(B u (Fv )×Rd>0−1 )/∼ → X v , replacing V by Vi /Vi −1 as in the above arguments for the archimedean

case. Suppose that πv (g , t ) = πv (1, t ′ ) for g ∈ B u (Fv ) and t , t ′ ∈ Rd>0−1 . We must show that
′

(g , t ) ∼ (1, t ′ ). Write πv (g , t ) = g µ(r ) and πv (1, t ′ ) = µ(r ) with r = (ri )i and r ′ = (ri′ )i ∈ Rd>0 such
Q
that r1 = 1 and r j /ri = ( i ≤k <j t k )−1 for all 1 ≤ i < j ≤ d , and similarly for r ′ and t ′ . It then
′

suffices to check that r ′ = r and ri |g i j | ≤ r j for all i < j . Since µ(r ) = g −1 µ(r ) , there exists c ∈ R>0

such that

max{ri |x i | | 1 ≤ i ≤ d } = c max{ri′ |(g x )i | | 1 ≤ i ≤ d }
for all x = (x i )i ∈ Fvd . Taking x = e 1 , we have g x = e 1 as well, so c = 1. Taking x = e i , we

obtain ri ≥ ri′ , and taking x = g −1 e i , we obtain ri ≤ ri′ . Thus r = r ′ , and taking x = e j yields

r j = max{ri |g i j | | 1 ≤ i ≤ j }, which tells us that r j ≥ ri |g i j | for i < j .
♯

Proposition 3.3.9. There is a unique topology on X̄ F,v (resp., X̄ F,v ) satisfying the following conditions (i) and (ii).
♯

(i) For every parabolic subgroup P of PGLV , the set X̄ F,v (P) (resp., X̄ F,v (P)) is open in X̄ F,v
♯

(resp., X̄ F,v ).
(ii) For every parabolic subgroup P of PGLV and basis (e i )i of V such that P contains the Borel
♯

subgroup with respect to (e i )i , the topology on X̄ F,v (P) (resp., X̄ F,v (P)) is the topology as a
quotient of P(Fv ) × Rd≥0−1(P) under the surjection of 3.3.8(1).
15

This topology is also characterized by (i) and the following (ii)’.
(ii)’ If B is a Borel subgroup of PGLV consisting of upper triangular matrices with respect to a
♯

basis (e i )i of V , then the topology on X̄ F,v (B ) (resp., X̄ F,v (B )) is the topology as a quotient
of B u (Fv ) × Rd≥0−1 under the surjection of 3.3.8(2).
Proof. The uniqueness is clear if we have existence of a topology satisfying (i) and (ii). Let
(e i )i be a basis of V , let B be the Borel subgroup of PGLV with respect to this basis, and let
P be a parabolic subgroup of PGLV containing B . It suffices to prove that for the topology
♯

on X̄ F,v (B ) (resp., X̄ F,v (B )) as a quotient of B u (Fv ) × Rd≥0−1 (B ), the subspace topology on X̄ F,v (P)
♯

(resp., X̄ F,v (P)) coincides with the quotient topology from P(Fv )×Rd≥0−1 (P). For this, it is enough
♯

to show that the action of the topological group P(Fv ) on X̄ F,v (P) (resp., X̄ F,v (P)) is continuous
♯

with respect to the topology on X̄ F,v (P) (resp., X̄ F,v (P)) as a quotient of B u (Fv ) × Rd≥0−1 (P). We
must demonstrate this continuity.

Let (Vi )−1≤i ≤m be the flag corresponding to P, and let c (i ) = dim(Vi ). For 0 ≤ i ≤ m , we
Lm
regard GLV (i ) as a subgroup of GLV via the decomposition V = i =0 V (i ) of 3.3.3.
Suppose first that v is archimedean. For 0 ≤ i ≤ m , let K i be the compact subgroup of

GLV (i ) (Fv ) that is the isotropy group of the norm on V (i ) given by the basis (e j )c (i −1)<j ≤c (i ) and
Q
(1, . . . , 1) ∈ c (i −1)<j ≤c (i ) R>0 . We identify Rd>0−1 with A v as in 3.3.7. By the Iwasawa decomposition 3.3.4 and its uniqueness in 3.3.5, the product on P(Fv ) induces a homeomorphism
!
m
Y
∼
K i /{z ∈ Fv× | |z | = 1}.
(a ,b, c ): P(Fv ) −
→ B u (Fv ) × Rd>0−1 ×
i =0

∆′ (P)

We also have a product map φ : P(Fv ) × B u (Fv ) × R>0

∆′ (P)

→ P(Fv ), where we identify t ′ ∈ R>0
1/2

1/2

with the diagonal matrix diag(r1 , . . . , rd )−1 if v is real and diag(r1 , . . . , rd )−1 if v is complex,
Q
with r j−1 = c (i −1)<k <j t k′ for c (i − 1) < j ≤ c (i ) as in 2.3.1. These maps fit in a commutative

diagram

∆(P)

P(Fv ) × R≥0

(φ, id)

∆′ (P)

∆(P)

P(Fv ) × B u (Fv ) × R>0 × R≥0

(id, π̄P,v )

π̄P,v

B u (Fv ) × Rd≥0−1 (P)

P(Fv ) × X̄ F,v (P)
X̄ F,v (P)

in which the right vertical arrow is the action of P(Fv ) on X̄ F,v (P), and the left vertical arrow is
the continuous map
(u , t ) 7→ (a (u ),b (u ) · (1, t )),
∆′ (P)

∆(P)

(u , t ) ∈ P(Fv ) × R≥0
∆(P)

for (1, t ) the element of Rd≥0−1 (P) with R>0 -component 1 and R≥0 -component t . (To see the
commutativity, note that c (u ) commutes with the block-scalar matrix determined by (1, t ).)
16

♯

We also have a commutative diagram of the same form for X̄ F,v . Since the surjective horizontal
arrows are quotient maps, we have the continuity of the action of P(Fv ).
Next, we consider the case that v is non-archimedean. For 0 ≤ i ≤ m , let S (i ) be the group

of permutations of the set

I i = {j ∈ Z | c (i − 1) < j ≤ c (i )},
and regard it as a subgroup of GLV (i ) (F ). Let A v be the subgroup of the diagonal torus of
PGLV (Fv ) with respect to the basis (e i )i with entries powers of a fixed uniformizer, as in 3.3.4.
Qm
Consider the action of A v i =0 S (i ) ⊂ P(Fv ) on Rd≥0−1 (P) that is compatible with the action

of P(Fv ) on X̄ F,v (P) via the embedding Rd≥0−1 (P) → X̄ F,v (P). This action is described as follows.

Any matrix a = diag(a 1 , . . . , a d ) ∈ A v sends t ∈ Rd≥0−1 (P) to (t j |a j +1 ||a j |−1 ) j ∈ Rd≥0−1 (P). The action
Qm
of i =0 S (i ) on Rd≥0−1 (P) is the unique continuous action which is compatible with the evident
Qm
action of i =0 S (i ) on Rd>0 via the map Rd>0 → Rd≥0−1 (P) that sends (ri )i to (t j ) j , where t j = r j /r j +1 .

That is, for

σ = (σi )0≤i ≤m ∈

m
Y

S (i ) ,

i =0

let f ∈ S d be the unique permutation with f |I i = σi−1 for all i . Then σ sends t ∈ Rd≥0−1 (P) to the

element t ′ = (t j′ ) j given by


Q
t j′

=



t
f (j )≤k < f (j +1) k

if f (j ) < f (j + 1),

Q

t −1
f (j +1)≤k < f (j ) k

if f (j + 1) < f (j ).

Let C be the compact subset of Rd≥0−1(P) given by
¨

«

C = t = (t j ) j ∈ Rd≥0−1 (P) ∩ [0, 1]d −1 |

Y
c (i −1)<j <c (i )

t j ≥ qv−1 for all 0 ≤ i ≤ m .

Qm
We claim that for each x ∈ Rd≥0−1 (P), there is a finite family (h k )k of elements of A v i =0 S (i ) such
S
that the union k h k C is a neighborhood of x . This is quickly reduced to the following claim.
Claim. Consider the natural action of H = A v S d ⊂ PGLV on the quotient space Rd>0 /R>0 ,

with the class of (a j ) j in A v acting as multiplication by (|a j |) j . Let C be the image of
{r ∈ Rd>0 | r1 ≤ r2 ≤ · · · ≤ rd ≤ qv r1 }

in Rd /R>0 . Then for each x ∈ Rd>0 /R>0 , there is a finite family (h k )k of elements of H such that
S >0
h C is a neighborhood of x .
k k

17

Proof of Claim. This is a well-known statement in the theory of Bruhat-Tits buildings: the
quotient Rd>0 /R>0 is called the apartment of the Bruhat-Tits building X v of PGLV , and the set
C is a (d − 1)-simplex in this apartment. Any (d − 1)-simplex in this apartment has the form

hC for some h ∈ H , for any x ∈ Rd>0 /R>0 there are only finitely many (d − 1)-simplices in this
apartment which contain x , and the union of these is a neighborhood of x in Rd>0 /R>0 .

S
By compactness of C , the topology on the neighborhood k h k C of x is the quotient topolQm
`
ogy from k h k C . Thus, it is enough to show that for each h ∈ A v i =0 S (i ) , the composition
(id,πP,v )

P(Fv ) × B u (Fv ) × hC −−−→ P(Fv ) × X̄ F,v (P) → X̄ F,v (P)
♯

(where the second map is the action) and its analogue for X̄ F,v are continuous.
For 0 ≤ i ≤ m , let Iwi be the Iwahori subgroup of GLV (i ) (Fv ) for the basis (e j )c (i −1)<j ≤c (i ).

By the the Iwasawa and Bruhat decompositions as in 3.3.6, the product on P(Fv ) induces a
continuous surjection
m
Y

B u (Fv ) × A v

i =0

S (i ) ×

m
Y
i =0

Iwi → P(Fv ),

and it admits continuous sections locally on P(Fv ). (Here, the middle group A v

Qm
i =0

S (i ) has

the discrete topology.) Therefore, there exist an open covering (Uλ )λ of P(Fv ) and, for each λ,
a subset Uλ of the above product mapping homeomorphically to Uλ , together with a contin-

uous map

(a λ ,b λ , c λ ): Uλ → B u (Fv ) × A v

m
Y

S

(i )

i =0

m
Y

×

Iwi

i =0
∼

→ Uλ . Let Uλ′ denote the
such that its composition with the above product map is the map Uλ −
inverse image of Uλ under

P(Fv ) × B u (Fv ) → P(Fv ),

(g , g ′ ) 7→ g g ′ h,

so that (Uλ′ )λ is an open covering of P(Fv ) × B u (Fv ). For any γ in the indexing set of the cover,

′
′
let Uλ,γ
be the inverse image of Uλ′ in Uγ × B u (Fv ). Then the images of the Uλ,γ
form an open
′
cover of P(Fv ) × B u (Fv ) as well. Let (a ′λ,γ ,b λ,γ
) be the composition
(a λ ,b λ )

′
Uλ,γ
→ Uλ −−−→ B u (Fv ) × A v

As

Qm
i =0

m
Y

S (i ) .

i =0

Iwi fixes every element of C under its embedding in X̄ F,v (P), we have a commutative

18

diagram
′
Uλ,γ
× hC

P(Fv ) × B u (Fv ) × hC

B u (Fv ) × Rd≥0−1 (P)

X̄ F,v (P)

in which the left vertical arrow is
′
(u , hx ) 7→ (a ′λ,γ (u ),b λ,γ
(u )x )
♯

for x ∈ C . We also have a commutative diagram of the same form for X̄ F,v . This proves the

continuity of the action of P(Fv ).

♯

3.3.10. We call the topology on X̄ F,v (resp., X̄ F,v ) in 3.3.9 the Borel-Serre topology. The BorelSerre topology on X̄ F,v coincides with the quotient topology of the Borel-Serre topology on
♯

X̄ F,v . This topology on X̄ F,v is finer than the subspace topology from X̄ v .
♭
We define the Borel-Serre topology on X̄ F,v
as the quotient topology of the Borel-Serre
♭
topology of X̄ F,v . This topology on X̄ F,v
is finer than the subspace topology from X̄ v♭ .

For a nonempty finite set S of places of F , we define the Borel-Serre topology on X̄ F,S (resp.,
Q
Q
♭
) for the
as the subspace topology for the product topology on v ∈S X̄ F,v (resp., v ∈S X̄ F,v

♭
X̄ F,S
)

♭
Borel-Serre topology on each X̄ F,v (resp., X̄ F,v
).

3.4 Satake topology
3.4.1. For a nonempty finite set of places S of F , we define the Satake topology on X̄ F,S and,
♭
under the assumption S contains all archimedean places, on X̄ F,S
.

The Satake topology is coarser than the Borel-Serre topology of 3.3.10. On the other hand,
the Satake topology and the Borel-Serre topology induce the same topology on the quotient
space by an arithmetic group (4.1.8). Thus, the Hausdorff compactness of this quotient space
can be formulated without using the Satake topology (i.e., using only the Borel-Serre topology). However, arguments involving the Satake topology appear naturally in the proof of this
property. One nice aspect of the Satake topology is that each point has an explicit base of
neighborhoods (3.2.5, 3.4.9, 4.4.9).
3.4.2. Let H be a finite-dimensional vector space over a local field E . Let H ′ and H ′′ be E subspaces of H such that H ′ ⊃ H ′′ . Then a norm µ on H induces a norm ν on H ′ /H ′′ as

follows. Let µ′ be the restriction of µ to H ′ . Let (µ′ )∗ be the norm on (H ′ )∗ dual to µ′ . Let ν ∗ be

the restriction of (µ′ )∗ to the subspace (H ′ /H ′′ )∗ of (H ′ )∗ . Let ν be the dual of ν ∗ . This norm ν is

19

given on x ∈ H ′ /H ′′ by
ν(x ) = inf {µ(x̃ ) | x̃ ∈ H ′ such that x̃ + H ′′ = x }.
3.4.3. For a parabolic subgroup P of PGLV , let (Vi )−1≤i ≤m be the corresponding flag. Set
X̄ F,S (P) = {(P ′ , µ) ∈ X̄ F,S | P ′ ⊃ P}.
For a place v of F , let us set
m
Y

ZF,v (P) =
i =0

X (Vi /Vi −1)v

and ZF,S (P) =

Y

ZF,v (P).
v ∈S

We let P(Fv ) act on ZF,v (P) through P(Fv )/Pu (Fv ), using the PGL(Vi /Vi −1 )v (Fv )-action on X (Vi /Vi −1)v
for 0 ≤ i ≤ m . We define a P(Fv )-equivariant map

φP,v : X̄ F,v (P) → ZF,v (P)
with the product of these over v ∈ S giving rise to a map φP,S : X̄ F,S (P) → ZF,S (P).

′ ( V ′ ( · · · ( V ′ = V corresponding
Let (P ′ , µ) ∈ X̄ F,v (P). Then the spaces in the flag 0 = V−1
m′
0

to P ′ form a subset of {Vi | −1 ≤ i ≤ m }. The image ν = (νi )0≤i ≤m of (P ′ , µ) under φP,v is as
follows: there is a unique j with 0 ≤ j ≤ m ′ such that

Vj ′ ⊃ Vi ) Vi −1 ⊃ Vj ′−1 ,
and νi is the norm induced from µ j on the subquotient (Vi /Vi −1 )v of (Vj ′ /Vj ′−1 )v , in the sense

of 3.4.2. The P(Fv )-equivariance of φP,v is easily seen using the actions on norms of 2.1.5 and
2.1.7.

Though the following map is not used in this subsection, we introduce it here by way of
♭
comparison between X̄ F,S and X̄ F,S
.

3.4.4. Let W be a nonzero F -subspace of V , and set
♭
♭
X̄ F,S
(W ) = {(W ′ , µ) ∈ X̄ F,S
| W ′ ⊃ W }.

For a place v of F , we have a map
♭
♭
φW,v
: X̄ F,v
(W ) → X Wv
♭
♭
which sends (W ′ , µ) ∈ X̄ F,v
(W ) to the restriction of µ to Wv . The map φW,v
is P(Fv )-equivariant,

for P the parabolic subgroup of PGLV consisting of all elements that preserve W . Setting
Q
♭
♭
: X̄ F,S
(W ) →
Z♭F,S (W ) = v ∈S X Wv , the product of these maps over v ∈ S provides a map φW,S

Z♭F,S (W ).

20

3.4.5. For a finite-dimensional vector space H over a local field E , a basis e = (e i )1≤i ≤d of H ,

and a norm µ on H , we define the absolute value |µ : e | ∈ R>0 of µ relative to e as follows.

Suppose that µ is defined by a basis e ′ = (e i′ )1≤i ≤d and a tuple (ri )1≤i ≤d ∈ Rd>0 . Let h ∈ GLH (E )

be the element such that e ′ = he . We then define

|µ : e | = | det(h)|−1

d
Y

ri .

i =1

This is independent of the choice of e ′ and (ri )i . Note that we have
|g µ : e | = | det(g )|−1 |µ : e |
for all g ∈ GLH (E ).
3.4.6. Let P and (Vi )i be as in 3.4.3, and let v be a place of F . Fix a basis e (i ) of (Vi /Vi −1 )v for
each 0 ≤ i ≤ m . Then we have a map

′
φP,v
: X̄ F,v (P) → Rm
,
≥0

(P ′ , µ) 7→ (t i )1≤i ≤m

where (t i )1≤i ≤m is defined as follows. Let (Vj ′ )−1≤j ≤m ′ be the flag associated to P ′ . Let 1 ≤ i ≤

m . If Vi −1 belongs to (Vj ′ ) j , let t i = 0. If Vi −1 does not belong to the last flag, then there is a

unique j such that Vj ′ ⊃ Vi ⊃ Vi −2 ⊃ Vj ′−1 . Let µ̃ j be a norm on (Vj ′ /Vj ′−1 )v which belongs to the
class µ j , and let µ̃ j ,i and µ̃ j ,i −1 be the norms induced by µ j on the subquotients (Vi /Vi −1 )v and
(Vi −1 /Vi −2 )v , respectively. We then let

t i = |µ̃ j ,i −1 : e (i −1) |1/d i −1 · |µ̃ j ,i : e (i ) |−1/d i ,
where d i := dim(Vi /Vi −1 ).

′
is P(Fv )-equivariant for the following action of P(Fv ) on Rm
The map φP,v
≥0 . For g ∈ P(Fv ),

let g̃ ∈ GLV (Fv ) be a lift of g , and for 0 ≤ i ≤ m , let g i ∈ GLVi /Vi −1 (Fv ) be the element induced by
m
′
g̃ . Then g ∈ P(Fv ) sends t ∈ Rm
≥0 to t ∈ R≥0 where

t i′ = | det(g i )|1/d i · | det(g i −1 )|−1/d i −1 · t i .
If we have two families e = (e (i ) )i and f = (f (i ) )i of bases e (i ) and f (i ) of (Vi /Vi −1 )v , and if the

′
map φP,v
defined by e (resp., f ) sends an element to t (resp., t ′ ), then the same formula also

describes the relationship between t and t ′ , in this case taking g i to be the element of GLVi /Vi −1

such that e (i ) = g i f (i ) .

3.4.7. Fix a basis e (i ) of Vi /Vi −1 for each 0 ≤ i ≤ m . Then we have a map
′
φP,S
: X̄ F,S (P) → Rm
,
≥0

where t i =

Q

t ,
v ∈S v,i

(P ′ , µ) 7→ (t i )1≤i ≤m

′
with (t v,i )i the image of (P ′ , µv ) under the map φP,v
of 3.4.6.

21

3.4.8. We define the Satake topology on X̄ F,S as follows.
For a parabolic subgroup P of PGLV , consider the map
′
ψP,S := (φP,S , φP,S
): X̄ F,S (P) → ZF,S (P) × Rm
≥0

from 3.4.3 and 3.4.7, which we recall depends on a choice of bases of the Vi /Vi −1 . We say that a
subset of X̄ F,S (P) is P-open if it is the inverse image of an open subset of ZF,S (P) × Rm
≥0 . By 3.4.6,

the property of being P-open is independent of the choice of bases.

We define the Satake topology on X̄ F,S to be the coarsest topology for which every P-open
set for each parabolic subgroup P of PGLV is open.
By this definition, we have:
3.4.9. Let a ∈ X̄ F,S be of the form (P, µ) for some µ. As U ranges over neighborhoods of the

image (µ, 0) of a in ZF,S (P) × Rm
≥0 , the inverse images of the U in X̄ F,S (P) under ψP,S form a base
of neighborhoods of a in X̄ F,S .

3.4.10. In §3.5 and §3.6, we explain that the Satake topology on X̄ F,S is strictly coarser than the
Borel-Serre topology for d ≥ 2.
3.4.11. The Satake topology on X̄ F,S can differ from the subspace topology of the product
topology for the Satake topology on each X̄ F,v with v ∈ S.
Example. Let F be a real quadratic field, let V = F 2 , and let S = {v 1 , v 2 } be the set of real places

of F . Consider the point (∞, ∞) ∈ (H ∪ {∞}) × (H ∪ {∞}) ⊂ X̄ v 1 × X̄ v 2 (see §3.2), which we regard
as an element of X̄ F,S . Then the sets

Uc := {(x 1 + y 1 i ,x 2 + y 2 i ) ∈ H × H | y 1 y 2 ≥ c } ∪ {(∞, ∞)}
with c ∈ R>0 form a base of neighborhoods of (∞, ∞) in X̄ F,S for the Satake topology, whereas
the sets

Uc′ := {(x 1 + y 1 i ,x 2 + y 2 i ) ∈ H × H | y 1 ≥ c , y 2 ≥ c } ∪ {(∞, ∞)}
for c ∈ R>0 form a base of neighborhoods of (∞, ∞) in X̄ F,S for the topology induced by the

product of Satake topologies on X̄ F,v 1 and X̄ F,v 2 .

3.4.12. Let G = PGLV , and let Γ be a subgroup of G (F ).
• For a parabolic subgroup P of G , let Γ(P) be the subgroup of Γ ∩ P(F ) consisting of all
elements with image in the center of (P/Pu )(F ).

• For a nonzero F -subspace W of V , let Γ(W ) denote the subgroup of elements of Γ that
can be lifted to elements of GLV (F ) which fix every element of W .
22

3.4.13. We let AF denote the adeles of F , let ASF denote the adeles of F outside of S, and let
Q
AF,S = v ∈S Fv so that AF = ASF × AF,S . Assume that S contains all archimedean places of F .

Let G = PGLV , let K be a compact open subgroup of G (ASF ), and let ΓK < G (F ) be the inverse

image of K under G (F ) → G (ASF ).
The following proposition will be proved in 3.5.15.
Proposition 3.4.14. For S, G , K and ΓK as in 3.4.13, the Satake topology on X̄ F,S is the coarsest
topology such that for every parabolic subgroup P of G , a subset U of X̄ F,S (P) is open if
(i) it is open for Borel-Serre topology, and
(ii) it is stable under the action of ΓK ,(P) (see 3.4.12).
The following proposition follows easily from the fact that for any two compact open subgroups K and K ′ of G (ASF ), the intersection ΓK ∩ ΓK ′ is of finite index in both ΓK and ΓK ′ .
♭
Proposition 3.4.15. For S, K and ΓK as in 3.4.13, consider the coarsest topology on X̄ F,S
such
♭
that for every nonzero F -subspace W , a subset U of X̄ F,S
(W ) is open if

(i) it is open for Borel-Serre topology, and
(ii) it is stable under the action of ΓK ,(W ) (see 3.4.12).
Then this topology is independent of the choice of K .
♭
3.4.16. We call the topology in 3.4.15 the Satake topology on X̄ F,S
.

Proposition 3.4.17.
(1) Let P be a parabolic subgroup of PGLV . For both the Borel-Serre and Satake topologies
on X̄ F,S , the set X̄ F,S (P) is open in X̄ F,S , and the action of the topological group P(AF,S ) on
X̄ F,S (P) is continuous.
(2) The actions of the discrete group PGLV (F ) on the following spaces are continuous: X̄ F,S
♭
and X̄ F,S
with their Borel-Serre topologies, X̄ F,S with the Satake topology, and assuming S
♭
contains all archimedean places, X̄ F,S
with the Satake topology.

Proposition 3.4.18. Let W be a nonzero F -subspace of V . Then for the Borel-Serre topology,
♭
and for the Satake topology if S contains all archimedean places of F , the subset X̄ F,S
(W ) is open
♭
in X̄ F,S
.

Part (1) of 3.4.17 was shown in §3.3 for the Borel-Serre topology, and the result for the
Satake topology on X̄ F,S follows from it. The rest of 3.4.17 and 3.4.18 is easily proven.
23

3.5 Properties of X̄ F,S
Let S be a nonempty finite set of places of F .
3.5.1. Let P and (Vi )−1≤i ≤m be as before. Fix a basis e (i ) of Vi /Vi −1 for each i . Set
Y0 = (RS>0 ∪ {(0)v ∈S })m ⊂ (RS≥0 )m .
′
The maps ψP,v := (φP,v , φP,v
): X̄ F,v (P) → ZF,v (P)×Rm
≥0 of 3.4.3 and 3.4.6 for v ∈ S combine to give

the map

ψP,S : X̄ F,S (P) → ZF,S (P) × Y0 .
3.5.2. In addition to the usual topology on Y0 , we consider the weak topology on Y0 that is the
product topology for the topology on RS>0 ∪ {(0)v ∈S } which extends the usual topology on RS>0
by taking the sets

¨

«
(t v )v ∈S ∈ RS>0

Y
|

v ∈S

t v ≤ c ∪ {(0)v ∈S }

for c ∈ R>0 as a base of neighborhoods of (0)v ∈S . In the case that S consists of a single place, we

have Y0 = Rm
≥0 , and the natural topology and the weak topology on Y0 coincide.
Proposition 3.5.3. The map ψP,S of 3.5.1 induces a homeomorphism
∼

Pu (AF,S )\X̄ F,S (P) −
→ ZF,S (P) × Y0
for the Borel-Serre topology (resp., Satake topology) on X̄ F,S and the usual (resp., weak) topology
on Y0 . This homeomorphism is equivariant for the action of P(AF,S ), with the action of P(AF,S )
on Y0 being that of 3.4.6.
This has the following corollary, which is also the main step in the proof.
Corollary 3.5.4. For any place v of F , the map
Pu (Fv )\X̄ F,v (P) → ZF,v (P) × Rm
≥0
is a homeomorphism for both the Borel-Serre and Satake topologies on X̄ F,v .
We state and prove preliminary results towards the proof of 3.5.3.
3.5.5. Fix a basis (e i )i of V and a parabolic subgroup P of PGLV which satisfies the condition
in 3.3.2 for this basis. Let (Vi )−1≤i ≤m be the flag corresponding to P, and for each i , set c (i ) =

dim(Vi ). We define two maps

ξ, ξ⋆ : Pu (Fv ) × ZF,v (P) × Rm
→ X̄ F,v (P).
≥0
24

3.5.6. First, we define the map ξ.
Set ∆(P) = {c (0), . . . , c (m − 1)}. Let ∆i = {j ∈ Z | c (i − 1) < j < c (i )} for 0 ≤ i ≤ m . We then

clearly have


{1, . . . , d − 1} = ∆(P) ∐
For 0 ≤ i ≤ m , let

V (i )

=

P
c (i −1)<j ≤c (i )

m
a


∆i .

i =0

F e j , so Vi = Vi −1 ⊕ V (i ) . We have

∆(P)
Rd≥0−1 (P) = R≥0 ×

m
Y

∼
×
= Rm
≥0

∆
R>0i

i =0

m
Y

∆

R>0i .

i =0

Let B be the Borel subgroup of PGLV consisting of all upper triangular matrices for the
basis (e i )i . Fix a place v of F . We consider two surjections
B u (Fv ) × Rd≥0−1(P) ։ X̄ F,v (P) and B u (Fv ) × Rd≥0−1(P) ։ Pu (Fv ) × ZF,v (P) × Rm
.
≥0
The first is induced by the surjection π̄v of 3.3.8.
The second map is obtained as follows. For 0 ≤ i ≤ m , let B i be the image of B in PGLV (i )
under P → PGLV /V ∼
= PGL (i ) . Then B i is a Borel subgroup of PGL (i ) , and we have a canonical
i

bijection

i −1

V

V

Pu (Fv ) ×

m
Y
i =0

∼

B i ,u (Fv ) −
→ B u (Fv ).

∆

By 3.3.8, we have surjections B i ,u (Fv )× R>0i ։ X (Vi /Vi −1 )v for 0 ≤ i ≤ m . The second (continuous)
surjection is then the composite

m
Y


∼
B u (Fv ) × Rd≥0−1(P) −
→

Pu (Fv ) ×


։ Pu (Fv ) ×

i =0


B i ,u (Fv ) ×

m
Y
i =0


Rm
×
≥0

m
Y


∆
R>0i

i =0


X (Vi /Vi −1 )v

× Rm
= Pu (Fv ) × ZF,v (P) × Rm
.
≥0
≥0

Proposition 3.5.7. There is a unique surjective continuous map
ξ: Pu (Fv ) × ZF,v (P) × Rm
։ X̄ F,v (P)
≥0
for the Borel-Serre topology on X̄ F,v (P) that is compatible with the surjections from B u (Fv ) ×
Rd≥0−1 (P) to these sets. This map induces a homeomorphism
∼

−
→ Pu (Fv )\X̄ F,v (P)
ZF,v (P) × Rm
≥0
that restricts to a homeomorphism of ZF,v (P) × Rm
>0 with Pu (Fv )\X v .
25

This follows from 3.3.8.
3.5.8. Next, we define the map ξ⋆ .
For g ∈ Pu (Fv ), (µi )i ∈ (X (Vi /Vi −1)v )0≤i ≤m , and (t i )1≤i ≤m ∈ Rm
≥0 , we let
ξ⋆ (g , (µi )i , (t i )i ) = g (P ′ , ν),
where P ′ and ν are as in (1) and (2) below, respectively.
(1) Let J = {c (i − 1) | 1 ≤ i ≤ m , t i = 0}. Write J = {c ′ (0), . . . , c ′ (m ′ − 1)} with c ′ (0) < · · · <
c ′ (m ′ − 1). Let c ′ (−1) = 0 and c ′ (m ′ ) = d . For −1 ≤ i ≤ m ′ , let
c ′ (i )
X

′

Vi =
j =1

F e j ⊂ V.

Let P ′ ⊃ P be the parabolic subgroup of PGLV corresponding to the flag (Vi ′ )i .
(2) For 0 ≤ i ≤ m ′ , set

J i = {j | c ′ (i − 1) < c (j ) ≤ c ′ (i )} ⊂ {1, . . . , m }.
L
We identify Vi ′ /Vi ′−1 with j ∈ J i V (j ) via the basis (e k )c ′ (i −1)<k ≤c ′(i ) . We define a norm ν̃i on

Vi ′ /Vi ′−1 as follows. Let µ̃ j be the unique norm on V (j ) which belongs to µ j and satisfies
P
|µ̃ j : (e k )c (j −1)<k ≤c (j )| = 1. For x = j ∈ J i x j with x j ∈ V (j ) , set
P

2

2 1/2

(r µ̃ j (x j ) )

P j ∈ J i j
ν̃i (x ) =
j ∈ J i r j µ̃ j (x j )


maxj ∈ J i (r j µ̃ j (x j ))
where for j ∈ J i , we set

Y
rj =

if v is real,
if v is complex,
if v is non-archimedean,

t ℓ−1 .

ℓ∈ J i
ℓ<j

Let νi ∈ X (Vi ′ /Vi ′−1 )v be the class of the norm ν̃i .
We omit the proofs of the following two lemmas.
Lemma 3.5.9. The composition
ξ⋆

ψP,v

Pu (Fv ) × ZF,v (P) × Rm
−
→ X̄ F,v (P) −→ ZF,v (P) × Rm
≥0
≥0
coincides with the canonical projection. Here, the definition of the second arrow uses the basis
(e j mod Vi −1 )c (i −1)<j ≤c (i ) of Vi /Vi −1 .
26

Lemma 3.5.10. We have a commutative diagram
Pu (Fv ) × ZF,v (P) × Rm
≥0
Pu (Fv ) × ZF,v (P) × Rm
≥0

ξ

X̄ F,v (P)

ξ⋆

X̄ F,v (P)

in which the left vertical arrow is (u , µ, t ) 7→ (u , µ, t ′ ), for t ′ defined as follows. Let I i : X (Vi /Vi −1 )v →
∆

R>0i be the unique continuous map for which the composition
Ii

∆

∆

B i ,u (Fv ) × R>0i → X (Vi /Vi −1 )v −
→ R>0i
is projection onto the second factor, and for j ∈ ∆i , let I i ,j : X (Vi /Vi −1)v → R>0 denote the composi∆

tion of I i with projection onto the factor of R>0i corresponding to j . Then
Y
Y
j −c (i −2)
c (i )−j
I i −1,j (µi ) c (i −1)−c (i −2) ·
t i′ = t i ·
I i ,j (µi ) c (i )−c (i −1)
j ∈∆i −1

j ∈∆i

for 1 ≤ i ≤ m .
3.5.11. Proposition 3.5.3 is quickly reduced to Corollary 3.5.4, which now follows from 3.5.7,
3.5.9 and 3.5.10.
3.5.12. For two topologies T1 , T2 on a set Z , we use T1 ≥ T2 to denote that the identity map of Z

is a continuous map from Z with T1 to Z with T2 , and T1 > T2 to denote that T1 ≥ T2 and T1 6= T2 .
In other words, T1 ≥ T2 if T1 is finer than T2 and T1 > T2 if T1 is strictly finer than T2 .

By 3.5.3, the map ψP,S : X̄ F,S (P) → ZF,S (P) × Y0 is continuous for the Borel-Serre topology on

X̄ F,S and usual topology on Y0 . On X̄ F,S , we therefore have

Borel-Serre topology ≥ Satake topology.
♭
♭
Corollary 3.5.13. For any nonempty finite set S of places of F , the map φW,S
: X̄ F,S
(W ) → Z♭F,S (W )
♭
of 3.4.4 is continuous for the Borel-Serre topology on X̄ F,S
. If S contains all archimedean places

of F , it is continuous for the Satake topology.
Proof. The continuity for the Borel-Serre topology follows from the continuity of ψP,S , noting
♭
that the Borel-Serre topology on X̄ F,S
is the quotient topology of the Borel-Serre topology on
♭
X̄ F,S . Suppose that S contains all archimedean places. As φW,S
is ΓK ,(W ) -equivariant, and ΓK ,(W )

acts trivially on Z♭F,S (W ), the continuity for the Satake topology is reduced to the continuity for
the Borel-Serre topology.
Remark 3.5.14. We remark that the map φP,v : X̄ F,v (P) → ZF,v (P) of 3.4.3 need not be continu♭
♭
ous for the topology on X̄ F,v as a subspace of X̄ v . Similarly, the map φW,v
: X̄ F,v
(W ) → X Wv of

♭
3.4.4 need not be continuous for the subspace topology on X̄ F,v
⊂ X̄ v♭ . See 3.6.6 and 3.6.7.

27

3.5.15. We prove Proposition 3.4.14.
Proof. Let α = (P, µ) ∈ X̄ F,S . Let U be a neighborhood of α for the Borel-Serre topology which is

stable under the action of ΓK ,(P). By 3.5.12, it is sufficient to prove that there is a neighborhood

W of α for the Satake topology such that W ⊂ U .

Let (Vi )−1≤i ≤m be the flag corresponding to P, and let V (i ) be as before. Let Γ1 = ΓK ∩ Pu (F ),

and let Γ0 be the subgroup of ΓK consisting of the elements that preserve V (i ) and act on V (i )

as a scalar for all i . Then Γ1 is a normal subgroup of ΓK ,(P) and Γ1 Γ0 is a subgroup of ΓK ,(P) of
finite index.
Let

)m

(
Y1 = (a v )v ∈S ∈ RS>0 |

Y

av = 1

,

v ∈S

and set s = ♯S. We have a surjective continuous map
1/s

′
(t , t ′ ) 7→ (t i t v,i
)v,i .

Rm
× Y1 ։ Y0 ,
≥0

m
The composition Rm
≥0 × Y1 → Y0 → R≥0 , where the second arrow is (t v,i )v,i 7→ (

cides with projection onto the first coordinate.

Q

t ),
v ∈S v,i i

coin-

Let
Φ = Pu (AF,S ) × Y1 and Ψ = ZF,S (P) × Rm
.
≥0
Consider the composite map
(ξ⋆v )v ∈S

f : Φ × Ψ → Pu (AF,S ) × ZF,S (P) × Y0 −−−→ X̄ F,S (P).
The map f is Γ1 Γ0 -equivariant for the trivial action on Ψ and the following action on Φ: for
(g , t ) ∈ Φ, γ1 ∈ Γ1 and γ0 ∈ Γ0 , we have
γ1 γ0 · (g , t ) = (γ1 γ0 g γ−1
, γ0 t ),
0
where γ0 acts on Y1 via the embedding ΓK → P(AF,S ) and the actions of the P(Fv ) described in
3.4.6. The composition

ψP,S

f

Φ×Ψ−
→ X̄ F,S (P) −→ Ψ
coincides with the canonical projection.
There exists a compact subset C of Φ such that Φ = Γ1 Γ0C for the above action of Γ1 Γ0 on
Φ. Let β = (µ, 0) ∈ Ψ be the image of α under ψP,S . For x ∈ Φ, we have f (x , β ) = α. Hence,

there is an open neighborhood U ′ (x ) of x in Φ and an open neighborhood U ′′ (x ) of β in Ψ
such that U ′ (x ) × U ′′ (x ) ⊂ f −1 (U ). Since C is compact, there is a finite subset R of C such that
28

C⊂

S

W =

U ′ (x ).
x ∈R
′′
ψ−1
P,S (U ) ⊂

Let U ′′ be the open subset

T
x ∈R

U ′′ (x ) of Ψ, which contains β . The P-open set

X̄ F,S (P) is by definition an open neighborhood of α in the Satake topology on

X̄ F,S . We show that W ⊂ f −1 (U ). Since the map Φ × Ψ → X̄ F,S (P) is surjective, it is sufficient to

prove that the inverse image Φ × U ′′ of W in Φ × Ψ is contained f −1 (U ). For this, we note that
!
[
U ′ (x ) × U ′′ ⊂ Γ1 Γ0 f −1 (U ) = f −1 (U ),
Φ × U ′′ = Γ1 Γ0C × U ′′ = Γ1 Γ0
x ∈R

the last equality by the stability of U under the action of ΓK ,(P) ⊃ Γ1 Γ0 and the Γ1 Γ0 -equivariance
of f .

♭
3.5.16. In the case d = 2, the canonical surjection X̄ F,S → X̄ F,S
is bijective. It is a homeomor-

phism for the Borel-Serre topology. If S contains all archimedean places of F , it is a homeomorphism for the Satake topology by 3.4.14.

3.6 Comparison of the topologies
♭
When considering X̄ F,v
, we assume that all places of F other than v are non-archimedean.
♭
3.6.1. For X̄ F,v (resp., X̄ F,v
), we have introduced several topologies: the Borel-Serre topology,

the Satake topology, and the subspace topology from X̄ v (resp., X̄ v♭ ), which we call the weak
topology. We compare these topologies below; note that we clearly have
Borel-Serre topology ≥ Satake topology and Borel-Serre topology ≥ weak topology.
♭
3.6.2. For both X̄ F,v and X̄ F,v
, the following hold:

(1) Borel-Serre topology > Satake topology if d ≥ 2,
(2) Satake topology > weak topology if d = 2,
(3) Satake topology 6≥ weak topology if d > 2.
We do not give full proofs of these statements. Instead, we describe some special cases
that give clear pictures of the differences between these topologies. The general cases can be
proven in a similar manner to these special cases.
♭
are equal, their Borel-Serre
Recall from 3.5.16 that in the case d = 2, the sets X̄ F,v and X̄ F,v

topologies coincide, and their Satake topologies coincide.
3.6.3. We describe the case d = 2 of 3.6.2(1).
Take a basis (e i )i =1,2 of V . Consider the point α = (B, µ) of X̄ F,v , where B is the Borel
subgroup of upper triangular matrices with respect to (e i )i , and µ is the unique element of

ZF,v (B ) = X Fv e 1 × X Vv /Fv e 1 .
29

Let π̄v be the surjection of 3.3.8(2), and identify B u (Fv ) with Fv in the canonical manner.
The images of the sets
{(x , t ) ∈ Fv × R≥0 | t ≤ c } ⊂ B u (Fv ) × R≥0
in X̄ F,v (B ) for c ∈ R>0 form a base of neighborhoods of α for the Satake topology. Thus, while
the image of the set

{(x , t ) ∈ Fv × R≥0 | t < |x |−1 }
is a neighborhood of α for Borel-Serre topology, it is not a neighborhood of α for the Satake
topology.
3.6.4. We prove 3.6.2(2) in the case that v is non-archimedean. The proof in the archimedean
♭
case is similar. Since all boundary points of X̄ F,v = X̄ F,v
are PGLV (F )-conjugate, to show 3.6.2(2),

it is sufficient to consider any one boundary point. We consider α of 3.6.3 for a fixed basis
(e i )i =1,2 of V .
For x ∈ Fv and y ∈ R>0 , let µy ,x be the norm on Vv defined by
µy ,x (a e 1 + b e 2) = max(|a − x b |, y |b |).
The class of µy ,x is the image of (x , y −1 ) ∈ B u (Fv ) × R>0 . Any element of X v is the class of the

norm µy ,x for some x , y . If we vary x ∈ F∞ and y ∈ R>0 , the classes of µy ,x in X̄ F,v converge

under the Satake topology to the point α if and only if y approaches ∞. In X̄ v , the point α is
the class of the semi-norm ν on Vv∗ defined by ν(a e 1∗ + b e 2∗ ) = |a |. By 2.1.7,
µ∗y ,x = µy ,0 ◦

1 −x

0

!!∗

1

= µ∗y ,0 ◦

!
1 0
x

1

,

from which we see that
µ∗y ,x (a e 1∗ + b e 2∗) = max(|a |, y −1 |x a + b |).
Then µ∗y ,x is equivalent to the norm νy ,x on Vv∗ defined by
νy ,x (a e 1∗ + b e 2∗ ) = min(1, y |x |−1 ) max(|a |, y −1 |x a + b |),
and the classes of the νy ,x converge in X̄ v to the class of the semi-norm ν as y → ∞. Therefore,

the Satake topology is finer than the weak topology.

Now, the norm µ∗1,x is equivalent to the norm ν1,x on Vv∗ defined above, which for sufficiently large x satisfies
ν1,x (a e 1 + b e 2) = max(|a /x |, |a + (b /x )|).
30

Thus, as |x | → ∞, the sequence µ1,x converges in X̄ v = X̄ v♭ to the class of the semi-norm ν.

♭
However, the sequence of classes of the norms µ1,x does not converge to α in X̄ F,v = X̄ F,v
for

the Satake topology, so the Satake topology is strictly finer than the weak topology.
3.6.5. We explain the case d = 3 of 3.6.2(3) in the non-archimedean case.
Take a basis (e i )1≤i ≤3 of V . For y ∈ R>0 , let µy be the norm on Vv defined by
µy (a e 1 + b e 2 + c e 3 ) = max(|a |, y |b |, y 2 |c |).
For x ∈ Fv , consider the norm µy ◦ g x , where


1 0 0





.
gx = 
0
1
x


0 0 1
If we vary x ∈ F∞ and let y ∈ R>0 approach ∞, then the class of µy ◦g x in X v converges under the

Satake topology to the class α ∈ X̄ F,v of the pair that is the Borel subgroup of upper triangular
Q2
matrices and the unique element of i =0 X (Vi /Vi −1 )v , where (Vi )−1≤i ≤2 is the corresponding flag.

♭
The quotient topology on X̄ F,v
of the Satake topology on X̄ F,v is finer than the Satake topology
♭
on X̄ F,v
by 3.4.14 and 3.4.15. Thus, if the Satake topology is finer than the weak topology on
♭
X̄ F,v or X̄ F,v
, then the composite µy ◦ g x should converge in X̄ v♭ to the class of the semi-norm ν

on Vv∗ that satisfies ν(a e 1∗ + b e 2∗ + c e 3∗ ) = |a |. However, if y → ∞ and y −2 |x | → ∞, then the class

of µy ◦ g x in X v converges in X̄ v♭ to the class of the semi-norm a e 1∗ + b e 2∗ + c e 3∗ 7→ |b |. In fact, by
2.1.7 we have

(µy ◦ g x )∗ (a e 1∗ + b e 2∗ + c e 3∗ ) = µ∗y ◦ (g x∗ )−1 (a e 1∗ + b e 2∗ + c e 3∗ )
= max(|a |, y −1 |b |, y −2 |−b x + c |) = y −2 |x |νy ,x
where νy ,x is the norm
a e 1∗ + b e 2∗ + c e 3∗ 7→ max(y 2 |x |−1 |a |, y |x |−1 |b |, |−b + x −1 c |)
on Vv∗ . The norms νy ,x converge to the semi-norm a e 1∗ + b e 2∗ + c e 3∗ 7→ |b |.
♭
♭
3.6.6. Let W be a nonzero subspace of V . We demonstrate that the map φW,v
: X̄ F,v
(W ) → X Wv

of 3.4.4 given by restriction to Wv need not be continuous for the weak topology, even though
by 3.5.13, it is continuous for the Borel-Serre topology and (if all places other than v are nonarchimedean) for the Satake topology.
For example, suppose that v is non-archimedean and d = 3. Fix a basis (e i )1≤i ≤3 of V , and

let W = F e 1 + F e 2 . Let µ be the class of the norm

a e 1 + b e 2 7→ max(|a |, |b |)
31

♭
on Wv , and consider the element (W, µ) ∈ X̄ F,v
. For x ∈ Fv and ε ∈ R>0 , let µx ,ε ∈ X v be the class

of the norm

a e 1 + b e 2 + c e 3 7→ max(|a |, |b |, ε−1 |c + b x |)
on Vv . Then µx∗ ,ε is the class of the norm
a e 1∗ + b e 2∗ + c e 3∗ 7→ max(|a |, |b − x c |, ε|c |)
on Vv∗ . When x → 0 and ε → 0, the last norm converges to the semi-norm
a e 1∗ + b e 2∗ + c e 3∗ 7→ max(|a |, |b |)
on Vv∗ , and this implies that µx ,ε converges to (W, µ) for the weak topology. However, the restriction of µx ,ε to Wv is the class of the norm
a e 1 + b e 2 7→ max(|a |, |b |, ε−1 |x ||b |).
If x → 0 and ε = r −1 |x | → 0 for a fixed r > 1, then the latter norms converge to the norm

a e 1 + b e 2 7→ max(|a |, r |b |), the class of which does not coincide with µ.

3.6.7. Let P be a parabolic subgroup of PGLV (F ). We demonstrate that the map φP,v : X̄ F,v (P) →

ZF,v (P) of 3.4.3 is not necessarily continuous for the weak topology, though by 3.5.4, it is con-

tinuous for the Borel-Serre topology and for the Satake topology.
Let d = 3 and W be as in 3.6.6, and let P be the parabolic subgroup of PGLV corresponding
to the flag
0 = V−1 ⊂ V0 = W ⊂ V1 = V.
♭
In this case, the canonical map X̄ F,v (P) → X̄ F,v
(W ) is a homeomorphism for the weak topology

on both spaces. It is also a homeomorphism for the Borel-Serre topology, and for the Satake
topology if all places other than v are non-archimedean. Since ZF,v (P) = X (V ) × X (V /V ) ∼
= XW ,
0 v

0 v

v

the argument of 3.6.6 shows that φP,v is not continuous for the weak topology.

♭
3.6.8. For d ≥ 3, the Satake topology on X̄ F,v
does not coincide with the quotient topology for

the Satake topology on X̄ F,v , which is strictly finer. This is explained in 4.4.12.

3.7 Relations with Borel-Serre spaces and reductive Borel-Serre spaces
3.7.1. In this subsection, we describe the relationship between our work and the theory of
♯

Borel-Serre and reductive Borel-Serre spaces (see Proposition 3.7.4). We also show that X̄ F,v is
not Hausdorff if v is a non-archimedean place.
32

3.7.2. Let G be a semisimple algebraic group over Q. We recall the definitions of the BorelSerre and reductive Borel-Serre spaces associated to G from [3] and [26, p. 190], respectively.
Let Y be the space of all maximal compact subgroups of G (R). Recall from [3, Proposition

1.6] that for K ∈ Y , the Cartan involution θK of G R := R⊗Q G corresponding to K is the unique

homomorphism G R → G R such that

K = {g ∈ G (R) | θK (g ) = g }.
Let P be a parabolic subgroup of G , let S P be the largest Q-split torus in the center of P/Pu ,
and let A P be the connected component of the topological group S P (R) containing the origin.
We have
AP ∼
= Rr>0 ⊂ S P (R) ∼
= (R× )r
for some integer r . We define an action of A P on Y as follows (see [3, Section 3]). For K ∈ Y ,

we have a unique subtorus S P,K of PR = R ⊗Q P over R such that the projection P → P/Pu

induces an isomorphism

∼

S P,K −
→ (S P )R := R ⊗Q S P
and such that the Cartan involution θK : G R → G R of K satisfies θK (t ) = t −1 for all t ∈ S P,K (R).
For t ∈ A P , let t K ∈ S P,K (R) be the inverse image of t . Then A P acts on Y by
AP × Y → Y ,

(t , K ) 7→ t K K t K−1 .

The Borel-Serre space is the set of pairs (P,Z ) such that P is a parabolic subgroup of G
and Z is an A P -orbit in Y . The reductive Borel-Serre space is the quotient of the Borel-Serre

space by the equivalence relation under which two elements (P,Z ) and (P ′ ,Z ′ ) are equivalent
if (P ′ ,Z ′ ) = g (P,Z ) (that is, P = P ′ and Z ′ = g Z ) for some g ∈ Pu (R).
3.7.3. Now assume that F = Q and G = PGLV . Let v be the archimedean place of Q.
We have a bijection between X v and the set Y of all maximal compact subgroups of G (R),

whereby an element of X v corresponds to its isotropy group in G (R), which is a maximal compact subgroup.
Suppose that K ∈ Y corresponds to µ ∈ X v , with µ the class of a norm that in turn cor-

responds to a positive definite symmetric bilinear form ( , ) on Vv . The Cartan involution
θK : G R → G R is induced by the unique homomorphism θK : GLVv → GLVv satisfying
(g x , θK (g )y ) = (x , y )

for all g ∈ GLV (R) and x , y ∈ Vv .

For a parabolic subgroup P of G corresponding to a flag (Vi )−1≤i ≤m , we have

 m
Y
Gm,Q /Gm,Q ,
SP =
i =0

33

where the i th term in the product is the group of scalars in GLVi /Vi −1 , and where the last Gm,Q is

embedded diagonally in the product. The above description of θK shows that S P,K is the lifting
of (S P )R to PR obtained through the orthogonal direct sum decomposition
Vv ∼
=

m
M
i =0

(Vi /Vi −1 )v

with respect to ( , ).
♯

Proposition 3.7.4. If v is the archimedean place of Q, then X̄ Q,v (resp., X̄ Q,v ) is the Borel-Serre
space (resp., reductive Borel-Serre space) associated to PGLV .
♯

Proof. Denote the Borel-Serre space by (X̄ Q,v )′ in this proof. We define a canonical map
♯

♯

X̄ Q,v → (X̄ Q,v )′ ,

(P, µ, s ) 7→ (P,Z ),

where Z is the subset of Y corresponding to the following subset Z ′ of X v . Let (Vi )−1≤i ≤m be

the flag corresponding to P. Recall that s is an isomorphism
s:

m
M
i =0

∼

(Vi /Vi −1 )v −
→ Vv .

Then Z ′ is the subset of X v consisting of classes of the norms
µ̃(s ) : x 7→

m
X

!1/2
µ̃i (s −1 (x )i )2

i =0

on Vv , where s −1 (x )i ∈ (Vi /Vi −1 )v denotes the i th component of s −1 (x ) for x ∈ Vv , and µ̃ =

(µ̃i )0≤i ≤m ranges over all families of norms µ̃i on (Vi /Vi −1 )v with class equal to µi . It follows

from the description of S P,K in 3.7.3 that Z is an A P -orbit.
For a parabolic subgroup P of G , let
♯

♯

(X̄ Q,v )′ (P) = {(Q,Z ) ∈ (X̄ Q,v )′ | Q ⊃ P}.
♯

♯

By [3, 7.1], the subset (X̄ Q,v )′ (P) is open in (X Q,v )′ .
Take a basis of V , and let B denote the Borel subgroup of PGLV of upper-triangular matrices for this basis. By 3.3.8(3), we have a homeomorphism
∼

♯

→ X̄ Q,v (B ).
B u (R) × Rd≥0−1 −
It follows from [3, 5.4] that the composition
♯

B u (R) × Rd≥0−1 → (X̄ Q,v )′ (B )
34

induced by the above map
♯

♯

X̄ Q,v (B ) → (X̄ Q,v )′ (B ),

(P, µ, s ) 7→ (P,Z )
♯

♯

is also a homeomorphism. This proves that the map X̄ Q,v → (X̄ Q,v )′ restricts to a homeomor∼

♯

♯

♯

♯

phism X̄ Q,v (B ) −
→ (X̄ Q,v )′ (B ). Therefore, X̄ Q,v → (X̄ Q,v )′ is a homeomorphism as well. It then

follows directly from the definitions that the reductive Borel-Serre space is identified with
X̄ Q,v .
3.7.5. Suppose that F is a number field, let S be the set of all archimedean places of F , and let
G be the Weil restriction ResF /Q PGLV , which is a semisimple algebraic group over Q. Then Y

is identified with X F,S , and X̄ F,S is related to the reductive Borel-Serre space associated to G but

does not always coincide with it. We explain this below.
♯

′
Let (X̄ F,S )′ and X̄ F,S
be the Borel-Serre space and the reductive Borel-Serre space associQ
♯
♯
ated to G , respectively. Let X̄ F,S be the subspace of v ∈S X̄ F,v consisting of all elements (x v )v ∈S

such that the parabolic subgroup of G associated to x v is independent of v . Then by similar
arguments to the case F = Q, we see that Y is canonically homeomorphic to X F,S and this
homeomorphism extends uniquely to surjective continuous maps
♯

♯

(X̄ F,S )′ → X̄ F,S ,

′
X̄ F,S
→ X̄ F,S .

However, these maps are not bijective unless F is Q or imaginary quadratic. We illustrate the
differences between the spaces in the case that F is a real quadratic field and d = 2.
Fix a basis (e i )i =1,2 of V . Let P̃ be the Borel subgroup of upper triangular matrices in PGLV
for this basis, and let P be the Borel subgroup Res F /Q P̃ of G . Then P/Pu ∼
= ResF /Q Gm ,F and
S P = Gm ,Q ⊂ P/Pu . We have the natural identifications Y = X F,S = H × H. For a ∈ R>0 , the set
Z a := {(y i , a y i ) ∈ H × H | y ∈ R>0 }
is an A P -orbit. If a 6= b , the images of (P,Z a ) and (P,Z b ) in (X̄ F,S )′ do not coincide. On the other
♯

hand, both the images of (P,Z a ) and (P,Z b ) in X̄ F,S coincide with (x v )v ∈S , where x v = (P, µv , s v )

with µv the unique element of X Fv e 1 × X Vv /Fv e 1 and s v the splitting given by e 2.
♯

In the case that v is non-archimedean, the space X̄ F,v is not good in the following sense.
♯

Proposition 3.7.6. If v is non-archimedean, then X̄ F,v is not Hausdorff.
Proof. Fix a ,b ∈ B u (Fv ) with a 6= b , for a Borel subgroup B of PGLV . When t ∈ Rd>0−1 is sufficiently near to 0 = (0, . . . , 0), the images of (a , t ) and (b, t ) in X v coincide by 3.3.8(4) applied to
B u (Fv ) × Rd>0−1 → X v . We denote this element of X v by c (t ). The images f (a ) of (a , 0) and f (b )
♯

of (b, 0) in X̄ F,v are different. However, c (t ) converges to both f (a ) and f (b ) as t tends to 0.
♯

Thus, X̄ F,v is not Hausdorff.
35

3.7.7. Let F be a number field, S its set of archimedean places, and G = ResF /Q PGLV , as in
3.7.5. Then X̄ F,S may be identified with the maximal Satake space for G of [23]. Its Satake
topology was considered by Satake (see also [2, III.3]), and its Borel-Serre topology was con♭
sidered by Zucker [27] (see also [14, 2.5]). The space X̄ F,S
is also a Satake space corresponding

to the standard projective representation of G on V viewed as a Q-vector space.

4

Quotients by S-arithmetic groups

As in §3, fix a global field F and a finite-dimensional vector space V over F .

4.1 Results on S-arithmetic quotients
4.1.1. Fix a nonempty finite set S 1 of places of F which contains all archimedean places of F ,
fix a finite set S 2 of places of F which is disjoint from S 1 , and let S = S 1 ∪ S 2 .
4.1.2. In the following, we take X̄ to be one of the following two spaces:
(i) X̄ := X̄ F,S 1 ,
♭
(ii) X̄ := X̄ F,S
.
1

We endow X̄ with either the Borel-Serre or the Satake topology.
4.1.3. Let G = PGLV , and let K be a compact open subgroup of G (ASF ), with ASF as in 3.4.13.
We consider the two situations in which (X, X̄) is taken to be one of the following pairs of
spaces (for either choice of X̄ ):
(I) X := X S × G (ASF )/K ⊂ X̄ := X̄ × X S 2 × G (ASF )/K ,
(II) X := X S ⊂ X̄ := X̄ × X S 2 .
We now come to the main result of this paper.
Theorem 4.1.4. Let the situations and notation be as in 4.1.1–4.1.3.
(1) Assume we are in situation (I). Let Γ be a subgroup of G (F ). Then the quotient space Γ\X̄
is Hausdorff. It is compact if Γ = G (F ).
(2) Assume we are in situation (II). Let ΓK ⊂ G (F ) be the inverse image of K under the canon-

ical map G (F ) → G (ASF ), and let Γ be a subgroup of ΓK . Then the quotient space Γ\X̄ is

Hausdorff. It is compact if Γ is of finite index in ΓK .

♭
4.1.5. The case Γ = {1} of Theorem 4.1.4 shows that X̄ F,S and X̄ F,S
are Hausdorff.

36

4.1.6. Let OS be the subring of F consisting of all elements which are integral outside S. Take
an OS -lattice L in V . Then PGL L (OS ) coincides with ΓK for the compact open subgroup K =
Q
PGLL (Ov ) of G (ASF ). Hence Theorem 1.6 of the introduction follows from Theorem 4.1.4.
v∈
/S
4.1.7. In the case that F is a number field and S (resp., S 1 ) is the set of all archimedean places
of F , Theorem 4.1.4 in situation (II) is a special case of results of Satake [23] (resp., of Ji, Murty,
Saper, and Scherk [14, Proposition 4.2]).
4.1.8. If in Theorem 4.1.4 we take Γ = G (F ) in part (1), or Γ of finite index in ΓK in part (2), then
the Borel-Serre and Satake topologies on X̄ induce the same topology on the quotient space
Γ\X̄. This can be proved directly, but it also follows from the compact Hausdorff property.
4.1.9. We show that some modifications of Theorem 4.1.4 are not good.
Consider the case F = Q, S = {p, ∞} for a prime number p , and V = Q2 , and consider the

S-arithmetic group PGL2 (Z[ p1 ]). Note that PGL2 (Z[ p1 ])\(X̄ Q,∞ × X p ) is compact Hausdorff, as
is well known (and follows from Theorem 4.1.4). We show that some similar spaces are not
Hausdorff. That is, we prove the following statements:
(1) PGL2 (Z[ p1 ])\(X̄ Q,p × X ∞ ) is not Hausdorff.
(2) PGL2 (Z[ p1 ])\(X̄ Q,∞ × PGL2 (Qp )) is not Hausdorff.
(3) PGL2 (Q)\(X̄ Q,∞ × PGL2 (A∞
Q )) is not Hausdorff.

Statement (1) shows that it is important to assume in 4.1.4 that S 1 , not only S, contains all
archimedean places. Statement (3) shows that it is important to take the quotient G (ASF )/K in
situation (I) of 4.1.3.
Our proofs of these statements rely on the facts that the quotient spaces Z[ p1 ]\R, Z[ p1 ]\Qp ,
and Q\A∞
Q are not Hausdorff.
Proof of statements (1)–(3). For an element x of a ring R, let
!
1 x
gx =
∈ PGL2 (R).
0 1
In (1), for b ∈ R, let h b be the point i + b of the upper half plane H = X ∞ . In (2), for b ∈ Qp ,

∞
let h b = g b ∈ PGL2 (Qp ). In (3), for b ∈ A∞
Q , let h b = g b ∈ PGL2 (AQ ). In (1) (resp., (2) and (3)),

let ∞ ∈ X̄ Q,p (resp., X̄ Q,∞ ) be the boundary point corresponding to the the Borel subgroup of

upper triangular matrices.

In (1) (resp., (2), resp., (3)), take an element b of R (resp., Qp , resp., A∞
Q ) which does not belong to Z[ p1 ] (resp., Z[ p1 ], resp., Q). Then the images of (∞, h 0 ) and (∞, h b ) in the quotient space
37

are different, but they are not separated. Indeed, in (1) and (2) (resp., (3)), some sequence of
elements x of Z[ p1 ] (resp., Q) will converge to b , in which case g x (∞, h 0 ) converges to (∞, h b )
since g x ∞ = ∞.

4.2 Review of reduction theory
We review important results in the reduction theory of algebraic groups: 4.2.2, 4.2.4, and a
variant 4.2.6 of 4.2.2. More details may be found in the work of Borel [1] and Godement [8] in
the number field case and Harder [12, 13] in the function field case.
Fix a basis (e i )1≤i ≤d of V . Let B be the Borel subgroup of G = PGLV consisting of all upper

triangular matrices for this basis. Let S be a nonempty finite set of places of F containing all
archimedean places.
4.2.1. For b = (b v ) ∈ A×F , set |b | =

Q
v

|b v |. Let A v be as in 3.3.4. We let a ∈

image of a diagonal matrix diag(a 1 , . . . , a d ) in GLd (AF ). The ratios

a i a −1
i +1

Q
v

A v denote the

are independent of

the choice. For c ∈ R>0 , we let B (c ) = B u (AF )A(c ), where
(
)
Y
A(c ) = a ∈
A v ∩ PGLd (AF ) | |a i a −1
| ≥ c for all 1 ≤ i ≤ d − 1 .
i +1
v

Let K 0 =

Q
v

K v0 < G (AF ), where K v0 is identified via (e i )i with the standard maximal com-

pact subgroup of PGLd (Fv ) of 3.3.4. Note that B u (Fv )A v K v0 = B (Fv )K v0 = G (Fv ) for all v .
We recall the following known result in reduction theory: see [8, Theorem 7] and [12, Satz
2.1.1].
Lemma 4.2.2. For sufficiently small c ∈ R>0 , one has G (AF ) = G (F )B (c )K 0 .
4.2.3. Let the notation be as in 4.2.1. For a subset I of {1, . . . , d − 1}, let PI be the parabolic
P
subgroup of G corresponding to the flag consisting of 0, the F -subspaces 1≤j ≤i F e j for i ∈ I ,

and V . Hence PI ⊃ B for all I , with P∅ = G and P{1,...,d −1} = B .
For c ′ ∈ R>0 , let B I (c , c ′ ) = B u (AF )A I (c , c ′ ), where

A I (c , c ′ ) = {a ∈ A(c ) | |a i a −1
| ≥ c ′ for all i ∈ I }.
i +1
Note that B I (c , c ′ ) = B (c ) if c ≥ c ′ .
The following is also known [12, Satz 2.1.2] (see also [8, Lemma 3]):
Lemma 4.2.4. Fix c ∈ R>0 and a subset I of {1, . . . , d − 1}. Then there exists c ′ ∈ R>0 such that
{γ ∈ G (F ) | B I (c , c ′ )K 0 ∩ γ−1 B (c )K 0 6= ∅} ⊂ PI (F ).
38

4.2.5. We will use the following variant of 4.2.3.
Q
Let A S = v ∈S A v . For c ∈ R>0 , let
A(c )S = A S ∩ A(c ) and B (c )S = B u (AF,S )A(c )S .
For c 1 , c 2 ∈ R>0 , set
A(c 1 , c 2 )S = {a ∈ A S | for all v ∈ S and 1 ≤ i ≤ d − 1,
|a v,i a −1
| ≥ c 1 and |a v,i a −1
| ≥ c 2 |a w,i a −1
| for all w ∈ S}.
v,i +1
v,i +1
w,i +1
Note that A(c 1 , c 2 )S is empty if c 2 > 1. For a compact subset C of B u (AF,S ), we then set
B (C ; c 1, c 2 )S = C · A(c 1 , c 2 )S .
Let DS =

Q
v ∈S

D v , where D v = K v0 < G (Fv ) if v is archimedean, and D v < G (Fv ) is identified

with S d Iw(Ov ) < PGLd (Fv ) using the basis (e i )i otherwise. Here, S d is the symmetric group of
degree d and Iw(Ov ) is the Iwahori subgroup of PGLd (Ov ), as in 3.3.6.
Lemma 4.2.6. Let K be a compact open subgroup of G (ASF ), let ΓK be the inverse image of K
under G (F ) → G (ASF ), and let Γ be a subgroup of ΓK of finite index. Then there exist c 1 , c 2 ,C as

above and a finite subset R of G (F ) such that

G (AF,S ) = ΓR · B (C ; c 1 , c 2 )S DS .
Proof. This can be deduced from 4.2.2 by standard arguments in the following manner. By the
Iwasawa decomposition 3.3.4, we have G (ASF ) = B (ASF )K 0,S where K 0,S is the non-S-component
of K 0 . Choose a set E of representatives in B (ASF ) of the finite set
B (F )\B (ASF )/(B (ASF ) ∩ K 0,S ).
Let D 0 = DS × K 0,S , and note that since A S ∩DS = 1, we can (by the Bruhat decomposition 3.3.6)

replace K 0 by D 0 in Lemma 4.2.2. Using the facts that E is finite, |a | = 1 for all a ∈ F × , and D 0

is compact, we then have that there exists c ∈ R>0 such that

G (AF ) = G (F )(B (c )S × E )D 0.
For any finite subset R of G (F ) consisting of one element from each of those sets G (F )∩K 0,S e −1
with e ∈ E that are nonempty, we obtain from this that
G (AF,S ) = ΓK R · B (c )S DS .
39

As ΓK is a finite union of right Γ-cosets, we may enlarge R and replace ΓK by Γ. Finally, we can
replace B (c )S by C · A(c )S for some C by the compactness of the image of
B u (AF,S ) → Γ\G (AF,S )/DS
and then by B (C ; c 1 , c 2 )S for some c 1 , c 2 ∈ R>0 by the compactness of the cokernel of
Γ ∩ B (AF,S ) → (B /B u )(AF,S )1 ,
where (B /B u )(AF,S )1 denotes the kernel of the homomorphism
(B /B u )(AF,S ) → Rd>0−1 ,

Y a
v,i
a B u (AF,S ) 7→
a v,i +1
v ∈S

!
.
1≤i ≤d −1

4.3 X̄ F,S and reduction theory
4.3.1. Let S be a nonempty finite set of places of F containing all archimedean places. We consider X̄ F,S . From the results 4.2.6 and 4.2.4 of reduction theory, we will deduce results 4.3.4 and
4.3.10 on X̄ F,S , respectively. We will also discuss other properties of X̄ F,S related to reduction
theory. Let G , (e i )i , and B be as in §4.2.
For c 1 , c 2 ∈ R>0 with c 2 ≥ 1, we define a subset T(c 1 , c 2 ) of (RS≥0 )d −1 by

T(c 1 , c 2 ) = {t ∈ (RS≥0 )d −1 | t v,i ≤ c 1 , t v,i ≤ c 2 t w,i for all v, w ∈ S and 1 ≤ i ≤ d − 1}.
Let Y0 = (RS>0 ∪ {(0)v ∈S })d −1 as in 3.5.1 (for the parabolic B ), and note that T(c 1 , c 2 ) ⊂ Y0 . Define

the subset S(c 1 , c 2 ) of X̄ F,S (B ) as the image of B u (AF,S ) × T(c 1 , c 2 ) under the map
πS = (πv )v ∈S : B u (AF,S ) × Y0 → X̄ F,S (B ),

with πv as in 3.3.3. For a compact subset C of B u (AF,S ), we let S(C ; c 1 , c 2 ) ⊂ S(c 1 , c 2 ) denote

the image of C × T(c 1 , c 2 ) under πS .

4.3.2. We give an example of the sets of 4.3.1.
Example. Consider the case that F = Q, the set S contains only the real place, and d = 2, as in
§3.2. Fix a basis (e i )1≤i ≤2 of V . Identify B u (R) with R in the natural manner. We have

S(C ; c 1 , c 2 ) = {x + y i ∈ H | x ∈ C , y ≥ c 1−1 } ∪ {∞},
which is contained in

S(c 1 , c 2 ) = {x + y i ∈ H | x ∈ R, y ≥ c 1−1 } ∪ {∞}.
40

4.3.3. Fix a compact open subgroup K of G (ASF ), and let ΓK ⊂ G (F ) be the inverse image of K
under G (F ) → G (ASF ).

Proposition 4.3.4. Let Γ be a subgroup of ΓK of finite index. Then there exist c 1 , c 2 ,C as in 4.3.1
and a finite subset R of G (F ) such that
X̄ F,S = ΓR · S(C ; c 1 , c 2 ).
Proof. It suffices to prove the weaker statement that there are c 1 , c 2 ,C and R such that
X S = ΓR · (X S ∩ S(C ; c 1 , c 2 )).
Indeed, we claim that the proposition follows from this weaker statement for the spaces in the
Q
product v ∈S X (Vi /Vi −1)v , where PI is as in 4.2.3 for a subset I of {1, . . . , d − 1} and (Vi )−1≤i ≤m is

the corresponding flag. To see this, first note that there is a finite subset R ′ of G (F ) such that
every parabolic subgroup of G has the form γPI γ−1 for some I and γ ∈ ΓR ′ . It then suffices to

consider a = (P, µ) ∈ X̄ F,S , where P = PI for some I , and µ ∈ ZF,S (P). We use the notation of 3.5.6

and 3.5.1. By Proposition 3.5.7, the set X̄ F,S (P) ∩ S(C ; c 1 , c 2 ) is the image under ξ of the image
of C × T(c 1 , c 2 ) in Pu (AF,S ) × ZF,S (P) × Y0 . Note that a has image (1, µ, 0) in the latter set (for 1

the identity matrix of Pu (AF,S )), and ξ(1, µ, 0) = a . Since the projection of T(c 1 , c 2 ) (resp., C ) to
(RS>0 )∆i (resp., B i ,u (AF,S )) is the analogous set for c 1 and c 2 (resp., a compact subset), the claim
follows.
For v ∈ S, we define subsets Q v and Q v′ of X v as follows. If v is archimedean, let Q v = Q v′ be

the one point set consisting of the element of X v given by the basis (e i )i and (ri )i with ri = 1

for all i . If v is non-archimedean, let Q v (resp., Q v′ ) be the subset of X v consisting of elements
given by (e i )i and (ri )i such that 1 = r1 ≤ · · · ≤ rd ≤ qv (resp., r1 = 1 and 1 ≤ ri ≤ qv for 1 ≤ i ≤ d ).

Then X v = G (Fv )Q v for each v ∈ S. Hence by 4.2.6, there exist c 1′ , c 2′ ,C as in 4.3.1 and a finite
subset R of G (F ) such that

X S = ΓR · B (C ; c 1′ , c 2′ )S · DSQ S ,
where Q S =

Q
v ∈S

Qv .

We have DSQ S = Q S′ for Q S′ =

Q
v ∈S

Q v′ , noting for archimedean (resp., non-archimedean)

v that K v0 (resp., Iw(Ov )) stabilizes all elements of Q v . We have B (C ; c 1′ , c 2′ )SQ S′ ⊂ S(C ; c 1 , c 2 ),

where

c 1 = max{qv | v ∈ S f }(c 1′ )−1 and c 2 = max{qv2 | v ∈ S f }(c 2′ )−1 ,
with S f the set of all non-archimedean places in S (and taking the maxima to be 1 if S f = ∅).

41

′
4.3.5. For v ∈ S and 1 ≤ i ≤ d −1, let t v,i : S(c 1 , c 2 ) → R≥0 be the map induced by φ B,v
: X̄ F,v (B ) →

Rd≥0−1 (see 3.4.6) and the i th projection Rd≥0−1 → R≥0 . Note that t v,i is continuous.

4.3.6. Fix a subset I of {1, . . . , d − 1}, and let PI be the parabolic subgroup of G defined in 4.2.3.
For c 1 , c 2 , c 3 ∈ R>0 , let

SI (c 1 , c 2 , c 3 ) = {x ∈ S(c 1 , c 2 ) | min{t v,i (x ) | v ∈ S} ≤ c 3 for each i ∈ I }.
4.3.7. For an element a ∈ X̄ F,S , we define the parabolic type of a to be the subset
{dim(Vi ) | 0 ≤ i ≤ m − 1}
of {1, . . . , d − 1}, where (Vi )−1≤i ≤m is the flag corresponding to the parabolic subgroup of G

associated to a .

Lemma 4.3.8. Let a ∈ X̄ F,S (B ), and let J be the parabolic type of a . Then the parabolic subgroup
of G associated to a is PJ .
This is easily proved.
4.3.9. In the following, we will often consider subsets of G (F ) of the form R 1 ΓK R 2 , ΓK R, or
RΓK , where R 1 , R 2 , R are finite subsets of G (F ). These three types of cosets are essentially the
same thing when we vary K . For finite subsets R 1, R 2 of G (F ), we have R 1 ΓK R 2 = R ′ ΓK ′ = ΓK ′′ R ′′
for some compact open subgroups K ′ and K ′′ of G (ASF ) contained in K and finite subsets R ′
and R ′′ of G (F ).
Proposition 4.3.10. Given c 1 ∈ R>0 and finite subsets R 1 , R 2 of G (F ), there exists c 3 ∈ R>0 such
that for all c 2 ∈ R>0 we have

{γ ∈ R 1 ΓK R 2 | γSI (c 1 , c 2 , c 3 ) ∩ S(c 1, c 2 ) 6= ∅} ⊂ PI (F ).
Proof. First we prove the weaker version that c 3 exists if the condition on γ ∈ R 1 ΓK R 2 is replaced by γSI (c 1 , c 2 , c 3 ) ∩ S(c 1 , c 2 ) ∩ X S 6= ∅.

Let Q v′ for v ∈ S and Q S′ be as in the proof of 4.3.4.

Claim 1. If c 1′ ∈ R>0 is sufficiently small (independent of c 2 ), then we have
X S ∩ S(c 1 , c 2 ) ⊂ B (c 1′ )SQ S′ .
Proof of Claim 1. Any x ∈ X S ∩ S(c 1 , c 2 ) satisfies t v,i (x ) ≤ c 1 for 1 ≤ i ≤ d − 1. Moreover, if
Q
′ −1 for all such i , then x ∈ B (c ′ ) Q ′ . The claim
v ∈S t v,i (x ) is sufficiently small relative to (c 1 )
1 S S

follows.

42

Let C v denote the compact set
C v = {g ∈ G (Fv ) | gQ v′ ∩Q v′ 6= ∅}.
If v is archimedean, then C v is the maximal compact open subgroup K v0 of 4.2.1. Set C S =
Q
S
v ∈S C v . We use the decomposition G (A F ) = G (A F,S ) × G (A F ) to write elements of G (A F ) as
pairs.

Claim 2. Fix c 1′ ∈ R>0 . The subset B (c 1′ )S C S × R 1 K R 2 of G (AF ) is contained in B (c )K 0 for

sufficiently small c ∈ R>0 .

Proof of Claim 2. This follows from the compactness of the C v for v ∈ S and the Iwasawa decomposition G (AF ) = B (AF )K 0 .

Claim 3. Let c 1′ be as in Claim 1, and let c ≤ c 1′ . Let c ′ ∈ R>0 . If c 3 ∈ R>0 is sufficiently small

(independent of c 2 ), we have

X S ∩ SI (c 1 , c 2 , c 3 ) ⊂ B I (c , c ′ )SQ S′ ,
where B I (c , c ′ )S = B (AF,S ) ∩ B I (c , c ′ ).
Proof of Claim 3. An element x ∈ B (c )SQ S′ lies in B I (c , c ′ )SQ S′ if

Q

t (x ) ≤ (c
v ∈S v,i

′ )−1

for all i ∈ I .

An element x ∈ X S ∩ S(c 1 , c 2 ) lies in X S ∩ SI (c 1 , c 2 , c 3 ) if min{t v,i (x ) | v ∈ S} ≤ c 3 for all i ∈ I . In
this case, x will lie in B I (c , c ′ )SQ S′ if c 3 ≤ (c ′ )−1 c 11−s , with s = ♯S.

Let c 1′ be as in Claim 1, take c of Claim 2 for this c 1′ such that c ≤ c 1′ , and let c ′ ∈ R>0 . Take

c 3 satisfying the condition of Claim 3 for these c 1′ , c , and c ′ .

Claim 4. If X S ∩ SI (c 1 , c 2 , c 3 ) ∩ γ−1 S(c 1 , c 2 ) is nonempty for some γ ∈ R 1ΓR 2 ⊂ G (F ), then

B I (c , c ′ ) ∩ γ−1 B (c )K 0 contains an element of G (AF,S ) × {1}.

Proof of Claim 4. By Claim 3, any x ∈ X S ∩ SI (c 1 , c 2 , c 3 ) ∩ γ−1 S(c 1 , c 2 ) lies in gQ S′ for some g ∈

B I (c , c ′ )S . By Claim 1, we have γx ∈ g ′Q S′ for some g ′ ∈ B (c 1′ )S . Since γx ∈ γgQ S′ ∩ g ′Q S′ , we have

(g ′ )−1 γg ∈ C S . Hence γg ∈ B (c 1′ )S C S , and therefore γ(g , 1) = (γg , γ) ∈ B (c )K 0 by Claim 2.

We prove the weaker version of 4.3.10: let x ∈ X S ∩ SI (c 1 , c 2 , c 3 ) ∩ γ−1 S(c 1 , c 2 ) for some

γ ∈ R 1 ΓR 2 . Then by Claim 4 and Lemma 4.2.4, with c ′ satisfying the condition of 4.2.4 for the

given c , we have γ ∈ PI (F ).

We next reduce the proposition to the weaker version, beginning with the following.
Claim 5. Let c 1 , c 2 ∈ R>0 . If γ ∈ G (F ) and x ∈ S(c 1 , c 2 ) ∩ γ−1S(c 1 , c 2 ), then γ ∈ PJ (F ), where J

is the parabolic type of x .

43

Proof of Claim 5. By Lemma 4.3.8, the parabolic subgroup associated to x is PJ and that associated to γx is PJ . Hence γPJ γ−1 = PJ . Since a parabolic subgroup coincides with its normalizer, we have γ ∈ PJ (F ).
Fix J ⊂ {1, . . . , d − 1}, ξ ∈ R 1, and η ∈ R 2 .
Claim 6. There exists c 3 ∈ R>0 such that if γ ∈ ΓK and x ∈ SI (c 1 , c 2 , c 3 ) ∩ (ξγη)−1S(c 1 , c 2 ) is

of parabolic type J , then ξγη ∈ PI (F ).

Proof of Claim 6. Let (Vi )−1≤i ≤m be the flag corresponding to PJ . Suppose that we have γ0 ∈ ΓK

and x 0 ∈ S(c 1 , c 2 )∩(ξγ0η)−1 S(c 1 , c 2 ) of parabolic type J . By Claim 5, we have ξγ0 η, ξγη ∈ PJ (F ).

Hence

ξγη = ξ(γγ−1
)ξ−1 ξγ0η ∈ ΓK ′ η′ ,
0
where K ′ is the compact open subgroup ξK ξ−1 ∩ PJ (ASF ) of PJ (ASF ), and η′ = ξγ0 η ∈ PJ (F ). The

claim follows from the weaker version of the proposition in which V is replaced by Vi /Vi −1 (for

0 ≤ i ≤ m ), the group G is replaced by PGLVi /Vi −1 , the compact open subgroup K is replaced

by the image of ξK ξ−1 ∩ PJ (ASF ) in PGLVi /Vi −1 (ASF ), the set R 1 is replaced by {1}, the set R 2 is

replaced by the image of {η′ } in PGLVi /Vi −1 (F ), and PI (F ) is replaced by the image of PI (F )∩PJ (F )

in PGLVi /Vi −1 (F ).

By Claim 6 for all J , ξ, and η, the result is proven.
Pi
Lemma 4.3.11. Let 1 ≤ i ≤ d − 1, and let V ′ = j =1 F e j . Let x ∈ X v for some v ∈ S, and let
g ∈ GLV (Fv ) be such that g V ′ = V ′ . For 1 ≤ i ≤ d − 1, we have
e (i ,j )
d −1 
Y
|det(g v : Vv′ → Vv′ )|(d −i )/i
t v,j (g x )
=
,
t v,j (x )
|det(g v : Vv /Vv′ → Vv /Vv′ )|
j =1

where


e (i , j ) =

 j (d −i )

if j ≤ i ,

d − j

if j ≥ i .

i

Proof. By the Iwasawa decomposition 3.3.4 and 3.4.5, it suffices to check this in the case that
g is represented by a diagonal matrix diag(a 1 , . . . , a d ). It follows from the definitions that
t v,j (g x )t v,j (x )−1 = |a j a −1
j +1 |, and the rest of the verification is a simple computation.
Lemma 4.3.12. Let i and V ′ be as in 4.3.11. Let R 1 and R 2 be finite subsets of G (F ). Then
there exist A, B ∈ R>0 such that for all γ ∈ GLV (F ) with image in R 1 ΓK R 2 ⊂ G (F ) and for which
γV ′ = V ′ , we have

A≤

Y |det(γ: V ′ → V ′ )|(d −i )/i
v
v
v ∈S

|det(γ: Vv /Vv′ → Vv /Vv′ )|
44

≤ B.

Proof. We may assume that R 1 and R 2 are one point sets {ξ} and {η}, respectively. Suppose
that an element γ0 with the stated properties of γ exists. Then for any such γ, the image of

−1 ∩ P (F ), and hence the image of γγ−1 in G (AS ) belongs to the
γγ−1
{i }
0 in G (F ) belongs to ξΓ K ξ
0
F

compact subgroup ξK ξ−1 ∩ P{i } (ASF ) of P{i } (ASF ). Hence

|det(γγ−1
: Vv′ → Vv′ )| = |det(γγ−1
: Vv /Vv′ → Vv /Vv′ )| = 1
0
0
for every place v of F which does not belong to S. By Lemma 4.3.11 and the product formula,
we have
Y
v ∈S

(|det(γγ−1
: Vv′ → Vv′ )|(d −i )/i · |det(γγ−1
: Vv /Vv′ → Vv /Vv′ )|−1 ) = 1,
0
0

so the value of the product in the statement is constant under our assumptions, proving the
result.
Proposition 4.3.13. Fix c 1 , c 2 ∈ R>0 and finite subsets R 1 , R 2 of G (F ). Then there exists A > 1

such that if x ∈ S(c 1 , c 2 ) ∩ γ−1S(c 1 , c 2 ) for some γ ∈ R 1ΓK R 2, then
A −1 t v,i (x ) ≤ t v,i (γx ) ≤ At v,i (x )
for all v ∈ S and 1 ≤ i ≤ d − 1.

Proof. By a limit argument, it is enough to consider x ∈ X S ∩ S(c 1 , c 2 ). Fix v ∈ S. For x ,x ′ ∈
X S ∩ S(c 1 , c 2 ) and 1 ≤ i ≤ d − 1, let s i (x ,x ′ ) = t v,i (x ′ )t v,i (x )−1 .

For each 1 ≤ i ≤ d − 1, take c 3 (i ) ∈ R>0 satisfying the condition in 4.3.10 for the set I = {i }

and both pairs of finite subsets R 1 , R 2 and R 2−1 , R 1−1 of G (F ). Let
c 3 = min{c 3 (i ) | 1 ≤ i ≤ d − 1}.

For a subset I of {1, . . . , d − 1}, let Y (I ) be the subset of (X S ∩ S(c 1 , c 2 ))2 consisting of all pairs

(x ,x ′ ) such that x ′ = γx for some γ ∈ R 1 ΓK R 2 and such that

I = {1 ≤ i ≤ d − 1 | min(t v,i (x ), t v,i (x ′ )) ≤ c 3 }.
For the proof of 4.3.13, it is sufficient to prove the following statement (S d ), fixing I .
(S d ) There exists A > 1 such that A −1 ≤ s i (x ,x ′ ) ≤ A for all (x ,x ′ ) ∈ Y (I ) and 1 ≤ i ≤ d − 1.
By Proposition 4.3.10, if γ ∈ R 1ΓK R 2 is such that there exists x ∈ X S with (x , γx ) ∈ Y (I ), then

γ ∈ P{i } (F ) for all i ∈ I . Lemmas 4.3.11 and 4.3.12 then imply the following for all i ∈ I , noting

that c 2−1 t w,i (y ) ≤ t v,i (y ) ≤ c 2 t w,i (y ) for all w ∈ S and y ∈ X S ∩ S(c 1 , c 2 ).
45

(Ti ) There exists B i > 1 such that for all (x ,x ′ ) ∈ Y (I ), we have
B i−1

≤

d −1
Y
j =1

s j (x ,x ′ )e (i ,j ) ≤ B i ,

where e (i , j ) is as in 4.3.11.
We prove the following statement (S i ) for 0 ≤ i ≤ d − 1 by induction on i .
′
′
(S i ) There exists A i > 1 such that A −1
i ≤ s j (x ,x ) ≤ A i for all (x ,x ) ∈ Y (I ) and all j such that

1 ≤ j ≤ i and j is not the largest element of I ∩ {1, . . . , i } (if it is nonempty).

That (S 0 ) holds is clear. Assume that (S i −1 ) holds for some i ≥ 1. If i ∈
/ I , then since c 3 ≤

t v,i (x ) ≤ c 1 and c 3 ≤ t v,i (x ′ ) ≤ c 1 , we have

c3
c1
≤ s i (x ,x ′ ) ≤ ,
c1
c3
and hence (S i ) holds with A i := max(A i −1 , c 1 c 3−1 ).

Assume that i ∈ I . If I ∩ {1, . . . , i − 1} = ∅, then (S i ) is evidently true with A i := A i −1 . If

I ∩ {1, . . . , i − 1} 6= ∅, then let i ′ be the largest element of this intersection. We compare (Ti ) and

(Ti ′ ). We have e (i , j ) = e (i ′ , j ) if j ≥ i and e (i , j ) < e (i ′ , j ) if j < i , so taking the quotient of the

equations in (Ti ′ ) and (Ti ), we have
(B i B i ′ )

−1

≤

i −1
Y
j =1

s j (x ,x ′ )e (i ,j )−e (i ,j ) ≤ B i B i ′ .
′

Since (S i −1 ) is assumed to hold, there then exists a ∈ R>0 such that
(B i B i ′ )−1 A −a
≤ s i ′ (x ,x ′ )e (i ,i )−e (i ,i ) ≤ B i B i ′ A ai−1
i −1
′

′

′

As the exponent e (i ′ , i ′ ) − e (i , i ′ ) is nonzero, this implies that (S i ) holds.

By induction, we have (S d −1 ). To deduce (S d ) from it, we may assume that I is nonempty,

and let i be the largest element of I . Then (S d −1 ) and (Ti ) imply (S d ).

Proposition 4.3.14. Let c 1 , c 2 ∈ R>0 and a ∈ X̄ F,S . Let I be the parabolic type (4.3.7) of a . Fix a

finite subset R of G (F ) and 1 ≤ i ≤ d − 1.

(1) If i ∈ I , then for any ε > 0, there exists a neighborhood U of a in X̄ F,S for the Satake
topology such that max{t v,i (x ) | v ∈ S} < ε for all x ∈ (ΓK R)−1U ∩ S(c 1 , c 2 ).

(2) If i ∈
/ I , then there exist a neighborhood U of a in X̄ F,S for the Satake topology and c ∈ R>0
such that min{t v,i (x ) | v ∈ S} ≥ c for all x ∈ (ΓK R)−1U ∩ S(c 1 , c 2 ).
46

Proof. The first statement is clear by continuity of t v,i and the fact that t v,i (γ−1 a ) = 0 for all
γ ∈ G (F ), and the second follows from 4.3.13, noting 4.3.4.
Proposition 4.3.15. Let a ∈ X̄ F,S , and let P be the parabolic subgroup of PGLV associated to a .

Let ΓK ,(P) ⊂ ΓK be as in 3.4.12. Then there are c 1 , c 2 ∈ R>0 and ϕ ∈ G (F ) such that ΓK ,(P)ϕ S(c 1 , c 2 )

is a neighborhood of a in X̄ F,S for the Satake topology.

Proof. This holds by definition of the Satake topology with ϕ = 1 if a ∈ X̄ F,S (B ). In general, let I

be the parabolic type of a . Then the parabolic subgroup associated to a has the form ϕPI ϕ −1

for some ϕ ∈ G (F ). We have ϕ −1 a ∈ X̄ F,S (PI ) ⊂ X̄ F,S (B ). By that already proven case, there exists
γ ∈ ΓK ,(PI ) such that ΓK ,(P) ϕγS(c 1 , c 2 ) is a neighborhood of a for the Satake topoology.

♭
The following result can be proved in the manner of 4.4.8 for X̄ F,S
below, replacing R by

{ϕ}, and ΓK ,(W ) by ΓK ,(P).
Lemma 4.3.16. Let the notation be as in 4.3.15. Let U ′ be a neighborhood of ϕ −1 a in X̄ F,S for the
Satake topology. Then there is a neighborhood U of a in X̄ F,S for the Satake topology such that
U ⊂ ΓK ,(P)ϕ(S(c 1 , c 2 ) ∩ U ′ ).
♭
4.4 X̄ F,S
and reduction theory

4.4.1. Let S be a finite set of places of F containing the archimedean places. In this subsection,
♭
we consider X̄ F,S
. Fix a basis (e i )1≤i ≤d of V . Let B ⊂ G = PGLV be the Borel subgroup of upper

triangular matrices for (e i )i . Let K be a compact open subgroup of G (ASF ).

♭
4.4.2. Let c 1 , c 2 ∈ R>0 . We let S♭ (c 1 , c 2 ) denote the image of S(c 1 , c 2 ) under X̄ F,S → X̄ F,S
. For

r ∈ {1, . . . , d − 1}, we then define

S♭r (c 1 , c 2 ) = {(W, µ) ∈ S♭ (c 1 , c 2 ) | dim(W ) ≥ r }.
Then the maps t v,i of 4.3.5 for v ∈ S and 1 ≤ i ≤ r induce maps
t v,i : S♭r (c 1 , c 2 ) → R>0 (1 ≤ i ≤ r − 1) and t v,r : S♭r (c 1 , c 2 ) → R≥0 .
For c 3 ∈ R>0 , we also set

S♭r (c 1 , c 2 , c 3 ) = {x ∈ S♭r (c 1 , c 2 ) | min{t v,r (x ) | v ∈ S} ≤ c 3 }.
Proposition 4.4.3. Fix c 1 ∈ R>0 and finite subsets R 1 , R 2 of G (F ). Then there exists c 3 ∈ R>0 such
that for all c 2 ∈ R>0 , we have

{γ ∈ R 1 ΓK R 2 | γS♭r (c 1 , c 2 , c 3 ) ∩ S♭r (c 1 , c 2 ) 6= ∅} ⊂ P{r } .
47

Proof. Take (W, µ) ∈ γS♭r (c 1 , c 2 , c 3 ) ∩ S♭r (c 1 , c 2 ), and let r ′ = dim W . Let P be the parabolic
Pr ′ +i
subgroup of V corresponding to the flag (Vi )−1≤i ≤d −r ′ with Vi = W + j =r ′+1 F e i for 0 ≤ i ≤

d − r ′ . Let µ′ ∈ ZF,S (P) be the unique element such that a = (P, µ′ ) ∈ X̄ F,S maps to (W, µ). Then
a ∈ γS{r } (c 1 , c 2 , c 3 ) ∩ S(c 1 , c 2 ), so we can apply 4.3.10.

Proposition 4.4.4. Fix c 1 , c 2 ∈ R>0 and finite subsets R 1 , R 2 of G (F ). Then there exists A > 1 such

that if x ∈ S♭r (c 1 , c 2 ) ∩ γ−1S♭r (c 1 , c 2 ) for some γ ∈ R 1 ΓK R 2 , then

A −1 t v,i (x ) ≤ t v,i (γx ) ≤ At v,i (x )
for all v ∈ S and 1 ≤ i ≤ r .
Proof. This follows from 4.3.13.
We also have the following easy consequence of Lemma 4.3.8.
♭
Lemma 4.4.5. Let a be in the image of X̄ F,S (B ) → X̄ F,S
, and let r be the dimension of the F Pr
subspace of V associated to a . Then the F -subspace of V associated to a is i =1 F e i .
♭
Proposition 4.4.6. Let a ∈ X̄ F,S
and let r be the dimension of the F -subspace of V associated to

a . Let c 1 , c 2 ∈ R>0 . Fix a finite subset R of G (F ).

♭
(1) For any ε > 0, there exists a neighborhood U of a in X̄ F,S
for the Satake topology such that

max{t v,r (x ) | v ∈ S} < ε for all x ∈ (ΓK R)−1U ∩ S♭r (c 1 , c 2 ).
♭
(2) If 1 ≤ i < r , then there exist a neighborhood U of a in X̄ F,S
for the Satake topology and

c ∈ R>0 such that min{t v,i (x ) | v ∈ S} ≥ c for all x ∈ (ΓK R)−1U ∩ S♭r (c 1 , c 2 ).

Proof. This follows from 4.4.4, as in the proof of 4.3.14.
Proposition 4.4.7. Let W be an F -subspace of V of dimension r ≥ 1. Let Φ be set of ϕ ∈ G (F )
Pr
such that ϕ( i =1 F e i ) = W .
♭
(1) There exists a finite subset R of Φ such that for any a ∈ X̄ F,S
(W ), there exist c 1 , c 2 ∈ R>0 for

which the set ΓK ,(W ) R S♭r (c 1 , c 2 ) is a neighborhood of a in the Satake topology.

♭
(2) For any ϕ ∈ Φ and a ∈ X̄ F,S
with associated subspace W , there exist c 1 , c 2 ∈ R>0 such that

a ∈ ϕ S♭r (c 1 , c 2 ) and ΓK ,(W ) ϕ S♭r (c 1 , c 2 ) is a neighborhood of a in the Satake topology.

Proof. We may suppose without loss of generality that W =

Pr
j =1

F e j , in which case Φ =

G (F )(W ) (see 3.4.12). Consider the set Q of all parabolic subgroups Q of G such that W is

contained in the smallest nonzero subspace of V preserved by Q. Any Q ∈ Q has the form
48

Q = ϕPI ϕ −1 for some ϕ ∈ G (F )(W ) and subset I of J := {i ∈ Z | r ≤ i ≤ d − 1}. There exists a
finite subset R of G (F )(W ) such that we may always choose ϕ ∈ ΓK ,(W ) R.

♭
By 4.4.5, an element of X̄ F,S (B ) has image in X̄ F,S
(W ) if and only if the parabolic subgroup

associated to it has the form PI for some I ⊂ J . The intersection of the image of X̄ F,S (B ) →

♭
♭
X̄ F,S
with X̄ F,S
(W ) is the union of the S♭r (c 1 , c 2 ) with c 1 , c 2 ∈ R>0 . By the above, for any a ∈
♭
X̄ F,S
(W ), we may choose ξ ∈ ΓK ,(W ) R such that ξ−1 a is in this intersection, and part (1) follows.

♭
Moreover, if W is the subspace associated to a , then ϕ −1 a ∈ X̄ F,S
(W ) is in the image of X̄ F,S (B )

for all ϕ ∈ G (F )(W ) , from which (2) follows.

♭
Lemma 4.4.8. Let W , Φ, R be as in 4.4.7, fix a ∈ X̄ F,S
(W ), and let c 1 , c 2 ∈ R>0 be as in 4.4.7(1) for

♭
this a . For each ϕ ∈ R, let Uϕ be a neighborhood of ϕ −1 a in X̄ F,S
for the Satake topology. Then
♭
there is a neighborhood U of a in X̄ F,S
for the Satake topology such that

U⊂

[
ϕ∈R

ΓK ,(W ) ϕ(S♭r (c 1 , c 2 ) ∩ Uϕ ).

Proof. We may assume that each ϕ(Uϕ ) is stable under the action of ΓK ,(W ) . Let
U = ΓK ,(W ) R S♭r (c 1 , c 2 ) ∩

\

ϕ(Uϕ ).

ϕ∈R

Then U is a neighborhood of a by 4.4.7(1). Let x ∈ U . Take γ ∈ ΓK ,(W ) and ϕ ∈ R such that

x ∈ γϕ S♭r (c 1 , c 2 ). Since ϕ(Uϕ ) is ΓK ,(W ) -stable, γ−1x ∈ ϕ(Uϕ ) and hence ϕ −1 γ−1x ∈ S♭r (c 1 , c 2 ) ∩

Uϕ .

♭
Proposition 4.4.9. Let a = (W, µ) ∈ X̄ F,S
, and let r = dim(W ). Take ϕ ∈ G (F ) and c 1 , c 2 ∈ R>0 as

♭
♭
in 4.4.7(2) such that ΓK ,(W ) ϕ S♭r (c 1 , c 2 ) is a neighborhood of a . Let φW,S
: X̄ F,S
(W ) → Z♭F,S (W ) be as
♭
in 3.4.4. For any neighborhood U of µ = φW,S
(a ) in Z♭F,S (W ) and any ε ∈ R>0 , set

♭
Φ(U , ε) = (φW,S
)−1 (U ) ∩ ΓK ,(W ) ϕ{x ∈ S♭r (c 1 , c 2 ) | t v,r (x ) < ε for all v ∈ S}.
♭
Then the set of all Φ(U , ε) forms a base of neighborhoods of a in X̄ F,S
under the Satake topology.

Proof. We may suppose that W =

Pr
i =1

F e i without loss of generality, in which case ϕ ∈

G (F )(W ) . Let P be the smallest parabolic subgroup containing B with flag (Vi )−1≤i ≤m such that
V0 = W and m = d − r . Let Q be the parabolic of all elements that preserve W . We then have

G ⊃ Q ⊃ P ⊃ B . Let B ′ be the Borel subgroup of PGLV /W that is the image of P and which we
regard as a subgroup of G using (e r +i )1≤i ≤m to split V → V /W .
Let

f v : Q u (Fv ) × X̄ V /W,F,v (B ′ ) × X Wv × R≥0 → X̄ F,v (P)
49

be the unique surjective continuous map such that ξ = f v ◦ h, where ξ is as in 3.5.6 and h is

defined as the composition
∼

−1
Pu (Fv ) × X Wv × Rm
−
→ Q u (Fv ) × B u′ (Fv ) × X Wv × R≥0 × Rm
→ Q u (Fv ) × X̄ V /W,F,v (B ′ ) × X Wv × R≥0
≥0
≥0
∼

of the map induced by the isomorphism Pu (Fv ) −
→ Q u (Fv ) × B u′ (Fv ) and the map induced by

−1
′
the surjection π̄ B ′ ,v : B u′ (Fv ) × Rm
≥0 → X̄ V /W,F,v (B ) of 3.3.8(2). The existence of f v follows from

3.3.8(4).

Set Y0 = RS>0 ∪ {(0)v ∈S }, and let
f S : Q u (AF,S ) × X̄ V /W,F,S (B ′ ) × Z♭F,S (W ) × Y0 → X̄ F,S (P)
be the product of the maps f v . Let t v,r : X̄ F,v (P) → R≥0 denote the composition
′
φB,v

X̄ F,v (P) → X̄ F,v (B ) −→ Rd≥0−1 → R≥0 ,
where the last arrow is the r th projection. The composition of f S with (t v,r )v ∈S is projection
onto Y0 by 3.5.7 and 3.4.6.

♭
Let X̄ F,S (W ) denote the inverse image of X̄ F,S
(W ) under the canonical surjection ΠS : X̄ F,S →

♭
X̄ F,S
. Combining f S with the action of G (F )(W ) , we obtain a surjective map

f S′ : G (F )(W ) × (Q u (AF,S ) × X̄ V /W,F,S (B ′ ) × Z♭F,S (W ) × Y0 ) → X̄ F,S (W ),

f S′ (g , z ) = g f S (z ).

♭
The composition of f S′ with φW,S
◦ ΠS is projection onto Z♭F,S (W ) by 3.5.9 and 3.5.10.

Applying 4.3.4 with V /W in place of V , there exists a compact subset C of Q u (AF,S ) ×

X̄ V /W,F,S (B ′ ) and a finite subset R of G (F )(W ) such that f S′ (ΓK ,(W ) R × C × Z♭F,S (W ) × Y0 ) = X̄ F,S (W ).
Consider the restriction of ΠS ◦ f S′ to a surjective map

♭
λS : ΓK ,(W ) R × C × Z♭F,S (W ) × Y0 → X̄ F,S
(W ).

We may suppose that R contains ϕ, since it lies in G (F )(W ) .
♭
Now, let U ′ be a neighborhood of a in X̄ F,S
(W ) for the Satake topology. It is sufficient to

prove that there exist an open neighborhood U of µ in Z♭F,S (W ) and ε ∈ R>0 such that Φ(U , ε) ⊂

U ′ . For ε ∈ R>0 , set Yε = {(t v )v ∈S ∈ Y0 | t v < ε for all v ∈ S}.

For any x ∈ C , we have λS (α,x , µ, 0) = (W, µ) ∈ U ′ for all α ∈ R. By the continuity of λS , there

exist a neighborhood D(x ) ⊂ Q u (AF,S ) × X̄ V /W,F,S (B ′ ) of x , a neighborhood U (x ) ⊂ Z♭F,v (W ) of µ,
and ε(x ) ∈ R>0 such that

λS (R × D(x ) × U (x ) × Yε(x )) ⊂ U ′ .
50

Since C is compact, some finite collection of the sets D(x ) cover C . Thus, there exist a neighborhood U of µ in Z♭F,v (W ) and ε ∈ R>0 such that λS (R ×C ×U ×Yε ) ⊂ U ′ . Since U ′ is ΓK ,(W ) -stable
by 3.4.15, we have λS (ΓK ,(W ) R × C × U × Yε ) ⊂ U ′ .

Let y ∈ Φ(U , ε), and write y = g x with g ∈ ΓK ,(W ) ϕ and x ∈ S♭r (c 1 , c 2 ) such that t v,r (x ) < ε

♭
for all v ∈ S. Since Φ(U , ε) ⊂ X̄ F,S
(W ), we may by our above remarks write y = λS (g , c , ν, t ) =

♭
g ΠS (f S (c , ν, t )), where c ∈ C , ν = φW,S
(y ), and t = (t v,r (x ))v ∈S . Since ν ∈ U and t ∈ Yε by

definition, y is contained in U ′ . Therefore, we have Φ(U , ε) ⊂ U ′ .

Example 4.4.10. Consider the case F = Q, S = {v } with v the archimedean place, and d = 3.

♭
We construct a base of neighborhoods of a point in X̄ Q,v
for the Satake topology.

♭
Fix a basis (e i )1≤i ≤3 of V . Let a = (W, µ) ∈ X̄ Q,v
, where W = Qe 1 , and µ is the unique element

of X Wv .

For c ∈ R>0 , let Uc be the subset of X v = PGL3 (R)/ PO3 (R) consisting of the elements



! 1 x 12 x 13
y1y2 0 0


1 0 
0 1 x 23   0

y
0
2


0 γ 
0 0
1
0
0 1
such that γ ∈ PGL2 (Z), x i j ∈ R, y 1 ≥ c , and y 2 ≥

these elements converge to a in

♭
X̄ Q,v

p
3
.
2

When γ, x i j and y 2 are fixed and y 1 → ∞,

under the Satake topology. When γ, x i j , and y 1 are fixed

and y 2 → ∞, they converge in the Satake topology to


! 1 x 12 0

1 0 
0 1 0 µ(y 1 ),
µ(γ,x 12 , y 1 ) :=

0 γ 
0 0 1
♭
where µ(y 1 ) is the class in X̄ Q,v
of the semi-norm a 1 e 1∗ + a 2 e 2∗ + a 3 e 3∗ 7→ (a 21 y 12 + a 22 )1/2 on Vv∗ .

The set of

Ūc := {a } ∪ {µ(γ,x , y ) | γ ∈ PGL2 (Z),x ∈ R, y ≥ c } ∪ Uc .
♭
is a base of neighborhoods for a in X̄ Q,v
under the Satake topology. Note that H = SL2 (Z){z ∈

H | Im(z ) ≥

p
3
},
2

which is the reason for the appearance of

any b ∈ R>0 such that b ≤

p
3
.
2

p

3
.
2

It can of course be replaced by

♭
4.4.11. We continue with Example 4.4.10. Under the canonical surjection X̄ Q,v → X̄ Q,v
, the

inverse image of a = (W, µ) in X̄ Q,v is canonically homeomorphic to X̄ (V /W )v = H ∪ P1 (Q) under

the Satake topology on both spaces. This homeomorphism sends x + y 2 i ∈ H (x ∈ R, y 2 ∈ R>0 )

to the limit for the Satake topology of



1 0 0
y1 y2 0 0



0 1 x   0
 ∈ PGL3 (R)/ PO3 (R)
y
0
2



0 0 1
0
0 1
51

♭
as y 1 → ∞. (This limit in X̄ Q,v depends on x and y 2 , but the limit in X̄ Q,v
is a .)
♭
4.4.12. In the example of 4.4.10, we explain that the quotient topology on X̄ Q,v
of the Satake
♭
topology on X̄ Q,v is different from the Satake topology on X̄ Q,v
.

For a map
1 Z

f : PGL2 (Z)/

0 1

!
→ R>0 ,

define a subset U f of X v as in the definition of Uc but replacing the condition on γ,x i j , y i by
γ ∈ PGL2 (Z), x i j ∈ R, y 1 ≥ f (γ), and y 2 ≥

p
3
.
2

Let

Ū f = {a } ∪ {µ(γ,x , y ) | γ ∈ PGL2 (Z),x ∈ R, y ≥ f (γ)} ∪ U f .
♭
When f varies, the Ū f form a base of neighborhoods of a in X̄ Q,v
for the quotient topology of

the Satake topology on X̄ Q,v . On the other hand, if inf{ f (γ) | γ ∈ PGL2 (Z)} = 0, then Ū f is not a

♭
neighborhood of a for the Satake topology on X̄ Q,v
.

4.5 Proof of the main theorem
In this subsection, we prove Theorem 4.1.4. We begin with the quasi-compactness asserted
therein. Throughout this subsection, we set Z = X S 2 × G (ASF )/K in situation (I) and Z = X S 2 in

situation (II), so X̄ = X̄ × Z .

Proposition 4.5.1. In situation (I) of 4.1.3, the quotient G (F )\X̄ is quasi-compact. In situation
(II), the quotient Γ\X̄ is quasi-compact for any subgroup Γ of ΓK of finite index.
♭
Proof. We may restrict to case (i) of 4.1.2 that X̄ = X̄ F,S 1 , as X̄ F,S
of case (ii) is a quotient of
1

X̄ F,S 1 (under the Borel-Serre topology). In situation (I), we claim that there exist c 1 , c 2 ∈ R>0 , a

compact subset C of B u (AF,S ), and a compact subset C ′ of Z such that X̄ = G (F )(S(C ; c 1 , c 2 ) ×

C ′ ). In situation (II), we claim that there exist c 1 , c 2 ,C ,C ′ as above and a finite subset R of G (F )
such that X̄ = ΓR(S(C ; c 1 , c 2 )×C ′ ). It follows that in situation (I) (resp., (II)), there is a surjective
continuous map from the compact space C × T(c 1 , c 2 ) × C ′ (resp., R × C × T(c 1 , c 2 ) × C ′ ) onto
the quotient space under consideration, which yields the proposition.

For any compact open subgroup K ′ of G (ASF1 ), the set G (F )\G (ASF1 )/K ′ is finite. Each X v for
v ∈ S 2 may be identified with the geometric realization of the Bruhat-Tits building for PGLVv ,

the set of i -simplices of which for a fixed i can be identified with G (Fv )/K v′ for some K ′ . So,
we see that in situation (I) (resp., (II)), there is a compact subset D of Z such that Z = G (F )D
(resp., Z = ΓD).

52

Now fix such a compact open subgroup K ′ of G (ASF1 ). By 4.3.4, there are c 1 , c 2 ∈ R>0 , a

compact subset C of Pu (AF,S 1 ), and a finite subset R ′ of G (F ) such that X̄ F,S = ΓK ′ R ′ S(C ; c 1 , c 2 ).
We consider the compact subset C ′ := (R ′ )−1 K ′ D of Z .
Let (x , y ) ∈ X̄, where x ∈ X̄ F,S and y ∈ Z . Write y = γz for some z ∈ D and γ ∈ G (F ) (resp.,

γ ∈ Γ) in situation (I) (resp., (II)). In situation (II), we write ΓΓK ′ R ′ = ΓR for some finite subset

R of G (F ). Write γ−1x = γ′ ϕs where γ′ ∈ ΓK ′ , ϕ ∈ R ′ , s ∈ S(C ; c 1 , c 2 ). We have
(x , y ) = γ(γ−1x , z ) = γ(γ′ ϕs , z ) = (γγ′ ϕ)(s , ϕ −1 (γ′ )−1 z ).

As γγ′ ϕ lies in G (F ) in situation (I) and in ΓR in situation (II), we have the claim.
4.5.2. To prove Theorem 4.1.4, it remains only to verify the Hausdorff property. For this, it is
sufficient to prove the following.
Proposition 4.5.3. Let Γ = G (F ) in situation (I) of 4.1.3, and let Γ = ΓK in situation (II). For
every a , a ′ ∈ X̄, there exist neighborhoods U of a and U ′ of a ′ such that if γ ∈ Γ and γU ∩U ′ 6= ∅,

then γa = a ′ .

In the rest of this subsection, let the notation be as in 4.5.3. It is sufficient to prove 4.5.3 for
the Satake topology on X̄. In 4.5.4–4.5.8, we prove 4.5.3 in situation (II) for S = S 1 . That is, we
suppose that X̄ = X̄ . In 4.5.9 and 4.5.10, we deduce 4.5.3 in general from this case.
Lemma 4.5.4. Assume that X̄ = X̄ F,S . Suppose that a , a ′ ∈ X̄ have distinct parabolic types (4.3.7).

Then there exist neighborhoods U of a and U ′ of a ′ such that γU ∩ U ′ = ∅ for all γ ∈ Γ.

Proof. Let I (resp., I ′ ) be the parabolic type of a (resp., a ′ ). We may assume that there exists
an i ∈ I with i ∈
/ I ′.

By 4.3.15, there exist ϕ, ψ ∈ G (F ) and c 1 , c 2 ∈ R>0 such that ΓK ϕ S(c 1 , c 2 ) is a neighbor-

hood of a and ΓK ψS(c 1 , c 2 ) is a neighborhood of a ′ . By 4.3.14(2), there exist a neighborhood
U ′ ⊂ ΓK ψS(c 1 , c 2 ) of a ′ and c ∈ R>0 with the property that min{t v,i (x ) | v ∈ S} ≥ c for all
x ∈ (ΓK ψ)−1U ′ ∩ S(c 1 , c 2 ). Let A ∈ R>1 be as in 4.3.13 for these c 1 , c 2 for R 1 = {ϕ −1 } and R 2 = {ψ}.
Take ε ∈ R>0 such that Aε ≤ c . By 4.3.14(1), there exists a neighborhood U ⊂ ΓK ϕ S(c 1 , c 2 ) of a

such that max{t v,i (x ) | v ∈ S} < ε for all x ∈ (ΓK ϕ)−1U ∩ S(c 1 , c 2 ).

We prove that γU ∩ U ′ = ∅ for all γ ∈ ΓK . If x ∈ γU ∩ U ′ , then we may take δ, δ′ ∈ ΓK such

that (δϕ)−1 γ−1x ∈ S(c 1 , c 2 ) and (δ′ ψ)−1x ∈ S(c 1 , c 2 ). Since

(δϕ)−1 γ−1x = ϕ −1 (δ−1 γ−1 δ′ )ψ(δ′ ψ)−1x ∈ ϕ −1 ΓK ψ · (δ′ ψ)−1x ,
we have by 4.3.13 that
c ≤ t v,i ((δ′ ψ)−1x ) ≤ At v,i ((δϕ)−1 γ−1x ) < Aε,
for all v ∈ S and hence c < Aε, a contradiction.
53

♭
Lemma 4.5.5. Assume that X̄ = X̄ F,S
. Let a , a ′ ∈ X̄ and assume that the dimension of the F -

subspace associated to a is different from that of a ′ . Then there exist neighborhoods U of a and
U ′ of a ′ such that γU ∩ U ′ = ∅ for all γ ∈ Γ.
Proof. The proof is similar to that of 4.5.4. In place of 4.3.13, 4.3.14, and 4.3.15, we use 4.4.4,
4.4.6, and 4.4.7, respectively.

Lemma 4.5.6. Let P be a parabolic subgroup of G . Let a , a ′ ∈ ZF,S (P) (see 3.4.3), and let R 1 and
R 2 be finite subsets of G (F ). Then there exist neighborhoods U of a and U ′ of a ′ in ZF,S (P) such

that γa = a ′ for every γ ∈ R 1ΓK R 2 ∩ P(F ) for which γU ∩ U ′ 6= ∅.
Proof. For each ξ ∈ R 1 and η ∈ R 2 , the set ξΓK η ∩ P(F ) is a ξΓK ξ−1 ∩ P(F )-orbit for the left
Qm
action of ξΓK ξ−1 . Hence its image in i =0 PGLVi /Vi −1 (AF,S ) is discrete, for (Vi )−1≤i ≤m the flag
Qm
corresponding to P, and thus the image of R 1 ΓK R 2 ∩ P(F ) in i =0 PGLVi /Vi −1 (AF,S ) is discrete as

well. On the other hand, for any compact neighborhoods U of a and U ′ of a ′ , the set
)
(
m
Y
′
PGLVi /Vi −1 (AF,S ) | g U ∩ U 6= ∅
g∈
i =0

is compact. Hence the intersection M := {γ ∈ R 1 ΓK R 2 ∩P(F ) | γU ∩U ′ 6= ∅} is finite. If γ ∈ M and

γa 6= a ′ , then replacing U and U ′ by smaller neighborhoods of a and a ′ , respectively, we have

γU ∩ U ′ = ∅. Hence for sufficiently small neighborhoods U and U ′ of a and a ′ , respectively,

we have that if γ ∈ M , then γa = a ′ .

Lemma 4.5.7. Let W be an F -subspace of V . Let a , a ′ ∈ Z♭F,S (W ) (see 3.4.4), and let R 1 and R 2

be finite subsets of G (F ). Let P be the parabolic subgroup of G consisting of all elements which
preserve W . Then there exist neighborhoods U of a and U ′ of a ′ in Z♭F,S (W ) such that γa = a ′ for
every γ ∈ R 1ΓK R 2 ∩ P(F ) for which γU ∩ U ′ 6= ∅.
Proof. This is proven in the same way as 4.5.6.
4.5.8. We prove 4.5.3 in situation (II), supposing that S = S 1 .
In case (i) (that is, X̄ = X̄ = X̄ F,S ), we may assume by 4.5.4 that a and a ′ have the same
♭
parabolic type I . In case (ii) (that is, X̄ = X̄ = X̄ F,S
), we may assume by 4.5.5 that the dimension

r of the F -subspace of V associated to a coincides with that of a ′ . In case (i) (resp., (ii)), take
c 1 , c 2 ∈ R>0 and elements ϕ and ψ (resp., finite subsets R and R ′ ) of G (F ) such that c 1 , c 2 , ϕ

(resp., c 1 , c 2 , R) satisfy the condition in 4.3.15 (resp., 4.4.7) for a and c 1 , c 2 , ψ (resp., c 1 , c 2 , R ′ )
satisfy the condition in 4.3.15 (resp., 4.4.7) for a ′ . In case (i), we set R = {ϕ} and R ′ = {ψ}.

Fix a basis (e i )1≤i ≤d of V . In case (i) (resp., (ii)), denote S(c 1 , c 2 ) (resp., S♭r (c 1 , c 2 )) by S. In
Pr
case (i), let P = PI , and let (Vi )−1≤i ≤m be the associated flag. In case (ii), let W = i =1 F e i , and

let P be the parabolic subgroup of G consisting of all elements which preserve W .
54

Note that in case (i) (resp., (ii)), for all ϕ ∈ R and ψ ∈ R ′ , the parabolic subgroup P is

associated to ϕ −1 a and to ψ−1 a ′ (resp., W is associated to ϕ −1 a and to ψ−1 a ′ ) and hence
these elements are determined by their images in ZF,S (P) (resp. Z♭F,S (W )).
In case (i) (resp., case (ii)), apply 4.5.6 (resp., 4.5.7) to the images of ϕ −1 a and ψ−1 a ′ for
ϕ ∈ R, ψ ∈ R ′ in ZF,S (P) (resp., Z♭F,S (W )). By this, and by 4.3.10 for case (i) and 4.4.3 for case
(ii), we see that there exist neighborhoods Uϕ of ϕ −1 a for each ϕ ∈ R and Uψ′ of ψ−1 a ′ for each

ψ ∈ R ′ for the Satake topology with the following two properties:

(A) {γ ∈ (R ′ )−1 ΓK R | γ(S ∩ Uϕ ) ∩ (S ∩ Uψ′ ) 6= ∅ for some ϕ ∈ R, ψ ∈ R ′ } ⊂ P(F ),
(B) if γ ∈ (R ′ )−1 ΓK R ∩ P(F ) and γUϕ ∩ Uψ′ 6= ∅ for ϕ ∈ R and ψ ∈ R ′ , then γϕ −1 a = ψ−1 a ′ .
In case (i) (resp., (ii)), take a neighborhood U of a satisfying the condition in 4.3.16 (resp.,
4.4.8) for (Uϕ )ϕ∈R , and take a neighborhood U ′ of a ′ satisfying the condition in 4.3.16 (resp.,
4.4.8) for (Uψ′ )ψ∈R ′ . Let γ ∈ ΓK and assume γU ∩ U ′ 6= ∅. We prove γa = a ′ . Take x ∈ U and

x ′ ∈ U ′ such that γx = x ′ . By 4.3.16 (resp., 4.4.8), there are ϕ ∈ R, ψ ∈ R ′ , and ε ∈ ΓK ,(ϕPϕ −1)

and δ ∈ ΓK ,(ψPψ−1) in case (i) (resp., ε ∈ ΓK ,(ϕW ) and δ ∈ ΓK ,(ψW ) in case (ii)) such that ϕ −1 ε−1 x ∈

S ∩ Uϕ and ψ−1 δ−1x ′ ∈ S ∩ Uψ′ . Since

(ψ−1 δ−1 γεϕ)ϕ −1 ε−1x = ψ−1 δ−1x ′ ,
we have ψ−1 δ−1 γεϕ ∈ P(F ) by property (A). By property (B), we have
(ψ−1 δ−1 γεϕ)ϕ −1 a = ψ−1 a ′ .
Since εa = a and δa ′ = a ′ , this proves γa = a ′ .
We have proved 4.5.3 in situation (II) under the assumption S = S 1 . In the following 4.5.9
and 4.5.10, we reduce the general case to that case.
Lemma 4.5.9. Let a , a ′ ∈ Z . In situation (I) (resp., (II)), let H = G (ASF1 ) (resp., H = G (AF,S 2 )).

Then there exist neighborhoods U of a and U ′ of a ′ in Z such that g a = a ′ for all g ∈ H for
which g U ∩ U ′ 6= ∅.

Proof. For any compact neighborhoods U of a and U ′ of a ′ , the set M := {g ∈ H | g U ∩U ′ 6= ∅}
is compact. By definition of Z , there exist a compact open subgroup N of H and a compact

neighborhood U of a such that g x = x for all g ∈ N and x ∈ U . For such a choice of U , the

set M is stable under the right translation by N , and M /N is finite because M is compact and
N is an open subgroup of H . If g ∈ M and if g a 6= a ′ , then by shrinking the neighborhoods U

and U ′ , we have that g U ∩U ′ = ∅. As M /N is finite, we have sufficiently small neighborhoods

U and U ′ such that if g ∈ M and g U ∩ U ′ 6= ∅, then g a = a ′ .
55

4.5.10. We prove Proposition 4.5.3.
Let H be as in Lemma 4.5.9. Write a = (a S 1 , a Z ) and a ′ = (a S′ 1 , a Z′ ) as elements of X̄ × Z . By

4.5.9, there exist neighborhoodsUZ of a Z and UZ′ of a Z′ in Z such that if g ∈ H and g UZ ∩UZ′ 6= ∅,

then g a = a ′ . The set K ′ := {g ∈ H | g a Z = a Z } is a compact open subgroup of H . Let Γ′ be
the inverse image of K ′ under Γ → H , where Γ = G (F ) in situation (I). In situation (II), the

group Γ′ is of finite index in the inverse image of the compact open subgroup K ′ × K under
S

G (F ) → G (AF1 ). In both situations, the set M := {γ ∈ Γ | γa Z = a Z′ } is either empty or a Γ′ -torsor
for the right action of Γ′ .

Assume first that M 6= ∅, in which case we may choose θ ∈ Γ such that M = θ Γ′ . Since we

have proven 4.5.3 in situation (II) for S 1 = S, there exist neighborhoods US 1 of a S 1 and US′ 1 of
θ −1 a S′ 1 such that if γ ∈ Γ′ satisfies γUS 1 ∩ US′ 1 6= ∅, then γa S 1 = θ −1 a S′ 1 . Let U = US 1 × UZ and

U ′ = θUS′ 1 × UZ′ , which are neighborhoods of a and a ′ in X̄, respectively. Suppose that γ ∈ Γ

satisfies γU ∩U ′ 6= ∅. Then, since γUZ ∩UZ′ 6= ∅, we have γa Z = a Z′ and hence γ = θ γ′ for some

γ′ ∈ Γ′ . Since θ γ′US 1 ∩ θUS′ 1 6= ∅, we have γ′US 1 ∩US′ 1 6= ∅, and hence γ′ a S 1 = θ −1 a S′ 1 . That is, we

have γa S 1 = a S′ 1 , so γa = a ′ .

In the case that M = ∅, take any neighborhoods US 1 of a S 1 and US′ 1 of a S′ 1 , and set U =
US 1 × UZ and U ′ = US′ 1 × UZ′ . Any γ ∈ Γ such that γU ∩ U ′ 6= ∅ is contained in M , so no such γ
exists.

4.6 Supplements to the main theorem
We use the notation of §4.1 throughout this subsection. We suppose that Γ = G (F ) in situation
(I), and we let Γ be a subgroup of ΓK of finite index in situation (II). For a ∈ X̄, let Γa < Γ denote
the stabilizer of a .

Theorem 4.6.1. For a ∈ X̄ (with either the Borel-Serre or the Satake topology), there is an open

neighborhood U of the image of a in Γa \X̄ such that the image U ′ of U under the quotient map

Γa \X̄ → Γ\X̄ is open and the map U → U ′ is a homeomorphism.

Proof. By the case a = a ′ of Proposition 4.5.3, there is an open neighborhood U ′′ ⊂ X̄ of a such

that if γ ∈ ΓK and γU ′′ ∩ U ′′ 6= ∅, then γa = a . Then the subset U := Γa \Γa U ′′ of Γa \X̄ is open
and has the desired property.

Proposition 4.6.2. Suppose that S = S 1 , and let a ∈ X̄.
(1) Take X̄ = X̄ F,S , and let P be the parabolic subgroup associated to a . Then Γ(P) (as in 3.4.12)
is a normal subgroup of Γa of finite index.

56

♭
(2) Take X̄ = X̄ F,S
, and let W be the F -subspace of V associated to a . Then Γ(W ) (as in 3.4.12)

is a normal subgroup of Γa of finite index.
Proof. We prove (1), the proof of (2) being similar. Let (Vi )−1≤i ≤m be the flag corresponding
Qm
to P. The image of Γ ∩ P(F ) in i =0 PGLVi /Vi −1 (AF,S ) is discrete. On the other hand, the stabiQm
lizer in i =0 PGLVi /Vi −1 (AF,S ) of the image of a in ZF,S (P) is compact. Hence the image of Γa in
Qm
PGLVi /Vi −1 (F ), which is isomorphic to Γa /Γ(P) , is finite.
i =0
Theorem 4.6.3. Assume that F is a function field and X̄ = X̄ F,S 1 , where S 1 consists of a single
place v . Then the inclusion map Γ\X ,→ Γ\X̄ is a homotopy equivalence.
Proof. Let a ∈ X̄. In situation (I) (resp., (II)), write a = (a v , a v ) with a v ∈ X̄ F,v and a v ∈ X S 2 ×
Q
G (ASF )/K (resp., X S 2 ). Let K ′ be the isotropy subgroup of a v in G (AvF ) (resp., w ∈S 2 G (Fw )), and
Q
let Γ′ < Γ be the inverse image of K ′ under the map Γ → G (AvF ) (resp., Γ → w ∈S 2 G (Fw )).

Let P be the parabolic subgroup associated to a . Let Γa be the isotropy subgroup of a in Γ,

which is contained in P(F ) and equal to the isotropy subgroup Γ′a v of a v in Γ′ . In situation (I)
(resp., (II)), take a Γa -stable open neighborhood D of a v in X S 2 × G (ASF )/K (resp., X S 2 ) that has

compact closure.

Claim 1. The subgroup ΓD := {γ ∈ Γa | γx = x for all x ∈ D} of Γa is normal of finite index.
Proof of Claim 1. Normality follows from the Γa -stability of D. For any x in the closure D̄ of D,
there exists an open neighborhood Vx of x and a compact open subgroup N x of G (AvF ) (resp.,
Q
G (Fw )) in situation (I) (resp., (II)) such that g y = y for all g ∈ N x and y ∈ Vx . For a finite
w ∈S 2
Tn
subcover {Vx 1 , . . . , Vx n } of D̄, the group ΓD is the inverse image in Γa of i =1 N x i , so is of finite

index.

Claim 2. The subgroup H := ΓD ∩ Pu (F ) of Γa is normal of finite index.
Proof of Claim 2. Normality is immediate from Claim 1 as Pu (F ) is normal in P(F ). Let H ′ =
Γ′(P) ∩ Pu (F ), which has finite index in Γ′(P) and equals Γ′ ∩ Pu (F ) by definition of Γ′(P) . Since

Γ′(P) ⊂ Γ′a v ⊂ Γ′ and Γ′a v = Γa , we have H ′ = Γa ∩ Pu (F ) as well. By Claim 1, we then have that H ′

contains H with finite index, so H has finite index in Γ′(P) . Proposition 4.6.2(1) tells us that Γ′(P)
is of finite index in Γ′a v = Γa .

Let (Vi )−1≤i ≤m be the flag corresponding to P. By Corollary 3.5.4, we have a homeomor-

phism

∼

χ : Pu (Fv )\X̄ F,v (P) −
→ ZF,v (P) × Rm
≥0
′
on quotient spaces arising from the P(Fv )-equivariant homeomorphism ψP,v = (φP,v , φP,v
) of

3.5.1 (see 3.4.3 and 3.4.6).
57

Claim 3. For a sufficiently small open neighborhood U of 0 = (0, . . . , 0) in Rm
≥0 , the map χ

induces a homeomorphism

∼

χU : H \X̄ F,v (P)U −
→ ZF,v (P) × U ,
′
where X̄ F,v (P)U denotes the inverse image of U under φP,v
: X̄ F,v (P) → Rm
≥0 .

Proof of Claim 3. By definition, χ restricts to a homeomorphism
∼

Pu (Fv )\X̄ F,v (P)U −
→ ZF,v (P) × U
for any open neighborhood U of 0. For a sufficiently large compact open subset C of Pu (Fv ),
we have Pu (Fv ) = HC . For U sufficiently small, every g ∈ C fixes all x ∈ X̄ F,v (P)U , which yields
the claim.

Claim 4. The map χU and the identity map on D induce a homeomorphism
∼

χU ,a : Γa \(X̄ F,v (P)U × D) −
→ (Γa \(ZF,v (P) × D)) × U .
Proof of Claim 4. The quotient group Γa /H is finite by Claim 2. Since the determinant of an
automorphism of Vi /Vi −1 of finite order has trivial absolute value at v , the Γa -action on Rm
≥0 is

trivial. Since H acts trivially on D, the claim follows from Claim 3.

m
Now let c ∈ Rm
>0 , and set U = {t ∈ R≥0 | t i < c for all 1 ≤ i ≤ m }. Set (X v )U = X v ∩ X̄ F,v (P)U . If

c is sufficiently small, then

(Γa \(ZF,v (P) × D)) × (U ∩ Rm
) ,→ (Γa \(ZF,v (P) × D)) × U
>0
is a homotopy equivalence, and we can apply χU−1,a to both sides to see that the inclusion map
Γa \((X v )U × D) ,→ Γa \(X̄ F,v (P)U × D)
is also a homotopy equivalence. By Theorem 4.6.1, this proves Theorem 4.6.3.
Remark 4.6.4. Theorem 4.6.3 is well-viewed as a function field analogue of the homotopy
equivalence for Borel-Serre spaces of [3].
4.6.5. Theorem 4.1.4 remains true if we replace G = PGLV by G = SLV in 4.1.3 and 4.1.4. It
also remains true if we replace G = PGLV by G = GLV and we replace X̄ in 4.1.4 in situation (I)
(resp., (II)) by
X̄ × X S 2 × (RS>0 × G (ASF )/K )1
58

(resp., X̄ × X S 2 × (RS>0 )1 ),

where ( )1 denotes the kernel of
((a v )v ∈S , g ) 7→ |det(g )|

Y
av
v ∈S

(resp., (a v )v ∈S 7→

Y
a v ),
v ∈S

and γ ∈ GLV (F ) (resp., γ ∈ ΓK ) acts on this kernel by multiplication by ((| det(γ)|v )v ∈S , γ) (resp.,

(|det(γ)|v )v ∈S ).

Theorems 4.6.1 and 4.6.3 also remain true under these modifications. These modified ver-

sions of the results are easily reduced to the original case G = PGLV .

4.7 Subjects related to this paper
4.7.1. In this subsection, as possibilities of future applications of this paper, we describe connections with the study of
• toroidal compactifications of moduli spaces of Drinfeld modules (4.7.2–4.7.5)
• the asymptotic behavior of Hodge structures and p -adic Hodge structures associated to
a degenerating family of motives over a number field (4.7.6, 4.7.7), and

• modular symbols over function fields (4.7.8, 4.7.9).
4.7.2. In [21], Pink constructed a compactification of the moduli space of Drinfeld modules
that is similar to the Satake-Baily-Borel compactification of the moduli space of polarized
abalian varieties. In a special case, it had been previously constructed by Kapranov [15].
In [20], Pink, sketched a method for constructing a compactification of the moduli space
of Drinfeld modules that is similar to the toroidal compactification of the moduli space of
polarized abelian varieties (in part, using ideas of K. Fujiwara). However, the details of the
construction have not been published. Our plan for constructing toroidal compactifications
seems to be different from that of [20].
4.7.3. We give a rough outline of the relationship that we envision between this paper and the
analytic theory of toroidal compactifications. Suppose that F is a function field, and fix a place
v of F . Let O be the ring of all elements of F which are integral outside v . In [6], the notion of
a Drinfeld O-module of rank d is defined, and the moduli space of such Drinfeld modules is
constructed.
Let Cv be the completion of an algebraic closure of Fv and let | |: Cv → R≥0 be the absolute

value which extends the normalized absolute value of Fv . Let Ω ⊂ Pd −1 (Cv ) be the (d − 1)dimensional Drinfeld upper half-space consisting of all points (z 1 : · · · : z d ) ∈ Pd −1 (Cv ) such
that (z i )1≤i ≤d is linearly independent over Fv .

59

For a compact open subgroup K of GLd (AvF ), the set of Cv -points of the moduli space M K
of Drinfeld O-modules of rank d with K -level structure is expressed as
M K (Cv ) = GLd (F )\(Ω × GLd (AvF )/K )
(see [6]).
Consider the case V = F d in §3 and §4. We have a map Ω → X v which sends (z 1 : · · · : z d ) ∈ Ω
Pd
to the class of the the norm Vv = Fvd → R≥0 given by (a 1 , . . . , a d ) 7→ | i =1 a i z i | for a i ∈ Fv . This

map induces a canonical continuous map

(1) M K (Cv ) = GLd (F )\(Ω × GLd (AvF )/K ) → GLd (F )\(X v × GLd (AvF )/K ).
The map (1) extends to a canonical continuous map
♭
(2) M̄ KKP (Cv ) → GLd (F )\(X̄ F,v
× GLd (AvF )/K ),

where M̄ KKP denotes the compactification of Kapranov and Pink of M K . In particular, M̄ KKP is
♭
related to X̄ F,v
. On the other hand, the toroidal compactifications of M K should be related to

X̄ F,v . If we denote by M̄ Ktor the projective limit of all toroidal compactifications of M K , then the
map of (1) should extend to a canonical continuous map
(3) M̄ Ktor (Cv ) → GLd (F )\(X̄ F,v × GLd (AvF )/K ).
that fits in a commutative diagram
M̄ Ktor (Cv )

GLd (F )\(X̄ F,v × GL(AvF )/K )

M KKP (Cv )

♭
GLd (F )\(X̄ F,v
× GLd (AvF )/K ).

4.7.4. The expected map of 4.7.3(3) is the analogue of the canonical continuous map from
the projective limit of all toroidal compactifications of the moduli space of polarized abelian
varieties to the reductive Borel-Serre compactification (see [10, 16]).
From the point of view of our study, the reductive Borel-Serre compactification and X̄ F,v
are enlargements of spaces of norms. A polarized abelian variety A gives a norm on the polarized Hodge structure associated to A (the Hodge metric). This relationship between a polarized abelian variety and a norm forms the foundation of the relationship between the toroidal
compactifications of a moduli space of polarized abelian varieties and the reductive BorelSerre compactification. This is similar to the relationship between M K and the space of norms
X v given by the map of 4.7.3(1), as well as the relationship between M̄ Ktor and X̄ F,v given by
4.7.3(3).
4.7.5. In the usual theory of toroidal compactifications, cone decompositions play an important role. In the toroidal compactifications of 4.7.3, the simplices of Bruhat-Tits buildings
60

♭
(more precisely, the simplices contained in the fibers of X̄ F,v → X̄ F,v
) should play the role of the

cones in cone decompositions.

4.7.6. We are preparing a paper in which our space X̄ F,S with F a number field and with S
containing a non-archimedean place appears in the following way.
Let F be a number field, and let Y be a polarized projective smooth variety over F . Let
m
m ≥ 0, and let V = H dR
(Y ) be the de Rham cohomology. For a place v of F , let Vv = Fv ⊗F V .

For an archimedean place v of F , it is well known that Vv has a Hodge metric. For v non-

archimedean, we can under certain assumptions define a Hodge metric on Vv by the method
illustrated in the example of 4.7.7 below. The [Fv : Qv ]-powers of these Hodge metrics for v ∈ S
Q
are norms and therefore provide an element of v ∈S X Vv . When Y degenerates, this element
Q
of v ∈S X Vv can converge to a boundary point of X̄ F,S .
4.7.7. Let Y be an elliptic curve over a number field F , and take m = 1.
Let v be a non-archimedean place of F , and assume that Fv ⊗F Y is a Tate elliptic curve

of q -invariant qv ∈ Fv× with |qv | < 1. Then the first log-crystalline cohomology group D of the

special fiber of this elliptic curve is a free module of rank 2 over the Witt vectors W (k v ) with a
basis (e 1 , e 2 ) on which the Frobenius ϕ acts as ϕ(e 1 ) = e 1 and ϕ(e 2 ) = p e 2 , where p = char k v .
The first ℓ-adic étale cohomology group of this elliptic curve is a free module of rank 2 over Zℓ

with a basis (e 1,ℓ , e 2,ℓ ) such that the inertia subgroup of Gal(F̄v /Fv ) fixes e 1 . The monodromy
operator N satisfies
N e 2 = ξ′v e 1 ,

N e 1 = 0,

N e 2,ℓ = ξ′v e 1,ℓ ,

N e 1,ℓ = 0

where ξ′v = ordFv (qv ) > 0. The standard polarization 〈 , 〉 of the elliptic curve satisfies 〈e 1 , e 2 〉 =

1 and hence

〈N e 2, e 2 〉 = ξ′v ,

〈e 1 , N −1 e 1 〉 = 1/ξ′v

〈N e 2,ℓ, e 2,ℓ 〉 = ξ′v ,

〈e 1,ℓ , N −1 e 1,ℓ 〉 = 1/ξ′v .

1
For V = H dR
(Y ), we have an isomorphism Vv ∼
= Fv ⊗W (k v ) D. The Hodge metric | |v on Vv is

defined by

|a 1 e 1 + a 2 e 2 |v = max(ξ−1/2
|a 1 |p , ξv1/2 |a 2 |p )
v
for a 1 , a 2 ∈ Fv , where | |p denotes the absolute value on Fv satisfying |p |p = p −1 and
ξv := −ξ′v log(|̟v |p ) = − log(|qv |p ) > 0,
where ̟v is a prime element of Fv . That is, to define the Hodge metric on Vv , we modify the
naive metric (coming from the integral structure of the log-crystalline cohomology) by using
61

ξv which is determined by the polarization 〈 , 〉, the monodromy operator N , and the integral
structures of the log-crystalline and ℓ-adic cohomology groups (for ℓ 6= p ).

This is similar to what happens at an archimedean place v . We have Y (C) ∼
= C× /qvZ with

qv ∈ Fv× . Assume for simplicity that we can take |qv | < e −2π where | | denotes the usual abso-

lute value. Then qv is determined by Fv ⊗F Y uniquely. Let ξ := − log(|qv |) > 2π. If v is real, we
further assume that qv > 0 and that we have an isomorphism Y (Fv ) ∼
= F × /q Z which is compatv

v

ible with Y (C) ∼
= C× /qvZ . Then in the case v is real (resp., complex), there is a basis (e 1 , e 2 ) of Vv

such that (e 1 , (2πi )−1 e 2 ) is a Z-basis of H 1 (Y (C), Z) and such that the Hodge metric | |v on Vv
−1/2

satisfies |e 1 |v = ξv

1/2

1/2

−1/2

and |e 2 |v = ξv (resp., ||e 2 |v − ξv | ≤ πξv

Consider for example the elliptic curves

y2

).

= x (x − 1)(x − t ) with t ∈ F = Q, t 6= 0, 1. As t

approaches 1 ∈ Qv for all v ∈ S, the elliptic curves Fv ⊗F Y satisfy the above assumptions for all
Q
v ∈ S, and each qv approaches 0, so ξv tends to ∞. The corresponding elements of v ∈S X Vv
defined by the classes of the | |v for v ∈ S converge to the unique boundary point of X̄ F,S with

associated parabolic equal to the Borel subgroup of upper triangular matrices in PGLV for the
basis (e 1 , e 2 ).
We hope that this subject about X̄ F,S is an interesting direction to be studied. It may be
related to the asymptotic behaviors of heights of motives in degeneration studied in [18].
4.7.8. Suppose that F is a function field and let v be a place of F . Let Γ be as in 1.3.
Kondo and Yasuda [17] proved that the image of H d −1(Γ\X v , Q) → H dBM
−1 (Γ\X v , Q) is gener-

ated by modular symbols, where H ∗BM denotes Borel-Moore homology. Our hope is that the

compactification Γ\X̄ F,v of Γ\X v is useful in further studies of modular symbols.
Let ∂ := X̄ F,v \ X v . Then we have an isomorphism
H ∗BM (Γ\X v , Q) ∼
= H ∗ (Γ\X̄ F,v , Γ\∂ , Q)
and an exact sequence

· · · → H i (Γ\X̄ F,v , Q) → H i (Γ\X̄ F,v , Γ\∂ , Q) → H i −1 (Γ\∂ , Q) → H i −1 (Γ\X̄ F,v , Q) → . . . .
Since Γ\X v → Γ\X̄ F,v is a homotopy equivalence by Thm. 4.6.3, we have
∼

H ∗ (Γ\X v , Q) −
→ H ∗ (Γ\X̄ F,v , Q).
Hence the result of Kondo and Yasuda shows that the kernel of
H dBM
(Γ\X v , Q) ∼
= H d −1 (Γ\X̄ F,v , Γ\∂ , Q) → H d −2 (Γ\∂ , Q)
−1
is generated by modular symbols.
62

If we want to prove that H dBM
−1 (Γ\X v , Q) is generated by modular symbols, it is now sufficient

to prove that the kernel of H d −2 (Γ\∂ , Q) → H d −2 (Γ\X̄ F,v , Q) is generated by the images (i.e.,

boundaries) of modular symbols.

4.7.9. In 4.7.8, assume d = 2. Then we can prove that H 1BM (Γ\X v , Q) is generated by modular
symbols. In this case, the map H 0 (Γ\∂ , Q) = Map(Γ\∂ , Q) → H 0 (X̄ F,v , Q) = Q is just summation,

and it is clear that the kernel of it is generated by the boundaries of modular symbols.

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