arXiv:1411.5327v3 [] 11 Jan 2017

Almost algebraic actions of algebraic groups and
applications to algebraic representations
Uri Bader∗1 , Bruno Duchesne†2 , and Jean Lécureux‡3
1

Uri Bader, Weizmann Insitute of Science, Rehovot, Israel.
Université de Lorraine, Institut Élie Cartan, B.P. 70239, 54506
Vandoeuvre-lès-Nancy Cedex, France.
3
Département de Mathématiques, Bâtiment 425, Faculté des Sciences
d’Orsay, Université Paris-Sud 11, 91405 Orsay, France.
2

April 4, 2018
Abstract
Let G be an algebraic group over a complete separable valued field k. We
discuss the dynamics of the G-action on spaces of probability measures on
algebraic G-varieties. We show that the stabilizers of measures are almost
algebraic and the orbits are separated by open invariant sets. We discuss
various applications, including existence results for algebraic representations
of amenable ergodic actions. The latter provides an essential technical step
in the recent generalization of Margulis-Zimmer super-rigidity phenomenon
[BF13].

1

Introduction

This work concerns mainly the dynamics of an algebraic group acting on the space
of probability measures on an algebraic variety. Most (but not all) of our results are
known for local fields (most times, under a characteristic zero assumption). Our
main contribution is giving an approach which is applicable also to a more general
class of fields: complete valued fields. On our source of motivation, which stems
from ergodic theory, we will elaborate in §1.2, and in particular Theorem 1.16.
First we describe our objects of consideration and our main results, put in some
historical context.
Setup 1.1. For the entire paper (k, | · |) will be a valued field, which is assumed
to be complete and separable as a metric space, and b
k will be the completion of
its algebraic closure, endowed with the extended absolute value.
∗ uri.bader@gmail.com
† bruno.duchesne@univ-lorraine.fr
‡ jean.lecureux@math.u-psud.fr

1

Note that b
k is separable and complete as well (see the proof of Proposition 2.2).
The most familiar examples of separable complete valued fields are of course R and
C, but one may also consider the p-adic fields Qp , as well as their finite extensions.
Considering k = Cp = “
Qp one may work over a field which is simultaneously complete, separable and algebraically closed. Another example of a complete valued
field is given by fields of Laurent series K((t)), where K is any field (this field is
local if and only if K is finite, and separable if and only if K is countable), or
more generally the field of Hahn series K((tΓ )), where Γ is a subgroup of R (see
for example [Poo93]). This field is separable if and only if K is countable and Γ is
discrete (see [hmb]).
Convention 1.2. Algebraic varieties over k will be identified with their b
k-points
and will be denoted by boldface letters. Their k-points will be denoted by corresponding Roman letters. In particular we use the following.
Setup 1.3. We fix a k-algebraic group G and we denote G = G(k).
We are interested in algebraic dynamical systems, which we now briefly describe. For a formal, pedantic description see §2.1 and in particular Proposition 2.2. By an algebraic dynamical system we mean the action of G on V , where
V is the space of k-points of a k-algebraic variety V on which G acts k-morphically.
Such a dynamical system is Polish: G is a Polish group, V a Polish space and the
action map G × V → V is continuous (see §2.1 for proper definitions). The point
stabilizers of such an action are algebraic subgroups, and by a result of BernsteinZelevinski [BZ76], the orbits of such an action are locally closed (see Proposition
2.2).
Following previous works of Furstenberg and Moore, Zimmer found a surprising
result: for the action of an algebraic group G on an algebraic variety V , all defined
over R, consider now the action of G on the space Prob(V ) of probability measures
on V . Then the point stabilizers are again algebraic subgroups and the orbits are
locally closed. However, this result does not extend trivially to other fields. For
example, with k = C, consider the Haar measure on the circle S 1 < C∗ . For
the action of C∗ on itself, the stabilizer of that measure is S 1 , which is not a
C-algebraic subgroup. Similarly, for k = Qp , consider the Haar measure on the
p-adic integers Zp < Qp . For the action of Qp on itself, the stabilizer of that
measure is Zp , which is not a Qp -algebraic subgroup.
Definition 1.4. A closed subgroup L < G is called almost algebraic if there
exists a k-algebraic subgroup H < G such that L contains H = H(k) as a normal
cocompact subgroup. A continuous action of G on a Polish space V is called
almost algebraic if the point stabilizers are almost algebraic subgroups of G and
the collection of G-invariant open sets separates the G-orbits, i.e the quotient
topology on G\V is T0 .
Remark 1.5. If k is a local field then G is locally compact and by [Eff65, Theorem 2.6] the condition G\V is T0 is equivalent to the (a priori stronger) condition
that every G-orbit is locally closed in V .

2

Remark 1.6. If k = R then every compact subgroup of G is the real points of
a real algebraic subgroup of G (see e.g. [VGO90, Chapter 4, Theorem 2.1]). It
follows that every almost algebraic subgroup is the real points of a real algebraic
subgroup of G. We get that a continuous action of G on a Polish space V is almost
algebraic if and only if the stabilizers are real algebraic and the orbits are locally
closed.
Two obvious classes of examples of almost algebraic actions are algebraic actions (by the previously mentioned result of Bernstein-Zelevinski) and proper actions (as the stabilizers are compact and the space of orbits is T2 , that is, Hausdorff). The notion of almost algebraic action is a natural common generalization.
It is an easy corollary of Prokhorov’s theorem (see Theorem 2.3 below) that if the
action of G on V is proper then so is its action on Prob(V ), see Lemma 2.7. The
main theorem of this paper is the following analogue.
Theorem 1.7. If the action of G on a Polish space V is almost algebraic then
the action of G on Prob(V ) is almost algebraic as well.
The following corollary was obtained by Zimmer, under the assumptions that
k is a local field of characteristic 0 and V is homogeneous, see [Zim84, Chapter 3].
Corollary 1.8. Assume G has a k-action on a k-variety V. Then the induced
action of G = G(k) on Prob(V(k)) is almost algebraic.
In the course of the proof of Theorem 1.7 we obtain in fact a more precise
information. A k-G-variety is a k-variety with a k-action of G.
Proposition 1.9. Fix a closed subgroup L < G. Then there exists a k-subgroup
H0 < G which is normalized by L such that L has a precompact image in the
Polish group (NG (H0 )/H0 )(k) and such that for every k-G-variety V, any Linvariant finite measure on V(k) is supported on the subvariety of H0 -fixed points,
VH0 ∩ V(k).
This proposition is a generalization of one of the main results of Shalom [Sha99],
who proves it under the assumptions that k is local and L = G. For the case L = G
the following striking corollary is obtained.
Corollary 1.10. If for every strict k-algebraic normal subgroup H/G, G(k)/H(k)
is non-compact, then every G-invariant measure on any k-G-algebraic variety
V(k) is supported on the G-fixed points.
In particular we can deduce easily the Borel density theorem.
Corollary 1.11. Let G be a k-algebraic group and Γ < G = G(k) be a closed
subgroup such that G/Γ has a G-invariant probability measure. If for every proper
k-algebraic normal subgroup H / G, G(k)/H(k) is non-compact, then Γ is Zariski
dense in G.
To deduce the last corollary from the previous one, consider the map
Z

G/Γ → (G/Γ )(k),
3

Z

where Γ denotes the Zariski closure of Γ, and push forward the invariant measure
Z
from G/Γ to obtain a G-invariant measure on (G/Γ )(k). The homogeneous space
Z
Z
G/Γ must contain a G-fixed point, hence must be trivial. That is Γ = G.

1.1

Applications: ergodic measures on algebraic varieties

A classical theme in ergodic theory is the attempt to classify all ergodic measures
classes, given a continuous action of a topological group on a Polish space. In this
regard, the axiom that the space of orbits is T0 has strong applications. Recall
that, given a group L acting by homeomorphisms on a Polish space V , a measure
on V is L-quasi-invariant if its class is L-invariant. The following proposition is
well known.
Proposition 1.12. Let V be a Polish G-space and assume that the quotient topology on G\V is T0 . Let L < G be a subgroup and µ an L-quasi-invariant ergodic
probability (or σ-finite) measure. Then there exists v ∈ V such that µ(V −Gv) = 0.
Indeed, G\V is second countable, as V is, and for a countable basis Bi , denoting
the push forward of µ to G\V by µ̄, the set
\
\
{Bi | µ̄(Bi ) = 1} ∩ {Bic | µ̄(Bi ) = 0}
is clearly a singleton, whose preimage in V is an orbit of full measure.
In particular, we get that for a subgroup L < G and an algebraic dynamical
system of G, every L-invariant measure is supported on a single G-orbit. Another
striking result is that an algebraic variety cannot support a weakly mixing probability measure. Recall that an L-invariant probability measure µ is weakly mixing
if and only if µ × µ is L-ergodic.
Corollary 1.13. Assume G has a k-action on the k-variety V. Fix a closed
subgroup L < G and let µ be an L-invariant weakly mixing probability measure on
V = V(k). Then there exists a point x ∈ V L such that µ = δx .
This corollary follows at once from Proposition 1.9, as the action of L on
VH0 ∩ V(k) is via a compact group.
We end this subsection with the following useful application, obtained by composing Proposition 1.12 with Theorem 1.7. This corollary is in fact our main
motivation for developing the material in this paper. It deals with measure on
spaces of measures, and is the main tool in deriving Theorem 1.16 below.
Corollary 1.14. Assume G has a k-action on the k-variety V. Denote V =
V(k). Let L < G be a subgroup and ν be an L-ergodic quasi-invariant measure on
Prob(V ). Then there exists µ ∈ Prob(V ) such that ν(Prob(V ) − Gµ) = 0.

1.2

Applications to algebraic representations of ergodic actions.

A main motivation for us to extend the foundation outside the traditional local
field zone is the recent developments in the theory of algebraic representations of
4

ergodic actions, and in particular its applications to rigidity theory. In [BF13] the
following theorem, as well as various generalizations, are proven.
Theorem 1.15 ([BF13, Theorem 1.1], Margulis super-rigidity for arbitrary fields).
Let l be a local field. Let T to be the l-points of a connected almost-simple algebraic
group defined over l. Assume that the l-rank of T is at least two. Let Γ < T be a
lattice.
Let k be a valued field. Assume that as a metric space k is complete. Let G be
the k-points of an adjoint simple algebraic group defined over k. Let δ : Γ → G be
a homomorphism. Assume δ(Γ) is Zariski dense in G and unbounded. Then there
exists a continuous homomorphism d : T → G such that δ = d|Γ .
The proofs in [BF13] are based on the following, slightly technical, theorem
which will be proven here.
Theorem 1.16. Let R be a locally compact group and Y be an ergodic, amenable
Lebesgue R-space. Let (k, | · |) be a valued field. Assume that as a metric space k is
complete and separable. Let G be a simple k-algebraic group. Let f : R×Y → G(k)
be a measurable cocycle.
Then either there exists a k-algebraic subgroup H
G and an f -equivariant
measurable map φ : Y → G/H(k), or there exists a complete and separable metric space V on which G acts by isometries with bounded stabilizers and an f equivariant measurable map φ0 : Y → V .
A more friendly, cocycle free, version is the following.
Corollary 1.17. Let R be a locally compact, second countable group. Let Y be an
ergodic, amenable R-space. Suppose that G is an adjoint simple k-algebraic group,
and there is a morphism R → G = G(k). Then :
• Either there exists a complete and separable metric space V , on which G acts
by isometries with bounded stabilizers, and an R-equivariant measurable map
Y →V;
• or there exists a strict k-algebraic subgroup H and an R-equivariant measurable map Y → G/H(k).
Taking Y to be a point in the above corollary, we obtain the following.
Corollary 1.18. Suppose R < GLn (k) is a closed amenable subgroup. Then the
Z
image of R in R modulo its solvable radical is bounded.
Z

Indeed, upon moding out the solvable radical of R , the latter is a product of
simple adjoint factors, and by the previous corollary the image of R in each factor
is bounded.
Note that over various fields, such as Cp and F̄p ((t)), every bounded group
is amenable, being the closure of an ascending union of compact groups, while
for other fields there exist bounded groups which are not amenable. For example
SL2 (Q[[t]]), which is bounded in SL2 (Q((t))), factors over the discrete group SL2 (Q)
which contains a free group.
5

1.3

The structure of the paper.

The paper has two halves: the first half consisting of §2,§3 is devoted to the proof
of Theorem 1.7 and the second half is devoted to the proof of Theorem 1.16.
In §2 we collect various needed preliminaries, in particular we discuss the Polish
structure on algebraic varieties, and on spaces of measures. The most important
results in this section are Proposition 2.2 that discusses algebraic varieties and
and Corollary 2.14 that uses disintegration as a replacement for a classical ergodic
decomposition argument (which is not applicable in our context, due to the lack of
compactness). The heart of the paper is §3, where the concept of almost algebraic
action is discussed. Theorem 1.7 is proven at §3.4.
In §4, we give a thorough discussion of bounded subgroups of algebraic groups,
and in §5, we discuss a suitable replacement of a compactification of coset spaces.
In §6, we prove Theorem 1.16.

Acknowledgements
U.B was supported in part by the ERC grant 306706. B.D. was supported in part
by Lorraine Region and Lorraine University. B.D & J.L. were supported in part
by ANR grant ANR-14-CE25-0004 GAMME.

2

Preliminaries

2.1

Algebraic varieties as Polish spaces

Recall that a topological space is called Polish if it is separable and completely
metrizable. For a good survey on the subject we recommend [Kec95]. We mention
that the class of Polish spaces is closed under countable disjoint unions and countable products. A Gδ -subset of a Polish space is Polish so, in particular, a locally
closed subset of a Polish space is Polish. A Hausdorff space which admits a finite
open covering by Polish open sets is itself Polish. Indeed, such a space is clearly
metrizable (e.g. by Urysohn metrization theorem [Kec95, Theorem 1.1]) so it is
Polish by Sierpinski theorem [Kec95, Theorem 8.19] which states that the image
of a continuous open map from a Polish space to a separable metrizable space is
Polish.
A topological group which underlying topological space is Polish is called a
Polish group. Sierpinski theorem also implies that for a Polish group K and a
closed subgroup L, the quotient topology on K/L is Polish. Effros Lemma [Eff65,
Lemma 2.5] says that the quotient topology on K/L is the unique K-invariant
Polish topology on this space. Another important result of Effros concerning
Polish actions (that are continuous actions of Polish groups on Polish spaces) is
the following.
Theorem 2.1 (Effros theorem [Eff65, Theorem 2.1]). For a continuous action of
a Polish group G on a Polish space V the following are equivalent.
1. The quotient topology on G\V is T0 .
6

2. For every v ∈ V , the orbit map G/ StabG (v) → Gv is a homeomorphism.
Our basic class of Polish actions will be given by actions of algebraic groups
on algebraic varieties. As mentioned in Setups 1.1 & 1.3, we fixed a complete and
separable valued field (k, | · |), that is a field k with an absolute value | · | which
is complete and separable (in the sense of having a countable dense subset). See
[EP05, BGR84]1 for a general discussion on these fields. It is a standard fact that a
complete absolute value on a field F has a unique extension to its algebraic closure
“
F [BGR84, §3.2.4, Theorem 2] and Hensel lemma implies that the completion F
of this algebraic closure is still algebraically closed [BGR84, §3.4.1, Proposition 3].
Recall that we identify each k-variety V with its set of b
k-points. In particular,
this identification yields a topology on V. Identifying the affine space An (b
k) with
b
k n , any affine k-variety can be seen as a closed subset of An (b
k). More generally, a
k-variety has a unique topology making its affine charts homeomorphisms. Observe
that with this topology, the set of k-points V of V is closed.
Topological notions, unless otherwise said, will always refer to this topology. In
particular, for the k-algebraic group G we fixed, G and G = G(k) are topological
groups. We note that V actually carries a structure of a k-analytic manifold, G is
a k-analytic group and the action of G on V is k-analytic. We will not make an
explicit use of the analytic structure here. The interested reader is referred to the
excellent text [Ser06], in which the theory of analytic manifolds and Lie groups
over complete valued fields is developed (see in particular [Ser06, Part II, Chapter
I]).
We will discuss the category of k-G-varieties. A k-G-variety is a k-variety
endowed with an algebraic action of G which is defined over k. A morphism of
such varieties is a k-morphism which commutes with the G-action.
Proposition 2.2. A k-variety V and its set of k-points V are Polish spaces. In
particular, G and G are Polish groups.
If V is a k-G-variety then the G-orbits in V are locally closed and the quotient
topology on G\V is T0 . For v ∈ V , the orbit Gv is a k-subvariety of V. There
exists a k-subgroup H < G contained in the stabilizer of v such that the orbit map
G/H → Gv is defined over k and the induced map G/H → Gv is a homeomorphism, where H = H(k), G/H is endowed with the quotient space topology and
Gv is endowed with the subspace topology.
Proof. Let us first explain how the extended absolute value makes b
k Polish. In
our situation k has a countable dense subfield k0 . The algebraic closure k0 of k0 is
still countable and thus its completion k“0 is separable and algebraically closed. By
the universal property of the algebraic closure, k embeds in k“0 and by uniqueness
of the extension of the absolute value, this embedding is an isometry. Thus b
k is
algebraically closed, complete and separable.
Since b
k is Polish, so is the affine space An (b
k) ' b
k n . It follows that V (respectively V ) is a Polish space, as this space is a Hausdorff space which admits
a finite open covering by Polish open sets — the domains of its k-affine charts
(respectively their k-points).
1 In

the second reference, the word valuation is used for what we call an absolute value.

7

The fact that the G-orbits in V are locally closed is proven in the appendix of
[BZ76]. Note that in [BZ76] the statement is claimed only for non-Archimedean
local fields, but the proof is actually correct for any field with complete non-trivial
absolute value, which is the setting of [Ser06, Part II, Chapter III] on which [BZ76]
relies. Another proof can be found in [GGMB13, §0.5]. It is then immediate that
the quotient topology on G\V is T0 .
For v ∈ V the orbit Gv is a k-subvariety of V by [Bor91, Proposition 6.7].
Z

We set H = StabG (v) (note that if char(k) = 0 then H = StabG (v)). By
[Bor91, AG, Theorem 14.4], H is defined over k, and it is straightforward that
H = H(k) = StabG (v). By [Bor91, Theorem 6.8] the orbit map G/H → Gv is
defined over k, thus it restricts to a continuous map from G/H onto Gv. The fact
that the latter map is a homeomorphism follows from Effros theorem (Theorem 2.1)
since G\V is T0 .
We emphasize that, as a special case of Proposition 2.2, we get that for every kalgebraic subgroup H of G, the embedding G/H → G/H(k) is a homeomorphism
on its image. We will use this fact freely in the sequel.

2.2

Spaces of measures as Polish spaces

In this subsection V denotes a Polish space. We let Prob(V ) be the set of Borel
probability measures on V , endowed with the weak*-topology (also called the topology of weak convergence). This topology comes from the embedding of Prob(V )
in the dual of the Banach space of bounded continuous functions on V . If d is a
complete metric on V which is compatible with the topology (the metric topology
coincides with the original topology on V ), the corresponding Prokhorov metric
d on Prob(V ) is defined as follows: for µ, ν ∈ Prob(V ), d(µ, ν) is the infimum of
ε > 0 such that for all Borel subset A ⊆ V , µ(A) ≤ ν(Aε ) + ε and symmetrically
ν(A) ≤ µ(Aε ) + ε, where Aε is the ε-neighborhood (for d) around A. The following theorem summarizes some standard results, see Chapter 6 and Appendix III
of [Bil99].
Theorem 2.3 (Prokhorov). The metric space (Prob(V ), d) is complete and separable and the topology induced by d on Prob(V ) is the weak*-topology. In particular
the space Prob(V ) endowed with the weak*-topology is Polish.
A subset C in Prob(V ) is precompact if and only if it is tight: for every  > 0
there exists compact K ⊂ V such that for every µ ∈ C, µ(K) > 1−. In particular
Prob(V ) is compact if V is.
Remark 2.4. Replacing if necessary d by a bounded metric, we note that there
is another metric on Prob(V ) with the same properties (metrizing the weak*topology and being invariant under isometries): the Wasserstein metric [Vil09,
Corollary 6.13].
We endow Homeo(V ) with the pointwise convergence topology. The following
is a standard application of the Baire category theorem, see [Kec95, Theorem 9.14].

8

Theorem 2.5. Assume G is acting by homeomorphisms on V . Then the action
map G × V → V is continuous if and only if the homomorphism G → Homeo(V )
is continuous.
Lemma 2.6. If G acts continuously on V then it also acts continuously on
Prob(V ) and if the action G y (V, d) is by isometries, the action G y (Prob(V ), d)
is also by isometries.
Proof. The fact that G acts by isometries on Prob(V ) when G acts by isometries
on V is straightforward from the definition of the Prokhorov metric. In order to
prove that G acts continuously on Prob(V ) when it acts continuously on V it is
enough, by Theorem 2.5, to show that for every µ ∈ Prob(V ) and every sequence
gn in G, gn → e in G implies gn µ → µ in Prob(V ). Fix µ ∈ Prob(V ) and assume
gn → e in G. For every bounded continuous function f on V , we have by Lebesgue
bounded convergence theorem
Z
Z
Z
f (x) d(gn µ)(x) = f (gn x) dµ(x) → f (x) dµ(x)
as for every x ∈ V , gn x → x. Thus, by the definition of the weak*-topology
gn µ → µ.
We observe that Lemma 2.6 and Proposition 2.2 show that if V is a k-G-variety
then G acts continuously on V = V(k) and on Prob(V ). The following is a nice
application of Prokhorov theorem (Theorem 2.3).
Lemma 2.7. If the action of G on V is proper then the action of G on Prob(V )
is proper as well.
Proof. For a compact C ⊂ Prob(V ) we can find a compact K ⊂ V with µ(K) >
1/2 for every µ ∈ C by Theorem 2.3. Then for g ∈ G and µ ∈ C such that gµ ∈ C
we get that both µ(K) > 1/2 and µ(gK) = gµ(K) > 1/2, thus gK ∩ K 6= ∅.
We conclude that {g ∈ G | gC ∩ C 6= ∅} is precompact, as it is a subset of the
precompact set {g ∈ G | gK ∩ K 6= ∅}.

2.3

Polish extensions and disintegration

Definition 2.8. A Polish fibration is a continuous map p : V → U where U is a
T0 -space and V a Polish space. An action of G on such a Polish fibration is a pair
of continuous actions on V and U such that p is equivariant.
Let p : V → U be a Polish fibration. Let ProbU (V ) be the set of probability
measures on V which are supported on one fiber. We denote p• : ProbU (V ) → U
the natural map.
Lemma 2.9. The map p• is a Polish fibration. If the group G acts on the Polish
fibration V → U , then it also acts on p• .

9

Proof. Since U is T0 , fibers of p are separated by a countable family (Cn ) of closed
saturated subsets of V . A probability measure µ is supported on one fiber if
and only if for all n, µ(Cn )µ(V \ Cn ) = 0. The set {µ ∈ Prob(V ), µ(Cn ) = 1}
is closed and {µ ∈ Prob(V ), µ(V \ Cn ) = 1} is Gδ since for all 0 < r < 1,
{µ ∈ Prob(V ), µ(V \ Cn ) > r} is open. So ProbU (V ) is a Gδ -subset of Prob(V )
and thus Polish.
Let us show that p• is continuous. Assume µn → µ in ProbU (V ). Let u = p• (µ)
and un = p• (µn ). Let O ⊆ U be an open set containing u. For n large enough,
µn (p−1 (O)) > 1/2 and thus un ∈ O.
If G acts on V → U , it is clear that G acts on ProbU (V ). The continuity of
the action on ProbU (V ) follows from Lemma 2.6.
Let (U, ν) be a probability space and X be a Polish space, we denote by
L0 (U, X) the space of classes of measurable maps from U to X, under the equivalence relation of equality ν-almost everywhere. Note that the dependence on ν is
implicit in our notation. We endow that space with the topology of convergence
in probability. Fixing a compatible metric d on X, this topology is metrized as
follows: for φ, φ0 ∈ L0 (U, X), the distance between φ and φ0 is
Z
0
δ(φ, φ ) =
min(d(φ(v), φ0 (v)), 1) dν(v).
X

This topology can be also defined using sequences: φn → φ if for any ε > 0, there
is A ⊆ U such that ν(A) > 1 − ε and for all n sufficiently large and all v ∈ A,
d(φ(v), φn (v)) < ε. We note that this topology on L0 (U, X) does not depend on
the choice of an equivalent metric on V . This turns L0 (U, X) into a Polish space.
Lemma 2.10. Assume (αn ) is a sequence converging to α in probability in L0 (U, X).
Then there exists a subsequence αnk which convergence ν-a.e. to α, that is for νalmost every u ∈ U , αnk (u) converges to α(u) in X.
The proof of the lemma is standard, but in most textbooks it appears only for
the cases X = R or X = C, see for example [Fol99, Theorem 2.30]. Even though
the standard proof works mutatis-mutandis, we give below a short argument, reducing the general case to the case X = R.
Proof. Observe that the sequence d(αn , α) (which denotes the map u 7→ d(αn (u), α(u)))
converges in probability to 0 in L0 (U, R). Thus there exists a subsequence d(αnk , α)
converging to 0 a.e, and we get that αnk converges to α a.e.
If p : V → U is a Polish fibration, and ν is a measure on U , we denote L0p (U, V )
the space of measurable (identified if agree almost everywhere) sections of p, i.e.
maps which associates to u ∈ U a point in p−1 (U ), endowed with the induced
topology from L0 (U, V ). If G acts on the Polish fibration p, it also acts on L0p (U, V )
via the formula (gf )(u) = gf (g −1 u) where u ∈ U and f ∈ L0p (U, V ).
The following theorem is a variation of the classical theorem of disintegration
of measures. It is essentially proven in [Sim12].

10

Theorem 2.11. Let p : V → U be a Polish fibration and ν be a probability measure
on U . Let P = {µ ∈ Prob(V ) | p∗ µ = ν}. For every α ∈ L0p• (U, ProbU (V )) the
R
formula U α(u) dν defines an element of P . The map thus obtained L0p• (U, ProbU (V )) →
P is a homeomorphism onto.
Definition 2.12. For µ ∈ P , the element
of L0p• (U, ProbU (V )) obtained by apR
plying to µ the inverse map of α 7→ U α(u) dν is denoted u 7→ µu . It is called the
disintegration of µ with respect to p : V → U .
R
Proof. We first claim that the map α 7→ U α(u) dν is continuous, and then we
argue to show that it is invertible, and its inverse is continuous as well.
For the continuity,
given a converging
sequence αn → α in L0p• (U, ProbU (V ))
R
R
with µn = U αn (u) dν, µ = U α(u) dν, it is enough to show that every subsequence of µn has a subsequence that converges to µ. Since every sequence
that converges in measure has a subsequence that converges almost everywhere,
abusing our notation and denoting again αn and µn for the resulting sub-subsequences, we may assume that αn converges to α ν-almost everywhere. Picking
bounded function f on V , we obtain that for ν-a.e u ∈ U ,
Ran arbitrary continuous
R
dα
(u)f
→
dα(u)f
. Thus by Lebesgue bounded convergence theorem we
n
V
V
get
Z
Z
Z
Z
Z
Z
dµn f =
dν
dαn (u)f →
dν
dα(u)f =
dµf.
V

U

V

U

V

V

This shows that indeed µn → µ.
R
We now argue that the map α 7→ U α(u) dν is invertible and its inverse is
continuous. Without loss of generality, we can assume that p is onto. Hence U
is second countable. Since it is also T0 , it follows that U is countably separated.
By [Zim84, Proposition A.1], there exists a Borel embedding φ : U → [0, 1]. We
consider [0, 1] with the measure
φ∗ ν. Precomposition by φ gives a homeomorphism

L0(φ◦p)∗ [0, 1], Prob[0,1] (V ) → L0p• (U, ProbU (V )). Thus, in what follows we may
and do assume that U ⊂ [0, 1].2 Under this assumption [Sim12, Theorem 2.1]
guarantees that the map L0p• (U, ProbU (V )) → P is invertible. We denote the
preimage of µ ∈ P by u 7→ µu . We are left to show that this association is
continuous. To this end we embed V in a compact metric space V 0 and extend
p by setting p(v 0 ) = 1 for v 0 ∈ V 0 − V . Then [Sim12, Theorem 2.2] proves that
for almost every u ∈ U , µu is obtained as the weak*-limit of the normalized
restrictions, denoted by µu,η , of µ on p−1 (u − η, u + η) as η → 0.
Assume that µn → µ is a converging sequence in P . We know that for ν-a.e.
u, d(µu,η , µu ) → 0 when η → 0 and similarly for all n ∈ N, d(µnu,η , µnu ) → 0 when
η → 0. Fix ε > 0. For n ∈ N, we set
An = {u ∈ U ∃η0 > 0 ∀k ≥ n ∀η ∈ (0, η0 ); d(µku,η , µu ) ≤ ε }.
Then ν(∪An ) = 1 and An ⊆ An+1 . Thus there is n such that ν(An ) ≥ 1 − ε and
for u ∈ An , d(µu , µku ) ≤ ε for all k ≥ n. This shows that the image sequence of
(µn ) in L0p• (U, ProbU (V )) indeed converges to the image of µ.
2 Since the embedding U → [0, 1] is only Borel, when we assume U ⊂ [0, 1], the fibration
V → U cannot be assumed to be Polish anymore. Since our argument does not depend on the
topology of U , this does not matter here.

11

We note that if G acts on the fibration V → U (that is, G acts on U and V and
p is equivariant) then the disintegration homeomorphism is also equivariant with
respect to the natural action of G on L0p (U, V ) given by (gf )(u) = g(f (g −1 u)).
Lemma 2.13. Let p : V → U be a Polish fibration with an action of G such
that the G-action on U is trivial. Let ν be a probability measure on U , and let
f ∈ L0p (U, V ). Then there exists U1 ⊂ U of full measure such that
\

Stab(f ) =

Stab(f (u)).

u∈U1

Proof. Let L be the stabilizer of f in G. If L0 is a countable dense subgroup of L,
T
then there is a full measure subset U1 ⊂ U such that L0 ⊂ u∈U1 Stab(f (u)) (for
any g ∈ L0 , there is such a subspace Ug . Choose U1 to be the intersection over
L0 ). T
Since all these stabilizers are closed, and L0 is dense in L, we actually have
L ⊂ u∈U1 Stab(f (u)). Since the reverse inclusion is clear, we conclude that
L=

\

Stab(f (u)).

u∈U1

Corollary 2.14. Assume G acts continuously on the Polish space V and the
quotient topology on G\V is T0 . Let L < G be a closed subgroup and µ be an
L-invariant probability measure on V . Then there exist a point v ∈ V and an
L-invariant probability measure on G · v ' G/ Stab(v).
Proof. Let ν be the pushforward measure of µ on U . By Theorem 2.11, we may
consider the disintegration of µ as an element (µu ) ∈ L0p• (U, ProbU (V )) and this
element is clearly L-invariant. By Lemma 2.13, the stabilizer of (µu ) is an intersection of stabilizers of the measures µu , for u in a subset of U . In particular L
stabilizes some µu , which is a measure supported on an orbit G · v. The latter
is equivariantly homeomorphic to G/ Stab(v) thanks to Effros theorem (Theorem 2.1).

3

Almost algebraic groups and actions

The goal of this section is the proof of Theorem 1.7. Starting with an almost
algebraic action of G on a Polish V , we aim to prove that the action G y Prob(V )
is algebraic as well. So we have to prove that stabilizers of probability measures
on V are almost algebraic and the quotient G\ Prob(V ) is T0 . Going toward wider
and wider generality, we prove the first point in §3.2 and the second one in §3.3

3.1

Almost algebraic groups

Recall that by our setup 1.1, (k, | · |) is a fixed complete and separable valued field
and G is a fixed k-algebraic group. By Proposition 2.2, G = G(k) has the structure

12

of a Polish group. Recall that a closed subgroup L < G is called almost algebraic
if there exists a k-algebraic subgroup H < G such that L contains H = H(k) as
a normal cocompact subgroup (Definition 1.4).
Lemma 3.1. An arbitrary intersection of almost algebraic subgroups is again
almost algebraic.
More precisely, let (Li )i∈I be a collection of almost algebraic subgroups and Hi
algebraic subgroups such that Hi = Hi (k) is normal and cocompact in Li .
Then one can find a finite subset I0 such that, defining H = ∩i∈I0 Hi , we have
that H = ∩i∈I Hi and H(k) is normal and cocompact in ∩i∈I Li .
Proof. Let L = ∩Li and H = ∩Hi which coincides with (∩i∈I Hi )(k). Then it is
straighforward to check that H C L. Thanks to the Noetherian property of G,
there exists a finite subset I0 ⊂ I such that ∩i Hi coincides with ∩i∈I0 Hi .
Let L be theQZariski closure of L and Li the one of Li . The diagonal image of L(k) in i∈I0 Li (k)/Hi is locally closed by Proposition 2.2 and it is a
group. Thus it is actually closed. Moreover it is homeomorphic to L(k)/H.
To conclude, it suffices to
 observe that L/H is closed in L(k)/H and lies in
T Q
(L(k)/H)
i∈I0 Li /Hi which is compact.
Remark 3.2. Actually the proof of this lemma shows that any almost algebraic
subgroup L has a minimal subgroup among all cocompact normal subgroups N
which can be written N = N(k) for some algebraic subgroup N ≤ G. This group
is actually the intersection of all such subgroups and it is invariant under the
normalizer NG (L) of L in G.
Lemma 3.3. Let H, L be closed subgroups of G such that H is almost algebraic,
H C L and L/H is compact. Then L is almost algebraic.
Proof. There is a algebraic subgroup N of G such that N = N(k) is normal
and cocompact in H. Moreover thanks to Remark 3.2, N may be choosen to be
invariant under NG (H) and thus N is cocompact and normal in L.

3.2

Almost algebraicity of stabilizers of probability measures

Let V be a Polish space endowed with a continuous G-action. Recall that the
action G y V is called almost algebraic if the stabilizers are almost algebraic
subgroups of G and the quotient topology on G\V is T0 (Definition 1.4).
Remark 3.4. For a continuous action of G on a Polish space V , the action is
almost algebraic if and only if the stabilizers are almost algebraic and for every
v ∈ V and any sequence gn ∈ G, gn v → v implies gn → e in G/ StabG (v).
This equivalent definition is much easier to check, and we will allow ourselves to
use it freely in the sequel. The two definitions are indeed equivalent by Effros’
Theorem 2.1.
Example 3.5. Let I be a k-algebraic group and φ : G → I a k-morphism. Let L
be an almost algebraic group in I = I(k). Then the action of G on I/L is almost
algebraic. This fact is proved after Lemma 3.7.
13

Lemma 3.6. Let K be a compact group acting continuously on a T0 -space X.
Then the orbit space K\X is T0 as well.
Proof. Continuity of the action means that the action map K ×X → K ×X which
associates (k, kx) to (k, x) is a homeomorphism. Compactness of K implies that
the projection (k, x) 7→ x from K × X to X is closed. Composing the two yields
closedness of the map (k, x) 7→ kx. This implies that if F ⊂ X is closed, then KF
is again closed.
Let x, y ∈ X in different K-orbits. Let us consider Y = Kx ∪ Ky with the
induced topology. This is a compact T0 -space. Now, consider the set of closed
non-empty subspaces of Y with the order given by inclusion. By compactness any
decreasing chain has a non-empty intersection and thus Zorn’s Lemma implies
there are minimal elements, that are points since Y is T0 . Thus Y has at least a
closed point.
Without loss of generality we may and shall assume that {x} is closed in Y .
This means that there exists a closed subset F of X such that F ∩ Y = {x}. In
particular F ∩ Ky = ∅, and therefore Ky ∩ KF = ∅. Finally, KF is a closed
K-invariant set separating Kx from Ky.
Lemma 3.7. Let J be a topological group acting continuously on a topological space
X. If N is a closed normal subgroup of J, the induced action of J/N on N \X is
continuous and the orbits spaces J\X and (J/N )\(N \X) are homeomorphic.
Proof. The map (g, x) 7→ N gx from J ×X to N \X is continuous and goes through
the quotient space J/N ×N \X which is the orbit space of N ×N acting diagonally
on J × X. Thus, (gN, N x) 7→ N gx is continuous, that is the action of J/N on
N \X is continuous.
By the universal property of the topological quotient, the continuous map
x 7→ (J/N )N x from X to (J/N )\(N \X) induces a continuous map J\X →
(J/N )\(N \X). Conversely, the continuous map N \X → J\X induces also a continuous map (J/N )\(N \X) → J\X which is the inverse of the previous one.
Proof of Example 3.5. Since φ−1 (L) and its conjugates are almost algebraic in G,
it is clear that the stabilizers are almost algebraic. So we are left to prove that
the topology on G\I/L is T0 . Let H be a cocompact normal subgroup in L with
H = H(k) for some k-algebraic subgroup H of I. By Lemma 3.7 the orbit space
G\I/L is homeomorphic to the space of orbits of the action of G × (L/H) on I/H.
Note that the action of G on I/H ⊂ I/H(k) has locally closed orbits (and therefore
G\I/H is T0 ) by Proposition 2.2, as the action of G on I/H is k-algebraic. Now
the T0 property of G\I/L follows from Lemma 3.6 for the compact group L/H
acting continuously on the T0 -space G\I/H.
Lemma 3.8. Let J be a countable set, (Li )i∈J a family of almost algebraic subQ
groups of G. Then the diagonal action of G on i∈J G/Li is almost algebraic.
Q
Proof. Stabilizers of points in i∈J G/Li are intersections of almost algebraic
subgroups of G. Hence
by Lemma 3.1 they are almost algebraic. So we just have
Q
to prove that G\ ( i∈J G/Li ) is T0 .
14

For i ∈ J, let Hi be an algebraic subgroup of GQsuch that Hi = Hi (k) is
a cocompact normal subgroup of Li . Consider V = i∈J G/Hi . We first prove
that the topology on G\V is T0 , by proving that orbit maps are homeomorphisms
(Theorem 2.1). Let (hi Hi )i∈J be an element of V and (gn ) be a sequence of
elements of G such that gn · (hi Hi ) converges to (hi Hi ) in V .
T
Let H = i∈J hi Hi h−1
= Stab((hi Hi )i∈J ). We have to prove that gn converges
i
to e in G/HT(see Remark 3.4). By Noetherianity,
there exists a finite J0 ⊂ J such
Q
that H = i∈J0 hi Hi h−1
.
Set
V
=
G/H
. We see that, in V0 , we have
0
i
i∈J0
i
that gn .(hi Hi )i∈J0 converges to (hi Hi )i∈J0 . By Proposition 2.2, it follows that gn
converges to the identity in G/H.
Q
Now let K be the compact group i∈J Li /Hi . The group K acts also continuously on V via the formula (li Hi ) · (gi Hi ) = (gi li−1 Hi ) and this action commutes
with the action of G. Thus we can apply Lemma 3.6 to KQacting on G\V and
get that the space of orbits for the G-action on V /K ' i∈J Gi /Li is T0 , as
desired.
Our main goal in this subsection is proving the following theorem, which is an
essential part of our main theorem, Theorem 1.7.
Theorem 3.9. Let V be a Polish space with an almost algebraic action of G.
Then stabilizers of probability measures on V are almost algebraic subgroups of G.
We first restate and prove Proposition 1.9, discussed in the introduction.
Proposition 3.10. Fix a closed subgroup L < G. Then there exists a k-subgroup
H0 < G which is normalized by L such that L has a precompact image in the Polish
group (NG (H0 )/H0 )(k) and such that for every k-G-variety V, any L-invariant
finite measure on V(k) is supported on the subvariety of H0 -fixed points.
Proof. Replacing G by the Zariski closure of L, we assume that L is Zariski-dense
in G and consider the collection
{H < G | H is a k-algebraic subgroup, Prob(G/H(k))L 6= ∅}.
By the Noetherian property of G there exists a minimal element H0 in this collection. We let µ0 be a corresponding L-invariant measure on G/H0 (k).
We first claim that H0 is normal in G. Assuming not, we let N G be the
normalizer of H0 and consider the set
U = {(xH0 , yH0 ) | y −1 x ∈
/ N} ⊂ G/H0 × G/H0 .
This set is a non-empty Zariski-open set which is invariant under the diagonal Gaction, as its complement is the preimage of the diagonal under the natural map
G/H0 × G/H0 → G/N × G/N. Since the support of µ0 × µ0 in G/H0 × G/H0 is
invariant under L×L which is Zariski-dense in G×G we get that (µ0 ×µ0 )(U(k)) 6=
0. It follows from Corollary 2.14 that there exist u ∈ U(k) and an L-invariant
finite measure on G/ StabG (u) ⊂ (G/ StabG (u))(k). By the definition of U we
get a contradiction to the minimality of H0 , as point stabilizers in U are properly
contained in conjugates of H0 . This proves that H0 is normal in G.
15

Next we let V be a k-G-variety and µ be an L-invariant measure on V(k). We
argue to show that µ is supported on VH0 ∩ V(k). Indeed, assume not. Let V0
be the Zariski-closure of V(k) ∩ VH0 , and V00 = V − V0 . Then we see that V0
is defined over k [Bor91, AG, 14.4]. Furthermore, H0 acts on V0 trivially, so that
we have V0 (k) = V(k) ∩ VH0 . Hence by assumption we get that µ(V00 (k)) > 0.
Replacing V by V00 and restricting and normalizing the measure, we may and
shall assume that VH0 ∩ V(k) = ∅.
We consider the variety G/H0 × V as a k-G-variety. The measure µ0 × µ is
an L-invariant measure on (G/H0 × V)(k). It follows from Corollary 2.14 that
there exists u ∈ (G/H0 × V)(k) and an L-invariant measure on G/ StabG (u). By
Proposition 2.2 there exist a k-algebraic subgroup H < G with H = H(k) =
StabG (u) and an orbit map G/H → Gu inducing a homeormorphism G/H →
G/ StabG (u). Thus we obtain an L-invariant probability measure on G/H(k).
Now, H is contained in some conjugate gH0 g −1 , for some g ∈ G. Hence we get
that g −1 Hg < H0 is such that G/g −1 Hg has an L-invariant probability measure.
By minimality, this implies that g −1 Hg = H0 , hence by normality of H0 , H = H0 .
Therefore u belongs to V(k) ∩ VH0 , which was assumed to be empty. Hence we
get a contradiction. This proves that µ is supported on VH0 .
We set S = (G/H0 )(k) and let T be the closure of the image of L in S.
We are left to show that T is compact. S is a Polish group and T is a closed
subgroup. The quotient topology on T \S is Hausdorff, and in particular T0 . The
measure µ0 is an L-invariant finite measure on S, hence it is also T -invariant.
Substituting S = V and T = G = L in Corollary 2.14 we find a finite measure µ1
on S which is supported on a unique T -coset, T s. The measure (Rs )∗ µ1 , given
by pushing µ1 by the right translation by s−1 is then a T -invariant probability
measure on T . It is well-known result due to A. Weil (see [Oxt46] where the result
is attributed to Ulam) that a Polish group that admits an invariant measure class is
locally compact, and a locally compact group that admits an invariant probability
measure is compact. Thus T is indeed compact.
Corollary 3.11. Fix a k-G-algebraic variety V, and set V = V(k). Let µ ∈
Prob(V ). Then Stab(µ) is almost algebraic.
Proof. Let L = Stab(µ). We may and shall assume L to be Zariski-dense in G,
and we can find H0 as in Proposition 1.9. We know that µ is supported on the
set of VH0 thus H0 = H0 (k) < L. Since G/H0 is acting on VH0 ∩ V(k) and the
stabilizer of µ is closed in G/H0 , we conclude that L has a closed image. We know
that the image of L is precompact, thus it is actually compact, and we conclude
that L is almost algebraic.
Lemma 3.12. Let L < G be an almost algebraic group, with H = H(k) a normal
cocompact algebraic subgroup of L. Then there is a G-equivariant continuous map
φ : Prob(G/L) → Prob(G/H). Furthermore, we have, for every µ ∈ Prob(G/L),
Stab(µ) = Stab(φ(µ)).
Proof. Let λ be a Haar probability measure on L/H. For a continuous bounded
function Rf on G/H let f be the continuous bounded function on G/L defined by
f (gL) = L/H f (gh) dλ(h) and finally φ(µ)(f ) = µ(f ).
16

Then it is clear that φ is equivariant, and we deduce that Stab(µ) ⊂ Stab(φ(µ)).
In the other direction, we note that if π : G/H → G/L is the projection, we have
π∗ (φ(µ)) = µ. Hence the other inclusion is also clear.
To check the continuity, let µn → µ ∈ Prob(G/L), and take f a continuous
bounded function on G/H. Then φ(µn )(f ) = µn (f ) → µ(f ) = φ(µ)(f ). Hence
φ(µn ) converges to φ(µ).
Proof of Theorem 3.9. Choose µ ∈ Prob(V ) and denote L = StabG (µ), H =
FixG (supp(µ)). Set U = G\V , and let ν = p• µ, where p : V → U is the projection.
Note that p is a Polish fibration. By Theorem 2.11, L is equal to the stabilizer of
an element f ∈ L0p• (U, ProbU (V )). By Lemma 2.13 there exists a ν-full measure
T
set U1 ⊂ U such that L = u∈U1 Stab(f (u)). For a fixed u ∈ U1 , f (u) is a
measure on a G-orbit in V which we identify with G/L0 for some almost algebraic
subgroup L0 < G. Let H0 < G be a k-algebraic subgroup such that H 0 = H0 (k)
is a cocompact normal subgroup of L0 . By Lemma 3.12, Stab(f (u)) is also the
stabilizer of a probability measure on G/H 0 ⊂ G/H0 (k). By Corollary 3.11, it
follows that Stab(f (u)) is almost algebraic. We conclude that L is almost algebraic
by Lemma 3.1.

3.3

Separating orbits in the space of probability measures

In this subsection, we prove the following theorem.
Theorem 3.13. Let L < G be an almost algebraic subgroup. Then the action of
G on Prob(G/L) is almost algebraic.
The proof of Theorem 3.13 consists in several steps, proving particular cases
of the theorem, each of them using the previous one. First we start with the case
when L = G (Lemma 3.14). Then we treat the case when L is a normal algebraic
subgroup of G (Lemma 3.15). The main step is then to deduce the theorem when
L is any algebraic subgroup of G (Proposition 3.20), before concluding with the
general case.
Lemma 3.14. The G-action on Prob(G) is almost algebraic.
Proof. The regular action of G on itself is proper, so by Lemma 2.7 it follows that
the action of G on Prob(G) is proper. Any proper action is almost algebraic.
Lemma 3.15. Let H < G be a normal k-algebraic subgroup. Then the G-action
on Prob((G/H)(k)) is almost algebraic.
Proof. Denoting I = G/H and I = I(k), we know that the I-action on Prob(I)
is almost algebraic (Lemma 3.14). Since G/H is a subgroup of I, G stabilizes
each I-orbit. It is thus enough to show that G acts almost algebraically on each
I-orbit. We know that such an orbit is of the form I/L where L is almost algebraic
(Theorem 3.9), so this follows from Example 3.5.
An essential technical tool for proving Theorem 3.13 and Theorem 1.7 is given
by the following proposition.
17

Proposition 3.16. Let V be a Polish space, with a continuous action of G. Assume that
• The quotient topology on G\V is T0 , and
• For any v ∈ V , the action of G on Prob(G.v) is almost algebraic.
Then the quotient topology on G\ Prob(V ) is T0 .
The proposition will directly follow from the following lemma.
Lemma 3.17. Let p : V → U be a Polish fibration with an action of G, and let
ν be a probability measure on U . Assume that the action of G on U is trivial and
that the action of G on Prob(p−1 (u)) is almost algebraic for almost every u ∈ U .
Let P = {µ ∈ Prob(V ) | p∗ µ = ν}. Then the topology on G\P is T0 .
This proof is similar to the proof presented in [Zim84, Proof of Proposition
3.3.1]; see also [AB94, Lemma 6.7].
Proof. The set P is Polish, as a closed subset of Prob(V ). By Theorem 2.1 we
need to show that the orbit maps are homeomorphisms. By Theorem 2.11, P is
equivariantly homeomorphic to L0p• (U, ProbU (V )).
Fixing f ∈ L0p• (U, ProbU (V )) and letting gn ∈ G be such that gn f → f , we
will show that gn converges to the identity in G/ Stab(f ) by proving that every
subsequence of (gn ) has a sub-subsequence which converges to the identity in
G/ Stab(f ). Doing so, we are free to replace (gn ) by any subsequence. Relying on
Lemma 2.10, we replace (gn ) by a subsequence such that gn f (u) → f (u) for every
u in some ν-full subset U0 ⊂ U . Let U1 ⊂ U0 be a full measure subset such that
the action of G on p−1 (u) is almost algebraic for every u ∈ U1 .
Let u ∈ U1 . By definition, we know that f (u) ∈ Prob(p−1 (u)) and that the
action of G on Prob(p−1 (u)) is almost algebraic. By Proposition 2.2, the orbit
map G/ Stab(f (u)) → Gf (u) is a homeomorphism thus gn f (u) → f (u) implies
that gn converges to the identity in G/ Stab(f (u)). By Lemma 2.13, there is also
a full measure subset U2 , that we may and do assume to be contained in U1 , such
that
\
Stab(f ) =
Stab(f (u))
u∈U2

and since G is second countable, one can find U3 countable in U2 such that
\
Stab(f ) =
Stab(f (u)).
u∈U3

By assumption, for every u ∈ U3 , theQgroup Stab(f (u)) is almost algebraic. Hence
by Lemma 3.8, the action of G on u∈U3 G/ Stab(f (u)) is almost algebraic. In
particular, we see that gn converges to e in G/ Stab(f ).
Proof of Proposition 3.16. Let U = G\V and p : V → U be the projection. Consider the G-invariant continuous map p∗ : Prob(V ) → Prob(U ). Clearly the
fibers of p∗ are closed and G-invariant, so it is enough to prove that for a given
ν ∈ Prob(U ), the quotient space G\p−1
∗ ({ν}) has a T0 -topology. This is precisely
Lemma 3.17.
18

Let π : V → V 0 be a continuous G-map between Polish spaces, µ ∈ Prob(V )
and ν = π∗ µ. Then ν has a unique decomposition ν = νc + νd where
P νc and
Pνd are
the continuous and discrete parts of ν. Moreover νd can be written λ∈Λ λ f ∈Fλ δf ,
where Λ = {λ ∈ R+ | ∃u ∈ V 0 , π∗ µ({u}) = λ} and Fλ = {u ∈ P
V 0 | ν({u}) = λ}.
−1
Defining µλ to be the restriction of µ to π (Fλ ) and µc = µ − λ∈Λ µλ , we have
P
a unique decomposition µ = µc + λ∈Λ µλ , where π∗ (µc ) is non-atomic and each
P
π∗ (µλ ) is a finitely supported, uniform measure of the form λ f ∈Fλ δf .
Lemma 3.18. Let π : V → V 0 be a continuous G-map between Polish spaces
and
T µ ∈ Prob(V ). Using the above decomposition, we have Stab(µ) = Stab(µc ) ∩
( λ Stab(µλ )). If gn µ → µ then gn µc → µc and for each λ ∈ Λ, gn µλ → µλ .
Proof. The statement about Stab(µ) is straightforward from the uniqueness of the
decomposition of µ. Let (gn ) be a sequence such that gn µ → µ. Once again, we
use a sub-subsequence argument: we prove that any subsequence of (gn ) contains
a sub-subsequence such that gn µλ → µλ for every λ. Hence we start by replacing
(gn ) by an arbitrary subsequence.
Observe that gn µ → µ implies gn ν → ν because π∗ : Prob(V ) → Prob(V 0 ) is
continuous. Let K 0 be a compact metrizable space in which V 0 is continuously
embedded as a Gδ -subset (see [Kec95, Theorem 4.14]). Then Prob(V 0 ) embeds as a
Gδ -subset in Prob(K 0 ) as well [Kec95, Proof of Theorem 17.23]. We begin with the
following observation. Assume νn is a sequence of probability measures converging
to ν ∈ Prob(V 0 ) and νn decomposes as δun + νn0 with un ∈ V 0 and νn0 ∈ Prob(V 0 ).
Up to extraction un converges to some k ∈ K 0 and thus ν({k}) > 0 which implies
that k ∈ V 0 .
Let λ1 be the maximum of Λ. The above observation implies that up to extraction we may assume that for any f ∈ Fλ1 , gn f converges to some l(f ) ∈ V 0 .
Since gn ν → ν, we have that l(f ) ∈ Fλ1 thus gn νλ1 converges to νλ1 , where
νλ1 = π∗ (µλ1 ). An induction on Λ (countable and well ordered with the reverse
order of R) shows that (after extraction) gn νλ → νλ for any λ ∈ Λ.
Once again, we embed V in some compact metrizable space K. Fix λ ∈ Λ
and let µ0 be an adherent point of (gn µλ ) in Prob(K). As π is G-equivariant, we
have that π∗ gn µλ = gn π∗ µλ = gn νλ which converges to νλ . Hence π∗ µ0 = νλ .
Furthermore, we also see that µ0 is supported on π −1 (Fλ ), hence µ0 ∈ Prob(V ).
The same argument proves that µ−µ0 , which is an adherent point of gn (µ−µλ ),
is supported on V \ π −1 (Fλ ).
As µ can be written uniquely as a sum of a measure supported on π −1 (Fλ ) and a
measure supported on V \π −1 (Fλ ), we see, writing µ = (µ−µ0 )+µ0 =
P(µ−µλ )+µλ ,
that necessarily µ0 = µλ . This concludes the proof since µc = µ − λ∈Λ µλ .
Lemma 3.19. Let H < G be a k-algebraic subgroup. Set N = NG (H), H = H(k)
and N = N(k). Let V = G/H, V0 = G/N, V = V(k) and V 0 = V0 (k).
P Consider
the map π : V → V 0 . Let F ⊂ V 0 be a finite set, ν = 1/|F | f ∈F δf and
µ ∈ Prob(V ) be a measure with π∗ µ = ν. Let (gi ) be a sequence with gi µ → µ.
Then gi → e ∈ G/ Stab(µ).
Proof. Denote m = |F |. We know that (V0 )m / Sym(m) is an algebraic variety,
hence by Proposition 2.2, every G-orbit in (V 0 )m / Sym(m) is locally closed. It
19

follows in particular that gi → e in G/ Stab(F ).
Again, it is enough to show that every subsequence of (gi ) contains a subsequence which tends to e modulo Stab(µ). We start by extracting an arbitrary
subsequence of (gi ).
Let us number f1 , f2 , . . . , fm the elements of F and denote F 0 = (f1 , . . . , fm ) ∈
0 m
(V ) . Since gi converges to e in G/ Stab(F ), it follows that, passing to a subsequence, there exists σ ∈ Sym(m) such that gi F 0 tends to σ(F 0 ) = (fσ(1) , . . . , fσ(m) ).
This means GF 0 ⊃ Gσ(F 0 ) and thus GF 0 ⊃ Gσ(F 0 ) ⊃ · · · ⊃ Gσ n (F 0 ) = GF 0 for
some n ∈ N. In particular GF 0 = Gσ(F 0 ) and since orbits are locally closed we
have that GF 0 = Gσ(F 0 ).
This shows that there exists g ∈ Stab(F ) such that gF 0 = σ(F 0 ). Hence we
have gi F 0 → gF 0 , and by almost algebraicity of the action on (V 0 )m it follows that
T
gi tends to g modulo Stab(F 0 ) = f ∈F Stab(f ).
Let us fix some notations. For f ∈ F we denote by µf the restriction of
µ to π −1 ({f }) and fix f ∈ G such that f N = f and denote by Hf ≤ G the
−1
conjugate of H by f . Observe that f Nf
= StabG (f ), Hf C StabG (f ) and
−1
π ({f }) ' StabG (f )/Hf where π : G/H → G/N is the projection and StabG (f )
is the stabilizer of f under the action of G on G/N. We also denote µ0f = g −1 µσ(f )
T
T
and gi0 = g −1 gi . Since gi0 → e ∈ G/ f ∈F Stab(f ) there exist ni ∈ f ∈F Stab(f )
such that gi0 n−1
converges to e (in G). We observe that ni µf = ni (gi0 )−1 gi0 µf . As
i
0
0
gi µf tends to µf and ni (gi0 )−1 tends to e, we have that ni µf converges to µ0f .
Those measures are supported on π −1 ({f }) ' (StabG (f )/Hf ) (k). By Lemma
3.15, Stab(f ) acts almost algebraically on Prob ((StabG (f )/Hf ) (k)). So we have
that ni tends to some n in Stab(f )/ Stab(µf ).
We conclude that gi0 = gi0 n−1
in G/ Stab(µf ). Arguing similarly for
i ni tends to nT
every f , it follows that gi T
tends to gn in G/ f ∈F Stab(µf ). Hence (gi ) converges
also in G/ Stab(µ), since f ∈F Stab(µf ) ≤ Stab(µ). Let h be the limit point of
(gi ) modulo Stab(µ). Then we have that gi µ converges to hµ by continuity of the
action. Hence h ∈ Stab(µ), meaning that h = e modulo Stab(µ). In other words,
gi converges to e in G/ Stab(µ).
Proposition 3.20. Let H < G be a k-algebraic subgroup and set H = H(k).
Then the action of G on Prob(G/H) is almost algebraic.
Proof. Assume the proposition fails for an algebraic subgroup H. We also assume,
as we may, that H is minimal in the collection of k-subgroup of G with the property
that the G-action on Prob(G/H) is not almost algebraic. By Theorem 3.9, G acts
on Prob(G/H) with almost algebraic stabilizers. Hence we have to show that for
every measure µ ∈ Prob(G/H) and sequence gn with gn µ → µ then gn tends to e
in G/ Stab(µ) (Remark 3.4). We fix such a measure µ and a sequence gn . We will
achieve a contradiction by showing that gn does tend to e in G/ Stab(µ).
We set N = NG (H), N = N(k), V = G/H, V0 = G/N, V = V(k) and V 0 =
0
V (k). We consider the natural inclusion G/H ⊂ V and view µ as a measure on V .
We consider the projection map π : V → V 0 and set ν = π∗ µ. We use the notation
introduced T
in the discussion before Lemma 3.18. The lemma gives: Stab(µ) =
Stab(µc )∩( λ∈Λ Stab(µλ )) where Λ is countable subset of [0, 1], gn µc → µc and for
20

each λ ∈ Λ, gn µλ → µλ . By Lemma 3.19, for each λ ∈ Λ, gi → e ∈ G/ Stab(µλ ).
Assume given also that gn → e ∈ G/ Stab(µc ). Since by Theorem 3.9 the groups
Stab(µλ ) and Stab(µc ) are almost
Q algebraic, we will get by Lemma 3.8 that the
action of G on G/ Stab(µc ) × λ G/ Stab(µλ ) is almost algebraic. Hence,
,
!!
\
Stab(µλ )
gn → e ∈ G
Stab(µc ) ∩
= G/ Stab(µ),
λ

achieving our desired contradiction. We are thus left to show that indeed gn →
e ∈ G/ Stab(µc ).
For the rest of the proof we will assume as we may µ = µc , that is ν ∈ Prob(V 0 )
is atom-free. We consider the measure µ × µ ∈ Prob(V × V ) and the subset
U = {(xH, yH) | y −1 x ∈
/ N} ⊂ G/H × G/H = V × V
defined and discussed in the proof of Proposition 1.9. We set U = U(k). Note
that the diagonal in V 0 × V 0 is ν × ν-null as ν is atom-free, thus U is µ × µ-full.
We view as we may µ × µ as a probability measure on U .
We now consider the G-action on U and claim that the G-orbits are locally
closed and for every u ∈ U , G acts almost algebraically on Prob(Gu). The fact that
the G-orbits are locally closed follows from Proposition 2.2, as U is a k-subvariety
of V. Fix now a point u = (xH, yH) ∈ U for some x, y ∈ G, and consider
−1
the G-action on Prob(Gu). By the definition of U, H ∩ Hy x H, thus by the
−1
minimality of H the G-action on Prob(G/H∩H y x ) ' Prob(G/H x ∩H y ) is almost
algebraic. Since by Proposition 2.2 G/H x ∩ H y is equivariantly homeomorphic to
Gu we conclude that indeed, G acts almost algebraically on Prob(Gu), and the
claim is proved.
By Proposition 3.16, we conclude that G acts on Prob(U ) almost algebraically.
Hence Effros’ Theorem 2.1 implies that gn → e in G/ Stab(µ × µ) as gn (µ × µ) →
µ × µ. Observing that Stab(µ × µ) = Stab(µ), the proof is complete.
Proof of Theorem 3.13. By Theorem 3.9 we know that the point stabilizers in
Prob(G/L) are almost algebraic. We are left to show that for every µ ∈ Prob(G/L),
for every sequence gn ∈ G satisfying gn µ → µ we have gn → e modulo Stab(µ)
(see Remark 3.4). Fix µ ∈ Prob(G/L) and a sequence gn ∈ G satisfying gn µ → µ.
Let H < G be a k-algebraic subgroup with H = H(k) normal and cocompact in L, and recall that by Lemma 3.12 we can find a G-equivariant continous map φ : Prob(G/L) → Prob(G/H) such that Stab(µ) = Stab(φ(µ)). We
get that gn φ(µ) → φ(µ). By Proposition 3.20, the G-action on Prob(G/H) is
almost algebraic, thus gn → e modulo Stab(φ(µ)). This finishes the proof, as
Stab(µ) = Stab(φ(µ)).

3.4

Proof of Theorem 1.7

For the convenience of the reader we restate Theorem 1.7.
Theorem 3.21. If the action of G on V is almost algebraic then the action of G
on Prob(V ) is almost algebraic as well.
21

Proof. By Theorem 3.9, we know that the G-stabilizers in Prob(V ) are almost
algebraic. We need to show that the quotient topology on G\ Prob(V ) is T0 .
By Proposition 3.16, it is enough to check that the quotient topology on G\V is
T0 , which is guaranteed by the assumption that the action of G on V is almost
algebraic, and, as we will see, that for any v ∈ V , the action of G on Prob(Gv) is
almost algebraic. We note that by Effros theorem (Theorem 2.1), the orbit Gv is
equivariantly homeomorphic to the coset space G/ StabG (v), and thus Prob(Gv) '
Prob(G/ StabG (v)). Since StabG (v) is an almost algebraic subgroup of G, the fact
that the G-action on Prob(Gv) is almost algebraic now follows from Theorem 3.13.

4

On bounded subgroups

In this section, we essentially retain the setup 1.1 & 1.3: we fix a complete (k, | · |)
valued field and a k-algebraic group G. Nevertheless there is no need for us to
assume that (k, | · |) is separable, so we will refrain from doing so.
Definition 4.1. A subset of k is called bounded if its image under | · | is bounded
in R. For a k-variety V, a subset of V(k) is called bounded if its image by any
regular function is bounded in k.
Remark 4.2. Note that the collection of bounded sets on a k-variety forms a
bornology.
Remark 4.3. For a k-variety V it is clear that a subset of V(k) is bounded if and
only if its intersection with every k-affine open set is bounded, so in what follows
we will lose nothing by considering exclusively k-affine varieties. We will do so.
Remark 4.4. Note that if (k 0 , | · |0 ) is a field extension of k endowed with an
absolute value extension of | · | and V is a k-variety, we may regard V(k) as a
subset V(k 0 ) and, as one easily checks, a subset of V(k) is k-bounded if and only
if it is k 0 -bounded. Thus it causes no loss of generality assuming k is algebraically
closed since b
k is so. Nevertheless, we will not assume that.
It is clear that every k-regular morphism of k-varieties is a bounded map in
the sense that the image of a bounded set is bounded. For a k-closed immersion
of k-varieties f : U → V also the converse is true: a subset of U(k) is bounded
if and only if its image is bounded, as f ∗ : k[V] → k[U] is surjective. This is a
special case of the following lemma.
Lemma 4.5. For a finite k-morphism f : U → V a subset of U(k) is bounded if
and only if its image is bounded.
Proof. Assume there exists an unbounded set L in U(k) with f (L) being bounded
in V(k). Then we could find p ∈ k[U] and a sequence un ∈ L with |p(un )| → ∞.
The function p is integral over f ∗ k[V] so there exist q1 , . . . qm ∈ f ∗ k[V] with
P
m−i
pm + m
= 0. Thus,
i=1 qi p
1=

m
m
X
X
|qi (un )|
qi (un )
≤
→ 0,
i
p (un )
|pi (un )|
i=1
i=1

22

as the sequences qi (un ) are uniformly bounded. This is a contradiction.
Recall that a seminorm on a k-vector space E is a function k · k : E → [0, ∞)
satisfying
1. kαvk = |α|kvk, for α ∈ k, v ∈ E.
2. ku + vk ≤ kuk + kvk, for u, v ∈ E.
A seminorm on E is a norm if furthermore we have
3. kvk = 0 ⇔ v = 0, for v ∈ E.
Two norms on a vector space, k · k, k · k0 , are called equivalent if there exists
some C ≥ 1 such that
C −1 k · k ≤ k · k0 ≤ Ck · k.
It is a general fact that any linear map between two Hausdorff topological
(k, | · |)-vector spaces of finite dimensions is continuous [Bou87, I, §2,3 Corollary
2] and thus we get easily the following.
Theorem 4.6. All the norms on a finite dimensional k-vector space are equivalent.
Proof. It suffices to use that the identity map (E, || · ||) → (E, || · ||0 ) is continuous
and observe that every continuous linear map is bounded. The latter is an easy
exercise in case | · | is trivial, and standard if it is not.
Recall that, if (e1P
, . . . , en ) is a basis, then the norm || · ||∞ (relative to this
basis) is defined as || xi ei ||∞ = max{|xi |}.
Corollary 4.7. For a subset B ⊂ E = k n the following properties are equivalent.
1. B is a bounded set of An .
2. All elements of E ∗ are bounded on B.
3. All the coordinates of the elements of B are uniformly bounded.
4. The norm || · ||∞ is bounded on B.
5. Every norm on E is bounded on B.
6. Some norm on E is bounded on B.
Theorem 4.8. For a subgroup L of GLn (k) the following are equivalent:
1. L is bounded in GLn (k).
2. L is bounded as a subset of Mn (k).
3. L preserves a norm on k n .
4. L preserves a spanning bounded set in k n .
For a subgroup L of G = G(k) the following are equivalent:
23

1. L is bounded.
2. L preserves a norm in all k-linear representations of G.
3. L preserves a norm in some injective k-linear representation of G.
4. L preserves a spanning bounded set in some injective k-linear representation
of G.
Proof. Note that the second part of the theorem follows from the first once we
recall that any injective homomorphism of algebraic groups is a closed immersion.
We prove the equivalence of the first four conditions.
(1) ⇔ (2) : Clearly, if L is bounded in GLn (k) then it is bounded in Mn (k).
Assume L is bounded in Mn (k). Then it has a bounded image under both morphisms
ι
det
GLn →
− GLn ,→ Mn , GLn ,→ Mn −−→ A1 ,
ι

where GLn →
− GLn is the group inversion. We conclude that L has a bounded
image under the product morphism GLn → Mn ×A1 . But the latter morphism
id ⊕ det−1

is the composition of the isomorphism ι and the closed immersion GLn −−−−−−→
Mn ⊕A1 . Thus L is bounded in GLn (k).
(2) ⇔ (3) : If L is bounded in Mn (k) then, by Corollary 4.7(3) all its matrix
elements are uniformly bounded, hence for all v ∈ k n , supg∈L kgvk∞ is finite. This
expression forms an L-invariant norm on k n . On the other hand, if L preserves a
norm on k n , by the equivalence of this norm with k · k∞ , all matrix elements of L
are uniformly bounded, thus it is bounded in Mn (k).
(3) ⇔ (4) : If L preserves a norm then it preserves its unit ball which is a
bounded spanning set. If L preserves a bounded spanning set B than it also
preserves its symmetric convex hull:
( n
)
n
X
X
αi vi vi ∈ B, αi ∈ k,
|αi | ≤ 1 .
i=1

i=1

The latter is easily seen to be the unit ball of an L-invariant norm.
Note that if L is a compact subgroup of G then L is bounded, as the k-regular
functions of G are continuous on G.
Corollary 4.9. Every bounded subgroup of G admits a bi-invariant metric.
Proof. Let L be a bounded subgroup of G. Fix an injective k-linear representation
G → GL(V ) and consider L as a subset of Endk (V ). Endk (V ) is a representation
of G × G, hence admits a norm which is invariant under the bounded group L × L
by Theorem 4.8. This norm gives an L × L-invariant metric on Endk (V ) and on
its subset L.
Proposition 4.10. Assume V is an affine k-variety with a k-affine action of G.
Z
Let B ⊂ V(k) be a bounded set and denote by B its Zariski closure. Then the
Z
Z
image of StabG (B) is bounded in the k-algebraic group StabG (B )/ FixG (B ).
24

Z

Proof. Without loss of generality we may replace G by StabG (B ) and then asZ
Z
Z
sume V = B . We then may further assume G = StabG (B )/ FixG (B ). We do
so. By [Bor91, Proposition 1.12] there exists a k-embedding of V into some vector
space, which we may assume having a spanning image, equivariant with respect
to some k-representation G → GLn , which we thus may assume injective. The
proof then follows from the implication (4) =⇒ (1) in the second of equivalence
of Theorem 4.8.
Corollary 4.11. Let L < G be a bounded subgroup. Then NG (L)/ZG (L) is
bounded.

5

The space of norms and seminorms

In this section we study a compact space on which an algebraic group over a
complete valued field acts by homeomorphisms, the space of seminorms. This
space was already considered in the case when k is local, in [Wer04].
We fix a finite dimensional vector space E over k. Given two norms n, n0 on E
we denote
™
ß
n(y)n0 (x)
x,
y
∈
E
\
{0}
.
d(n, n0 ) = log sup
n0 (y)n(x)
This number is finite by the fact that n and n0 are equivalent norms. Recall
that two seminorms on E are called homothetic if they differ by a multiplicative
positive constant. The relation of being homothetic is an equivalence relation. We
denote the set consisting of all homothety classes of norms on E by I(E). Observe
that d(n, n0 ) only depends on the homothety classes of n and n0 and thus define a
function on I(E).
Lemma 5.1. The function d : I(E) × I(E) → [0, ∞) defines a metric on I(E).
The group PGL(E) acts continuously and isometrically on I(E) and the stabilizers
in PGL(E) of bounded subsets in I(E) are bounded as well.
Proof. The fact that d is a metric and PGL(E) acts by isometries on I(E) is
a straightforward verification. To prove the continuity part, it suffices to show
that the orbit map g 7→ gn is continuous for all n ∈ I(E). Fix a norm n on
E. Let (gi ) be a sequence converging to e in PGL(E). By an abuse of notation we identify gi with an element of GL(E) such that gi → e ∈ GL(E), and
also still
is n. Using that d(gi n, n) =
n denote n a norm whose homothety class o
log sup

n(gi−1 y)n(x)
n(gi−1 x)n(y)

| x, y ∈ E \ {0}, n(x), n(y) < 1 and that gi−1 converges uni-

formly to e on the unit ball of E with respect to n, we see that indeed d(gi n, n) → 0.
Let L be the stabilizer of some bounded set N ⊆ I(E). Fix v 6= 0 and identify
N with a set N 0 of norms on E satisfying n(v) = 1 for every n ∈ N 0 . The set
B = {x ∈ E | ∀n ∈ N 0 , n(x) ≤ 1} is clearly bounded in E. By Theorem 4.8, its
stabilizer L0 ∈ GL(E) is bounded, hence also its image in PGL(E), namely L.
Remark 5.2. The space I(E) actually contains the affine Bruhat-Tits building
I(E) associated to PGL(E) [Par00] and there is a metric d0 on I(E) such that
25

(I(E), d0 ) is CAT(0) —not necessarily complete. The metric d is similar to the
one considered by Goldman and Iwahori in [GI63]. The two metrics d and d0
are Lipschitz-equivalent. This can be checked first on an apartment and extended
to the whole building using that any two points actually lie in some apartment.
Thus, Lemma 5.1 and Theorem 4.8 are a reminiscence of the Bruhat-Tits fixed
point theorem.
Let S 0 (E) be the space of non-zero seminorms on E, and S(E) be its quotient
by homotheties. We endow S 0 (E) with the topology of pointwise convergence and
S(E) with the quotient topology.
Proposition 5.3. The space S(E) is compact and metrizable. The action of
PGL(E) on S(E) is continuous.
Proof. Let m be the dimension of E. Fix a basis (e1 , . . . , em ) of E. Let S1 (E) be
the set of all s ∈ S 0 (E) such that s(ei ) ≤ 1 for every 1 ≤ i ≤ d, and s(ej ) = 1 for
some j.
We first claim that the quotient map S 0 (E) → S(E) restricts to a surjection
S1 (E) → S(E). This follows from the fact that if a seminorm is zero on all the
vectors ei , then it is zero everywhere, by triangle inequality. Furthermore, the
map S1 (E) → S(E) is actually an injection. Indeed if s ∈ S1 (E) and λs ∈ S1 (E)
it is easy to conclude that λ = 1.
We now claim
Indeed, let k · k1 be the norm
P that the
Pspace S1 (E) is compact.
P
defined as k xi ei k1 = i |xi |. Let v =
xi ei . Then we see that s(v) ≤ kvk1
for every v ∈ E. So we get that S1 (E) is homeomorphic to a closed subset of
Q
v∈E [0, kvk1 ]. This proves the compactness of S1 (E) and therefore of S(E).
Note that it also proves that every element of S1 (E) is 1-Lipschitz with respect to
k · k1 .
It follows that S1 (E) is homeomorphic to S(E). The metrizability of S1 (E)
comes from the fact that S1 (E) is a closed subset of the space of continuous
functions on E, which is metrizable because E is separable.
Now, let (gn , sn ) be a sequence converging to (e, s) ∈ GL(E)×S1 (E) then gn sn
tends to s. Indeed, for every v ∈ E,
|sn (gn v)−s(v)| ≤ |sn (gn v)−sn (v)|+|sn (v)−s(v)| ≤ kgn v−vk1 +|sn (v)−s(v)| → 0.

Each non-zero seminorm s has a kernel ker(s) = {v ∈ E | s(v) = 0}, which is a
proper linear subspace of E depending only of the homothety class of s. The map
S(E) → N, s 7→ dim(ker(s)) is obviously PGL(E)-invariant. Denote by Sm (E)
the space of homothety classes of seminorms s such that dim(ker(s)) = m. Note
that S0 (E) = I(E). We denote by Grm (E) the Grassmannian of m-dimensional
linear subspaces of E. The map Sm (E) → Grm (E), s 7→ ker(s) is clearly PGL(E)equivariant. Grm (E) is the k-points of a k-algebraic variety, thus carries a Polish
topology by Proposition 2.2.
Proposition 5.4. The maps S(E) → N, s 7→ dim(ker(s)) and Sm (E) → Grm (E),
s 7→ ker(s) are measurable.
26

Proof. We first note that the space S(E) is covered by (countably many) open sets
which are homeomorphic images of sets of the form {s ∈ S 0 (E) | s(v) = 1}, for
v ∈ E, under the quotient map S 0 (E) → S(E). It is therefore enough to establish
0
that the corresponding maps S 0 (E) → N, Sm
(E) → Grm (E) are measurable
0
(where Sm (E) denotes the preimage of Sm (E)).
Fix a basis for E and a countable dense subfield k0 < k. Let E0 = E(k0 ) be
the k0 -span of the fixed basis of E. A subspace of E is said to be defined over k0
if it has a basis in E0 . E0 is a k0 -vector space and it is a countable dense subset
of E. Note that for every d, Grm (E0 ) is countable. Observe that for s ∈ S 0 (E),
dim(ker(s)) ≤ m if and only if we can find a codimension m subspace F < E which
is defined over k0 , such that s restricts to a norm on F . The latter condition is
equivalent by Theorem 4.6 to the condition that there exists n ∈ N such that for
every v ∈ F , s(v) ≥ |v|/n for some fixed norm | · |. Note that it is enough to check
this for every v ∈ F0 = F (k0 ), thus we obtain
[
[ \
{s ∈ S 0 (E) | dim(ker(s)) ≤ m} =
{s ∈ S 0 (E) | s(v) ≥ |v|/n}.
F0 ∈Grdim(E)−m (E0 ) n v∈F0

This shows that the map s 7→ dim(ker(s)) is measurable.
0
(E) → Grm (E) is measurable, we make two
In order to prove that the map Sm
observations. We first observe that the topologies of pointwise convergence and
uniform convergence give the same Borel structure on S 0 (E). In fact, for every
separable topological space X, the pointwise and uniform convergence topologies
on Cb (X) give the same Borel structure (as uniform balls are easily seen to be
Borel for the pointwise convergence topology), and S 0 (E) could be identified with
a closed (for both topologies) subspace of bounded continuous functions on the
unit ball of E. Our second observation is that we may identify Grm (E) with a
subset of the space of closed subsets of the unit ball of E. Endowing it with
the Hausdorff metric topology, we get a PGL(E)-invariant Polish topology on
Grm (E). Since the Polish group PGL(E) acts transitively on Grm (E), by Effros
Lemma [Eff65, Lemma 2.5] the quotient topology is the unique PGL(E)-invariant
Polish topology on this space, thus the topology on Grm (E) given by the Hausdorff
metric coincides with the one discussed in Proposition 2.2.
The proof is now complete, observing further that with respect to the uniform
0
convergence topology on Sm
(E) and the Hausdorff metric topology on Grm (E),
the map s 7→ ker(s) is in fact continuous (moreover, it is C-Lipschitz on {s ∈
0
Sm
(E) | s is C-Lipschitz}).

6

Existence of algebraic representations

This section is devoted to the proof of Theorem 1.16, which we restate below. The
reader who is unfamiliar with the notion of measurable cocycles and amenable
actions might consult with profit Zimmer’s book [Zim84, Chapter 4]. The following
theorem provides a so-called algebraic representation of the space R, thus allowing
to start the machinery developed in [BF13] and prove cocycle super-rigidity for
the group G.
27

Theorem 6.1. Let R be a locally compact group and Y an ergodic, amenable
Lebesgue R-space. Let (k, | · |) be a valued field. Assume that as a metric space k is
complete and separable. Let G be a simple k-algebraic group. Let f : R×Y → G(k)
be a measurable cocycle.
Then either there exists a k-algebraic subgroup H
G and an f -equivariant
measurable map φ : Y → G/H(k), or there exists a complete and separable metric space V on which G acts by isometries with bounded stabilizers and an f equivariant measurable map φ0 : Y → V .
Furthermore, in case k is a local field the G-action on V is proper and in case
k = R and G is non-compact the first alternative always occurs.
Proof. We first note that the isogeny G → G, where G is the adjoint group
associated to G, is a finite morphism. Thus, by Lemma 4.5 we may assume that
G is an adjoint group. We do so. By [Bor91, Proposition 1.10] we can find a
k-closed immersion from G into some GLn . By the fact that G is simple, we may
assume that this representation is irreducible. By the fact that G is adjoint, the
associated morphism G → PGLn is a closed immersion as well. We will denote for
convenience E = k n . Via this representation, G acts continuously and faithfully
on the metric space of homothety classes of norms, I(E), and on the compact
space of homothety classes of seminorms, S(E), introduced in §5.
By the amenability of the action of R on Y there exists a f -map, that is a
f -equivariant map, φ : Y → Prob(S(E)), which we now fix. By Proposition 5.4,
there is a measurable partition S(E) = ∪n−1
d=0 Sd (E), given by the dimension of
the kernels of the seminorms. For a given d, the function Y → [0, 1] given by
y 7→ φ(y)(Sd (E)) is R-invariant, hence almost everywherePequal to some constant,
by ergodicity. We denote this constant by αd . Note that n−1
d=0 αd = 1. We choose
d such that αd > 0 and define
ψ : Y → Prob(Sd (E)),

ψ(y) =

1
φ(y)|Sd (E) .
αd

Note that ψ is a f -map. We will consider two cases: either d > 0 or d = 0. This is
a first bifurcation leading to the two alternatives in the statement of the theorem.
We first consider the case d > 0. We use the map Sd (E) → Grd (E) discussed
in Proposition 5.4 to obtain the push forward map Prob(Sd (E)) → Prob(Grd (E)).
By post-composition, we obtain a f -map Ψ : Y → Prob(Grd (E)). By Theorem 1.7 the action of G on Prob(Grd (E)) is almost algebraic (as the action of
G on Grd (E) is almost algebraic by Proposition 2.2), and the quotient topology
on G\ Prob(Grd (E)) is T0 . We claim that there exists µ ∈ Prob(Grd (E)) such
that the set Ψ−1 (Gµ) has full measure in Y . The standard argument is similar to the prof of Proposition 1.12: for a countable basis Bi for the topology of
G\ Prob(Grd (E)), the set
\
\
{Bi | Ψ−1 (Bi ) is full in Y } ∩ {Bic | Ψ−1 (Bi ) is null in Y }
is clearly a singleton, whose preimage is of full measure in Y . Let µ be a preimage
of this singleton in Prob(Grd (E)).

28

By the fact that G acts almost algebraically on Prob(Grd (E)), we may identify
Gµ with a coset space G/L, for some almost algebraic subgroup L = StabG (µ) <
G, and view Ψ as an f -map from Y to G/L. By Proposition 1.9, there exists
a k-subgroup H0 < G which is normalized by L such that L has a precompact
image in the Polish group (NG (H0 )/H0 )(k) and such that µ is supported on the
subvariety of H0 fixed points in Grd (E). Note that by the irreducibility of the
representation G → GLn we have no G-fixed points in Grd (E), thus H0 G.
Assume moreover that H0 6= {e} and let H be the Zariski-closure of L. By
[Bor91, Theorem AG14.4], H is a k-subgroup of G. By the simplicity of G, H G,
as H normalizes H0 . Post-composing the f -map Ψ with the map G/L → G/H(k)
we obtain a k-algebraic subgroup H G and an f -equivariant measurable map
φ : Y → G/H(k), as desired.
Assume now H0 = {e}. In that case L is compact, and in particular bounded
in G. It follows by Theorem 4.8 that L fixes a norm on E. Thus we may map
the coset space G/L G-equivariantly into S0 (E) = I(E). Using the δ-measure
embedding I(E) ,→ Prob(I(E)) and obtain a new f -map Y → Prob(I(E)). We
are then reduced to the case d = 0, to be discussed below.
We consider now the case d = 0, that is we assume having an f -map Y →
Prob(I(E)). We set V = Prob(I(E)). By Lemma 5.1, G acts isometrically and
with bounded stabilizers on I(E). By Lemma 2.6, G acts isometrically on V .
Let us check that stabilizers are bounded. Fix µ ∈ Prob(I(E)), and let L be its
stabilizer in G. Since I(E) is Polish there is a ball B of I(E) such that µ(B) > 1/2.
It follows that for any g ∈ L, gB intersects B. Thus the set LB is bounded in I(E),
and by Lemma 5.1 its stabilizer is bounded in G. It follows that L is bounded.
Thus we have found an f -map from Y to a complete and separable metric space
V on which G acts by isometries with bounded stabilizers as desired.

Acknowledgement
U.B was supported in part by the ERC grant 306706. B.D. is supported in part
by Lorraine Region and Lorraine University. B.D & J.L. are supported in part by
ANR grant ANR-14-CE25-0004 GAMME.

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