arXiv:1612.07964v1 [math.QA] 23 Dec 2016

FINITE-DIMENSIONAL POINTED HOPF ALGEBRAS
OVER FINITE SIMPLE GROUPS OF LIE TYPE IV.
UNIPOTENT CLASSES IN CHEVALLEY AND STEINBERG
GROUPS
NICOLÁS ANDRUSKIEWITSCH, GIOVANNA CARNOVALE AND
GASTÓN ANDRÉS GARCÍA
Abstract. We show that all unipotent classes in finite simple Chevalley
or Steinberg groups, different from PSLn (q) and PSp2n (q), collapse
(i.e. are never the support of a finite-dimensional Nichols algebra), with
a possible exception on one class of involutions in PSUn (2m ).

Contents
1. Introduction
2. Preliminaries
3. Unipotent classes in finite groups of Lie type
4. Unipotent classes in Chevalley groups
5. Unipotent classes in Steinberg groups
References

1
4
7
10
22
36

1. Introduction
1.1. The main result and the context. This is the fourth paper of our
series on finite-dimensional complex pointed Hopf algebras whose group of
group-likes is isomorphic to a finite simple group of Lie type G. See Part I
[1] for a comprehensive Introduction. As we explain in loc. cit., the primary
2010 Mathematics Subject Classification: 16T05, 20D06.
Keywords: Nichols algebra; Hopf algebra; rack; finite group of Lie type; conjugacy class.
The work of N. A. was partially supported by CONICET, Secyt (UNC) and the MathAmSud project GR2HOPF. The work of G. C. was partially supported by Progetto di
Ateneo dell’Università di Padova: CPDA125818/12. The work of G. A. G. was partially
supported by CONICET, Secyt (UNLP) and ANPCyT-Foncyt 2014-0507. The results
were obtained during visits of N. A. and G. A. G. to the University of Padova, and of G.
C. to the University of Córdoba, partially supported by the bilateral agreement between
these Universities, the Erasmus Mundus Action 2 Programme AMIDILA and the Visiting
Scientist program of the University of Padova.
1

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

task is to study Nichols algebras over G with support in a conjugacy class
O of G. Actually there are group-theoretical criteria allowing to conclude
that every Nichols algebra with support in a given conjugacy class O has
infinite dimension. These criteria were developed in [4, 1, 3] and are recalled
in §2.1. The verification of any of these criteria in any conjugacy class might
be difficult. Let p be a prime number, m ∈ N, q = pm , Fq the field with q
elements and k := Fq . There are three families of finite simple groups of Lie
type (according to the shape of the Steinberg endomorphism): Chevalley,
Steinberg and Suzuki-Ree groups; see the list in [1, p. 38] and [16, 22.5] for
details. Here are the contents of the previous papers:
⋄ In [1] we dealt with unipotent conjugacy classes in PSLn (q), and as a consequence with the non-semisimple ones (since the centralizers of semisimple elements are products of groups with root system Aℓ ).
⋄ The paper [2] was devoted to unipotent conjugacy classes in PSp2n (q).
⋄ The subject of [3] was the semisimple conjugacy classes in PSLn (q). But
we also introduced the criterium of type C, and applied it to some of the
classes not reached with previous criteria in [1, 2].
In this paper we consider unipotent conjugacy classes in Chevalley and
Steinberg groups, different from PSLn (q) and PSp2n (q). Concretely, these
are the groups in Table 1. Notice that PSU3 (2) is not simple but needed
for recursive arguments.
Table 1. Finite groups considered in this paper; q odd for
PΩ2n+1 (q); q ≥ 3 for G2 (q)
Chevalley
G

Root system

PΩ2n+1 (q)
PΩ+
2n (q)
G2 (q)
F4 (q)
Ej (q)

Bn , n ≥ 3
Dn , n ≥ 4
G2
F4
E6 , E7 , E8

Steinberg
G
Root system
PSUn (q) An−1 , n ≥ 3
PΩ−
Dn , n ≥ 4
2n (q)
3 D (q)
D4
4
2 E (q)
E6
6

As in [4, 2.2], we say that a conjugacy class O of a finite group G collapses
if the Nichols algebra B(O, q) has infinite dimension for every finite faithful
2-cocycle q. Our main result says:
Main Theorem. Let G be as in Table 1. Let O be a non-trivial unipotent
conjugacy class in G. Then either O collapses, or else G = PSUn (q) with
q even and (2, 1, . . . , 1) the partition of O.
In the terminology of §2.1, the classes not collapsing in the Main Theorem
are austere, see Lemma 5.16. This means that the group-theoretical criteria
do not apply for it; however, we ignore whether these classes collapse by

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

3

other reasons. The classes in PSLn (q) or PSp2n (q) not collapsing (by these
methods) are listed in Table 3.
1.2. The scheme of the proof and organization of the paper. Let
G be a finite simple group of Lie type. Then there is q as above, a simple
simply connected algebraic group Gsc defined over Fq and a Steinberg endomorphism F of Gsc such that G = GFsc /Z(GFsc ). We refer to [16, Chapter 21]
for details. Conversely, G = GFsc /Z(GFsc ) is a simple group, out of a short
list of exceptions, see [16, Theorem 24.17]. For our inductive arguments,
it is convenient to denote by G the quotient GFsc /Z(GFsc ) even when it is
not simple. Often there is a simple algebraic group G with a projection
π : Gsc → G such that F descends to G and [GF , GF ]/π(Z(GFsc )) ≃ G.
The proof of the Main Theorem is by application of the criteria of type C,
D or F (see §2.1), that hold by a recursive argument on the semisimple rank
of Gsc . The first step of the induction is given by the results on unipotent
classes of PSLn (q) and PSp2n (q), while the recursive step is a reduction to
Levi subgroups. Then we proceed group by group and class by class. The
experience suggests that a general argument is not possible. There are some
exceptions in low rank for which Levi subgroups are too small and we need
the representatives of the classes to apply ad-hoc arguments.
Here is the organization of the paper: We recall some notations and facts
in §2, where we also state the needed notation for groups of Lie type. In §3
we describe the reduction to Levi subgroups and collect the known results
on unipotent classes of PSLn (q) and PSp2n (q).
Let O be a non-trivial unipotent class in a group G listed in Table 1. If
O is not kthulhu then O collapses, cf. Theorem 2.4. The proof that O is not
kthulhu is given in §4, respectively §5, when G is a Chevalley, respectively
Steinberg, group.
Indeed, if G = PΩ2n+1 (q), n ≥ 3, and q odd, the claim is Proposition
4.3. If G = PΩ+
2n (q), n ≥ 4, E6 (q), E7 (q), or E8 (q), then the claim is
Proposition 4.2. If G = F4 (q), the result follows from Lemmata 4.4 and 4.5;
and if G = G2 (q), q ≥ 3, the assertion follows from Lemmata 4.6 and 4.7.
In turn, PSUn (q) is settled in Proposition 5.1; PΩ−
2n (q) in Proposition
5.17; 2 E6 (q) in Proposition 5.19; and 3 D4 (q) in Proposition 5.20.
In this way, the Theorem is proved.
1.3. Applications and perspectives. The results in this paper will be applied to settle the non-semisimple classes in Chevalley and Steinberg groups.
Next we will deal with unipotent and non-semisimple classes in SuzukiRee groups. These are too small to apply the recursive arguments introduced
in this paper.
The semisimple conjugacy classes in G different from PSLn (q) are more
challenging. We expect that reducible classes would collapse while the irreducible ones would be kthulhu, as is the case for PSL2 (q) and PSL3 (q) (with
some exceptions). Both cases require a deeper understanding of the classes,

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

and in addition the irreducible case seems to need an inductive argument
on the maximal subgroups.
Acknowledgements. At different stages of this project, Mauro Costantini,
Paolo Papi and Jay Taylor, helped us with interesting conversations and
precise references. We thank them a lot.
2. Preliminaries
If a ≤ b ∈ N, then Ia,b denotes {a, a + 1, . . . , b}; for simplicity Ia = I1,a .
2.1. Glossary of racks. See [3] for details and more information.
2.1.1. A rack is a set X 6= ∅ with a self-distributive operation ⊲ : X × X →
is bijective for every x ∈ X. The archetypical example
X such that x ⊲
is the conjugacy class OzG of an element z in a group G with the operation
x ⊲ y = xyx−1 , x, y ∈ OzG . A rack X is abelian if x ⊲ y = y, for all x, y ∈ X.
2.1.2. [4, Definition
3.5] A rack X is of type D if it has a decomposable
`
subrack Y = R S with elements r ∈ R, s ∈ S such that r ⊲ (s ⊲ (r ⊲ s)) 6= s.
Lemma 2.1. [1, Lemma 2.10] Let X and Y be racks, y1 6= y2 ∈ Y , x1 6=
x2 ∈ X such that x1 ⊲ (x2 ⊲ (x1 ⊲ x2 )) 6= x2 , y1 ⊲ y2 = y2 . Then X × Y is of
type D.


Remark 2.2. One of the hypothesis of Lemma 2.1 holds in the following
setting. Let O be a real conjugacy class, i.e. O = O−1 , with no involutions.
Then y1 6= y2 = y1−1 , that obviously commute.
2.1.3. [1, Definition 2.4] A rack X is of type F if it has a family of subracks
(Ra )a∈I4 and elements ra ∈ Ra , a ∈ I4 , such that Ra ⊲ Rb = Rb , for a, b ∈ I4 ,
and Ra ∩ Rb = ∅, ra ⊲ rb 6= rb for a 6= b ∈ I4 .
2.1.4. [3, Definition 2.3]
` A rack X is of type C when there are a decomposable subrack Y = R S and elements r ∈ R, s ∈ S such that r ⊲ s 6= s,
R = OrInn Y ,

S = OsInn Y ,

min{|R|, |S|} > 2 or max{|R|, |S|} > 4.

Here Inn Y is the subgroup of SY generated by y ⊲

, y ∈Y.

2.1.5. Being of type C, D or F can be phrased in group terms, see [3]. Here
is a new formulation suitable for later applications.
Lemma 2.3. Let O be a conjugacy class in a group H. If there are r, s ∈ O
hr,si
hr,si
such that r 2 s 6= sr 2 , s2 r 6= rs2 and Or
6= Os , then O is of type C.
Proof. We check that the conditions in [3, Lemma 2.8] hold with H =
hr,si
hr,si
hr,si
hr,si
hr, si = hOr , Os i. By hypothesis, rs 6= sr and Or
6= Os . Now
hr,si
hr,si
r, s ⊲ r, s2 ⊲ r are all distinct, so |Or | > 2, and similarly for Os .


NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

5

Theorem 2.4. [4, Theorem 3.6], [1, Theorem 2.8], [3, Theorem 2.9]. A
rack X of type D, F or C collapses.
The proof rests on results from [7, 12, 13].
2.1.6. A rack is
◦ kthulhu if it is neither of type C, D nor F;
◦ sober if every subrack is either abelian or indecomposable;
◦ austere if every subrack generated by two elements is either abelian or
indecomposable.
Clearly, sober implies austere and austere implies kthulhu.
The criteria of type C, D, F are very flexible:
Lemma 2.5. [4, 1, 3] Let Y be either a subrack or a quotient rack of a rack
X. If Y is not kthulhu, then X is not kthulhu.

2.2. Conjugacy classes.
2.2.1. Let q = pm be as above. We fix a simple algebraic group G defined
over Fq , a maximal torus T, with root system denoted by Φ, and a Borel
subgroup B containing T. We denote by U the unipotent radical of B and
by ∆ ⊂ Φ+ the corresponding sets of simple and positive roots. Also U−
is the unipotent radical of the opposite Borel subgroup B− corresponding
to Φ− . We shall use the realisation of the associated root system and the
numbering of simple roots in [5]. The coroot system of G is denoted by
Φ∨ = {β ∨ | β ∈ Φ} ⊂ X∗ (T), where hα, β ∨ i = 2(α,β)
(β,β) , for all α ∈ Φ. Hence
α(β ∨ (ζ)) = ζ

2(α,β)
(β,β)

,

α, β ∈ Φ, ζ ∈ F×
q .

We denote by Gsc the simply connected group covering G.
For Π ⊂ ∆, we denote by ΦΠ the root subsystem with base Π and ΨΠ :=
Φ+ − ΦΠ . For α ∈ Φ, we denote by sα ∈ W = NG (T)/T the reflection
with respect to α. Also, si = sαi , if αi is a simple root with the alluded
numeration. Also, there is a monomorphism of abelian groups xα : k → U;
the image Uα of xα is called a root subgroup. We adopt the normalization
of xα and the notation for the elements in T from [20, 8.1.4]. We recall the
commutation rule: t ⊲ xα (a) = txα (a)t−1 = xα (α(t)a), for t ∈ T and α ∈ Φ.
In particular, if t = β ∨ (ξ) for some ξ ∈ k× , then t⊲xα (a) = xα (α(β ∨ (ξ))a) =
2(α,β)

xα (ξ (β,β) a).
We denote by P a standard parabolic subgroup of G, with standard Levi
subgroup L and unipotent radical V. Thus there exists Π ⊂ ∆ such that
L = hT, U±γ | γ ∈ Πi.
If u ∈ Q
U then for every ordering of Φ+ , there exist unique cα ∈ k such
that u = α∈Φ+ xα (cα ). We define supp u = {α ∈ Φ+ | cα 6= 0}. In general
the support depends on the chosen ordering of Φ+ . However, if u ∈ V as
above, then supp u ⊂ ΨΠ for every ordering of Φ+ .

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

2.2.2. In this paper we deal with Chevalley and Steinberg groups. Let
F be a Steinberg endomorphism of G; it is the composition of the split
endomorphism Frq (the q-Frobenius map) with an automorphism induced
by a Dynkin diagram automorphism ϑ. So, Chevalley groups correspond to
ϑ = id. We assume that T and B are F-stable. Let W F = NGF (T)/TF . Thus
W F ≃ W for Chevalley groups. For each w ∈ W F , there is a representative
ẇ of w in NGF (T), cf. [16, Proposition 23.2]. Notice that ẇ ⊲ (Uα ) = Uw(α)
for all α ∈ Φ. Hence, if α, β ∈ Φ are ϑ-stable and have the same length,
then UFα and UFβ are conjugated by an element in NGF (T) by [14, Lemma
10.4 C]. Let G = [GF , GF ]/Z(GF ).
2.2.3. We shall often use the Chevalley’s commutator formula (2.1), see
[22, pp. 22 and 24]. Let α, β ∈ Φ. If α + β is not a root, then Uα and Uβ
commute. Assume that α + β ∈ Φ. Fix a total order in the set Γ of pairs
(i, j) ∈ N2 such that iα + jβ ∈ Φ. Then there exist cαβ
ij ∈ Fq such that
Y
i j
(2.1) xα (ξ)xβ (η)xα (ξ)−1 xβ (η)−1 =
xiα+jβ (cαβ
∀ξ, η ∈ k.
ij ξ η ),
(i,j)∈Γ

Definition 2.6. [2, Definition 3.3] Let α, β ∈ Φ+ such that α+β ∈ Φ but the
pair α, β does not appear in Table 2. We fix an ordering of Φ+ . A unipotent
conjugacy class O in G has the αβ-property if there exists u ∈ O ∩ UF such
P
that α, β ∈ supp u and for any expression α + β = 1≤i≤r γi , with r > 1
and γi ∈ supp u, necessarily r = 2 and {γ1 , γ2 } = {α, β}.
Table 2.

Φ
G2

p=3
α
β
α1
2α1 + α2
2α1 + α2
α1
α1 + α2 2α1 + α2
2α1 + α2 α1 + α2

p=2
Φ
α
β
Bn , Cn , F4 orthogonal to each other
G2
α1
α1 + α2
α1 + α2
α1

Let α, β ∈ Φ+ . The scalar cα,β
1,1 6= 0 in (2.1) if α + β ∈ Φ and the pair does
not appear in Table 2.
Proposition 2.7. [2, Proposition 3.5] Let G be a finite simple group of Lie
type, with q odd. Assume O has the αβ-property, for some α, β ∈ Φ+ such
that q > 3 when (α, β) = 0. Then O is of type D.

Remark 2.8. Assume u satisfies the conditions in Definition 2.6. Then it is
never an involution. Indeed if q is odd this is never the case. If q is even then

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

7

the argument in the proof of [2, Proposition 3.5] shows that the coefficient
of xα+β in the expression of u2 is nonzero.
2.2.4. Let us choose an ordering of the positive roots and let w ∈ W and
u ∈ U be such that Σ := w(supp u) ⊂ Φ+ . Then, ẇ ⊲ u ∈ U and there is
an ordering of the positive roots for which Σ is the support of ẇ ⊲ u. If,
in addition, wΣ 6⊂ ΨΠ for some Π ⊂ ∆, then by the discussion in 2.2.1,
ẇ ⊲ u ∈ U − V.
2.2.5. We shall need a fact on root systems. Recall that there is a partial
ordering  on the root lattice ZΦ given by α  β if β − α ∈ N0 Φ+ = N0 ∆.
Lemma 2.9. Let γ, β ∈ Φ+ with β  γ. Then there exists a sequence
αi1 , . . . , αik ∈ ∆ such that
(1) ∀j ∈ Ik we have γj := β + αi1 + · · · + αij ∈ Φ+ ;
(2) γ = γk .
If, in addition, Φ is simply-laced, then γj = sij · · · si1 β for every j ∈ Ik .
Proof. (1) and (2) are consequences of [19, Lemma 3.2], with α1 = β, and
the αj being simple. Assume that Φ is simply-laced. Clearly, it is enough to
prove it for a couple of roots. If α, δ ∈ Φ and α + δ ∈ Φ, then Φ ∩ (Zα + Zδ)
is a root system of type A2 , so sα (δ) = α + δ. The last claim follows.

3. Unipotent classes in finite groups of Lie type
3.1. Reduction to Levi subgroups. We start by Lemma 3.2, that is behind the inductive step in most proofs below. We consider the following
setting and notation, that we will use throughout the paper:
P1 , . . . , Pk are standard F -stable parabolic subgroups of G;
Pi = Li ⋉ Vi are Levi decompositions, with Li F -stable;
Ui := (U ∩ Li )F , Ui− := (U− ∩ Li )F , Pi := PFi , Li := LFi , Vi := VFi ;
πi : Pi → Li is the natural projection;
Mi = hUi , Ui− i ≤ Li ; for i ∈ Ik .
Remark 3.1. Assume that G = Gsc . If Li is standard, then Mi = [Li , Li ]F .
Proof. Since G is simply-connected, so is [Li , Li ] (Borel-Tits, see [21, Corollary 5.4]). Then [16, Theorem 24.15] applies.

Lemma 3.2. Let u ∈ UF . Then πi (u) ∈ Mi for all i ∈ Ik .
F

i
(a) If OπMi (u)
is not kthulhu for some i ∈ Ik , then OπLii(u) , OuPi , and OuG are
not kthulhu either.

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

(b) Assume that
(3.1)

No non-trivial unipotent class in Mi is kthulhu, ∀i ∈ Ik .
F

If u ∈
/ ∩i∈Ik Vi , then OuG is not kthulhu.
(c) Assume that (3.1) holds. Let O be a unipotent conjugacy class in GF .
If O ∩ UF 6⊂ ∩i∈Ik Vi , then O is not kthulhu, hence collapses.
Proof. Since U ≤ Pi , u = u1 u2 with u1 ∈ Li and u2 ∈ Vi ≤ U. Hence
u1 ∈ Li ∩ U. Since clearly u1 and u2 are F -invariant, u1 = πi (u) ∈ Mi . Now
(a) follows from Lemma 2.5 and implies (b), since πj (u) 6= 1 for some j ∈ Ik .
(c) follows from (b) and Theorem 2.4 because O ∩ UF 6= ∅.

3.2. Unipotent classes in PSLn (q) and PSp2n (q). We recall now results
in the previous papers of the series that constitute the basis of the induction
argument. We will also need some of the non-simple groups of Lie type of
small rank and small characteristic listed in [2, 3.2.1].
The following Theorem collects information from [1, Table 2], [2, Lemma
3.12 & Tables 3, 4, 5] and [3, Tables 2 & 3].
Theorem 3.3. Let G be either PSLn (q) or PSp2n (q) and let O =
6 {e} be a
unipotent conjugacy class in G, not listed in Table 3. Then it is not kthulhu.
Table 3. Kthulhu classes in PSLn (q) and PSp2n (q)
G
class
q
PSL2 (q)
(2)
even, or 9, or odd not a square
PSL3 (2)
(3)
2
n−1
PSp2n (q), n ≥ 2 W (1)
⊕ V (2)
even
PSp2n (q), n ≥ 2
(2, 12n−2 )
9, or odd not a square
PSp4 (q)
W (2)
even
We explain the notation of Table 3, see [1, 2] for further details:
(i) Unipotent classes in PSLn (k) are parametrized
by partitions of n; i.e.
P
λ = (λ1 , λ2 , . . . ) with λ1 ≥ λ2 ≥ . . . and j λj = n. Thus, (n) is the
regular unipotent class of PSLn (k). Unipotent classes in PSLn (q)
with the same partition are isomorphic as racks.
(ii) Unipotent classes in PSp2n (k), for q odd, are also parametrized by
suitable partitions.
(iii) Unipotent classes in PSp2n (k), for q even, are parametrized by their
label, which is the decomposition of the standard representation as a
module for the action of an element in the conjugacy class:
(3.2)

V =

k
M
i=1

W (mi )ai ⊕

r
M
j=1

V (2kj )bj ,

0 < ai , 0 < bj ≤ 2,

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

9

for mi , kj ≥ 1. The block W (mi ) corresponds to a unipotent class
with partition (mi , mi ), whereas the block V (2kj ) corresponds to a
unipotent class with partition (2kj ).
(iv) The unipotent class in PSp4 (k) with label W (2), respectively, in
PSp2n (k) with label W (1)n−1 ⊕ V (2) contain a unique unipotent class
in PSp4 (q), respectively, PSp2n (q).
Remark 3.4. Assume q is even. If O is a unipotent conjugacy class in Sp2n (q)
enjoying the αβ-property, for some α and β, then O is of type C, D, or F.
Indeed, by Theorem 3.3 the kthulhu unipotent classes in Sp2n (q) for q even
consists of involutions. Remark 2.8 applies.
3.3. Further remarks. If a product X = X1 × X2 of racks has a factor X1
that is not kthulhu, then neither is X. Indeed, pick x ∈ X2 ; then X1 × {x}
is a subrack of X and Lemma 2.5 applies (here as usual X2 can be realized
as a subrack of a group, so that x ⊲ x = x). The following results will be
needed in order to deal with products of possibly kthulhu racks.
Lemma 3.5. Let O be a unipotent conjugacy class in Table 3.
(a) There exist x1 , x2 ∈ O such that (x1 x2 )2 6= (x2 x1 )2 .
(b) If G 6= PSL2 (2), PSL2 (3), then there exist y1 , y2 ∈ O such that y1 6= y2
and y1 y2 = y2 y1 .
Proof. By the isogeny argument [1, Lemma 1.2], we may reduce to classes
in SLn (q) or Sp2n (q). Also, the classes in PSp4 (q) with label W (2) and
W (1) ⊕ V (2) are isomorphic as racks, [2, Lemma 4.26], so we need not to
deal with the last row in Table 3.
If O is the class in SL3 (2), then x1 = id +e1,2 + e2,3 and x2 = σ ⊲ x1 ,
where σ = e1,2 + e2,1 + e3,3 , do the job for (a). For (b), take y1 = x1 and
y2 = x31 = x−1
1 , that belongs to O by [1, Lemma 3.3].
If O is the class in SL2 (q), then x1 = id +e1,2 ∈ O and x2 = σ ⊲ x1 , where
σ = e1,2 − e2,1 do the job for (a); while y1 = x1 , and y2 = id +a2 e1,2 , for
a ∈ Fq , a2 6= 0, 1, are as needed in (b) when q > 3.
Finally, let O be one of the classes in Sp2n (q), cf. Table 3. Then x1 =
P
id +e1,2n ∈ O and x2 = σ ⊲ x1 , where σ = e1,2n − e2n,1 + j6=1,2n ejj do
the job for (a). Let τ be the block-diagonal matrix τ = diag(J2 , id2n−2 , J2 ),
with J2 = ( 01 10 ). Then τ ∈ Sp2n (q) and y1 := x1 , y2 := τ ⊲ y1 fulfil (b). 
Here are results on regular unipotent classes needed later. Let Gsc be a
simply connected simple algebraic group and F a Steinberg endomorphism
as before; let G = GFsc /Z(GFsc ) but we do not assume that G is simple.

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Proposition 3.6. [2, 3.7, 3.8, 3.11] Let O be a regular unipotent class in
G. If any of the conditions below is satisfied, then O is of type D, or F.
(1)
(2)
(3)
(4)

G 6= PSL2 (q) is Chevalley and q 6= 2, 4;
G = PSU3 (q), with q 6= 2, 8;
G = PSU4 (q), with q 6= 2, 4;
G = PSUn (q), with n ≥ 5 and q =
6 2;

In addition, every regular unipotent class in GUn (q), where 1 < n is odd
and q = 22h+1 , h ∈ N0 , is of type D.

Finally, we quote [2, Lemma 4.8]:
Lemma 3.7. Let O be a regular unipotent class in either SLn (q), SUn (q)
or Sp2n (q), q even. Then there are x1 , x2 ∈ O such that (x1 x2 )2 6= (x2 x1 )2 .
4. Unipotent classes in Chevalley groups
In this Section we deal with unipotent conjugacy classes in a finite simple Chevalley group G = GFsc /Z(GFsc ), different to PSLn (q) and PSp2n (q),
treated in [1, 2], see §3.2. For convenience, we shall work in GFsc , cf. [1,
Lemma 1.2]. Let
(4.1)

Ψ(β) = {γ ∈ Φ | β  γ},

β ∈ Φ.

Let u ∈ U and β ∈ Φ+ . Then the support supp u depends on a fixed
ordering of Φ+ , but the assertion supp u ⊂ Ψ(β) does not. Indeed, passing from one order to another boils down to successive applications of the
Chevalley formula (2.1), that do not affect the claim.
We denote by O a non-trivial unipotent conjugacy class in G.
4.1. Unipotent classes in PΩ+
2n (q), n ≥ 4; E6 (q), E7 (q) and E8 (q). We
first deal with the case when Φ simply-laced, i.e. G is one of PΩ+
2n (q),
n ≥ 4; E6 (q), E7 (q) and E8 (q).
Lemma 4.1. Given β ∈ Φ+ − ∆, there is x ∈ O ∩ UF with supp x 6⊂ Ψ(β).
Proof. Let u ∈ O ∩ UF . If supp u 6⊂ Ψ(β), then we are done. Assume that
supp u ⊂ Ψ(β). We claim that there is τ ∈ NGFsc (T) such that
x := τ ⊲ u ∈ O ∩ UF

and

supp x 6⊂ Ψ(β).

For every γ ∈ Ψ(β) there is a unique k such that γ = β + αi1 + · · · + αik
as in Lemma 2.9. Let m be the minimum k for γ ∈ supp u. We call m
the bound of u. We will prove the claim by induction on the bound m. If
m = 0 then β ∈ supp u and since β 6∈ ∆, there is a simple reflection si
such that si β = β − αi ∈ Φ+ − Ψ(β). Also, si γ ∈ Φ+ for every γ ∈ supp u

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

11

because si (Φ+ − {αi }) = Φ+ − {αi }. In this case we take τ = s˙i to be any
representative of si in NGFsc (T).
Let now m > 0 and assume that the statement is proved for unipotent
elements with bound m − 1. Let γ ∈ supp u reach the minimum, i.e., be
such that γ = β + αi1 + · · · + αim for some αij ∈ ∆ chosen as in Lemma 2.9.
Then γ ′ = sim γ = β + αi1 + · · · + αim−1 ∈ Ψ(β), and sim α ∈ Φ+ for every
α ∈ Ψ(β) by construction. Let ṡim be a representative of sim in NGFsc (T).
Then u′ = ṡim ⊲ u ∈ O ∩ UF and either supp u′ 6⊂ Ψ(β), or supp u′ ⊂ Ψ(β),
with bound at most m − 1. In the first case, we conclude by setting x = u′ .
In the second case, we use the inductive hypothesis.

Proposition 4.2. O is not kthulhu.
Proof. The basic idea of the proof is to apply Lemma 3.2 (c) to a series of
standard F -stable parabolic subgroups Pi of Gsc for which (3.1) holds. We
show that for every O and for every G, we have O ∩ UF 6⊂ ∩i Vi . This follows
from Lemma 4.1 by observing that in each case ∩i Vi is a product of root
subgroups corresponding to roots in Ψ(β) for some β ∈ Φ+ − ∆. We analyze
the different cases according to Φ.
Dn , n ≥ 4. We consider the parabolic subgroups P1 and P2 such that L1 and
L2 have root systems An−1 , generated respectively by ∆ − αn−1 and ∆ − αn .
Since n ≥ 4, (3.1) holds by Theorem 3.3. Let u ∈ V1 ∩ V2 . Then α ∈ supp u
if and only if α contains αn−1 and αn in its expression, i.e. α ∈ Ψ(β) for
β = αn−2 + αn−1 + αn . By Lemma 4.1, O ∩ UF 6⊂ ∩i Vi .
E6 . We consider the parabolic subgroups P1 , P2 and P3 such that L1 , L2
and L3 have root systems D5 , D5 and A5 , generated respectively by ∆ − α1 ,
∆ − α6 and ∆ − α2 . By Theorem 3.3 and the result for Dn , (3.1) holds. Let
P
u ∈ V1 ∩ V2 ∩ V3 . Then α ∈ supp u if and only if α ∈ Ψ(β) for β = 6i=1 αi .
By Lemma 4.1, O ∩ UF 6⊂ ∩i Vi .
E7 . We consider the parabolic subgroups P1 , P2 and P3 such that L1 , L2
and L3 have root systems D6 , E6 and A6 , generated respectively by ∆ − α1 ,
∆ − α7 and ∆ − α2 . By Theorem 3.3 and the results for Dn and E6 , (3.1)
holds. Let u ∈ V1 ∩ V2 ∩ V3 . Then α ∈ supp u if and only if α ∈ Ψ(β) for
P
β = 7i=1 αi . By Lemma 4.1, O ∩ UF 6⊂ ∩i Vi .
E8 . We consider the parabolic subgroups P1 , P2 and P3 such that L1 , L2
and L3 have root systems D7 , E7 and A7 , generated respectively by ∆ − α1 ,
∆ − α8 and ∆ − α2 . By Theorem 3.3 and the results for Dn and E7 , (3.1)

12

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

holds. Let u ∈ V1 ∩ V2 ∩ V3 . Then α ∈ supp u if and only if α ∈ Ψ(β) for
P
β = 8i=1 αi . By Lemma 4.1, O ∩ UF 6⊂ ∩i Vi .


4.2. Unipotent classes in PΩ2n+1 (q). Here we deal with PΩ2n+1 (q), i.e.
Φ is of type Bn , n ≥ 3. In this case, q is always odd.
Proposition 4.3. O is not kthulhu.

Proof. We consider the standard F -stable parabolic subgroups P1 and P2
such that L1 and L2 have root systems An−1 and C2 , generated respectively
by Π1 := ∆ − αn and Π2 = {αn−1 , αn }. By Lemma 3.2 (a) and Theorem
3.3, if O ∩ UF 6⊂ V1 then O is not kthulhu. Let us thus consider u ∈ O ∩ V1 .
Then supp u ⊂ ΨΠ1 = {εi , εj + εl | i, j, l ∈ In , j < l}, since it must contain
αn . We will apply the argument in 2.2.4.
Assume first that supp u ⊂ {εj + εl | j, l ∈ In , j < l}. Let ℓ be the
maximum l such that εj +εl ∈ supp u for some j ∈ In−1 . Then sεℓ (supp u) ⊂
Φ+ . Let ṡεℓ be a representative of sεℓ in NGFsc (T). Then ṡεℓ ⊲ u ∈ O ∩ UF
and εj − εℓ ∈ supp(ṡεℓ ⊲ u) for every j such that εj + εℓ ∈ supp u. Hence
ṡεℓ ⊲ u ∈ O ∩ UF − V1 . By the previous argument, O is not kthulhu.
Assume next that there is some i such that εi ∈ supp u. We can always
assume i = n. Indeed, if εn 6∈ supp u, we may replace u by ṡεi −εn ⊲u ∈ O∩UF ,
where ṡεi −εn is a representative of sεi −εn in NGFsc (T). Then π2 (u) ∈ M2 lies
in a non-trivial unipotent conjugacy class in a group isomorphic to Sp4 (q)
and the short simple root lies in the support. A direct computation shows
that a representative of this class in Sp4 (q) is as follows:
1 a ∗ ∗ 
01 0 ∗
a 6= 0.
0 0 1 −a ,
00 0 1

Thus, its Jordan form has partition (2, 2) and this class is not kthulhu by
Theorem 3.3 (recall that q is odd). Then Lemma 3.2 applies.

4.3. Unipotent classes in F4 (q). Here we deal with unipotent classes in
F4 (q). In this case the approach in Section 4.1 is not effective. Indeed,
in characteristic 2, (3.1) does not hold for any of the standard parabolic
subgroups. For this reason we shall use explicit representatives of unipotent
classes and apply results from Theorem 3.3 and Proposition 4.3 for B3 ,
where q is assume to be odd.
We use the list of representatives of unipotent classes in F4 (q) in [18,
Tables 5,6] for q odd, see Table 4, respectively in [17, Theorem 2.1] for q
even, see Table 5. We indicate the roots as in [17]: εi is indicated by i, εi −εj
is indicated by i − j, and 21 (ε1 ± ε2 ± ε3 ± ε4 ) is indicated by 1 ± 2 ± 3 ± 4.
Thus the simple roots are α1 = 2 − 3, α2 = 3 − 4, α3 = 4, α4 = 1 − 2 − 3 − 4.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

13

Table 4. Representatives of unipotent classes in F4 (q) in
odd characteristic; η, ξ and ζ are suitable elements in F×
q
x1 = x1+2 (1)
x2 = x1−2 (1)x1+2 (−1)
x3 = x1−2 (1)x1+2 (−η)
x4 = x2 (1)x3+4 (1)
x5 = x2−3 (1)x4 (1)x2+3 (1)
x6 = x2−3 (1)x4 (1)x2+3 (η)
x7 = x2 (1)x1−2+3+4 (1)
x8 = x2−3 (1)x4 (1)x1−2 (1)
x9 = x2−3 (1)x3−4 (1)x3+4 (−1)
x10 = x2−3 (1)x3−4 (1)x3+4 (−η)
x11 = x2+3 (1)x1+2−3−4 (1)x1−2+3+4 (1)
x12 = x2−3 (1)x4 (1)x1−4 (1)
x13 = x2−3 (1)x4 (1)x1−4 (η)
x14 = x2−4 (1)x3+4 (1)x1−2 (−1)x1−3 (−1)
x15 = x2−4 (1)x3+4 (1)x1−2 (−η)x1−3 (−1)
x16 = x2−4 (1)x2+4 (−η)x1−2+3+4 (1)x1−3 (−1)
x17 = x2−4 (1)x3+4 (1)x1−2−3+4 (1)x1−2 (−η)x1−3 (ξ)
x18 = x2 (1)x3+4 (1)x1−2+3−4 (1)x1−2 (−1)x1−3 (ζ)
x19 = x2−3 (1)x3−4 (1)x4 (1)
x20 = x2 (1)x3+4 (1)x1−2−3−4 (1)
x21 = x2−4 (1)x3 (1)x2+4 (1)x1−2−3+4 (1)
x22 = x2−4 (1)x3 (1)x2+4 (η)x1−2−3+4 (1)
x23 = x2−3 (1)x3−4 (1)x4 (1)x1−2 (1)
x24 = x2−3 (1)x3−4 (1)x4 (1)x1−2 (η)
x25 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)
x26 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)x1−2+3+4 (ζ)
x27 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)x1−2+3+4 (−ζ)

If q is odd, then the possible representatives are xi , i ∈ I25 , for p 6= 3, with
two additional representatives x26 , x27 when p = 3.
Lemma 4.4. If q is odd, then O is not kthulhu.
Proof. A direct verification shows that all representatives for i ≥ 7 enjoy
the αβ-property with (α, β) 6= 0; we list in Table 6 the roots α and β for
each representative. By Proposition 2.7, O is of type D.
We next consider the representative x1 , that equals xγ (1) for a long root
γ. By the discussion in §2.2.2, Ox1 contains an element in UFα1 , that lies in
the subgroup of type A2 generated by U±α1 , U±α2 . Theorem 3.3 applies.

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Table 5. Representatives of unipotent classes in F4 (q) in
even characteristic; η and ζ are suitable elements in F×
q
x1 = x1 (1)
x2 = x1+2 (1)
x3 = x1 (1)x1+2 (1)
x4 = x2+3 (1)x1 (1)
x5 = x2 (1)x2+3 (1)x1−3 (1)
x6 = x2 (1)x2+3 (1)x1−3 (1)x1+3 (η)
x7 = x2 (1)x2+3 (1)x1−2+3+4 (1)
x8 = x2 (1)x2+3 (1)x1−2+3+4 (1)x1+4 (η)
x9 = x2 (1)x1−2 (1)
x10 = x2 (1)x1−2 (1)x1+2 (η)
x11 = x2 (1)x3+4 (1)x1−4 (1)
x12 = x2 (1)x1−2+3+4 (1)x1−4 (1)
x13 = x2 (1)x2+3 (1)x1−2 (1)
x14 = x2 (1)x3+4 (1)x1−2 (1)
x15 = x2 (1)x2+3 (1)x1−2+3+4 (1)x1−3 (1)
x16 = x2 (1)x2+3 (1)x1−2+3+4 (1)x1−2 (1)
x17 = x2 (1)x2+3 (1)x1−2−3+4 (1)x1−2 (1)
x18 = x2 (1)x2+3 (1)x1−2−3+4 (1)x1−2 (1)x1−4 (η)
x19 = x2 (1)x3+4 (1)x1−2+3−4 (1)x1−2 (1)x1−3 (ζ)
x20 = x1−2 (1)x2−3 (1)x3 (1)
x21 = x1−2 (1)x2−3 (1)x3 (1)x2+3 (η)
x22 = x4 (1)x2−4 (1)x1−2+3−4 (1)
x23 = x4 (1)x2−4 (1)x2+4 (η)x1−2+3−4 (1)
x24 = x2−4 (1)x3+4 (1)x1−2−3−4 (1)x1−2−3+4 (1)
x25 = x2−4 (1)x3+4 (1)x1−2−3−4 (1)x1−2−3+4 (1)x1−2 (η)
x26 = x2−4 (1)x3+4 (1)x1−2−3−4 (1)x1−2−3+4 (1)x1−2 (η)x1−3 (η)
x27 = x2−4 (1)x3 (1)x3+4 (1)x2+4 (η)x1−2−3+4 (1)
x28 = x2−4 (1)x3 (1)x3+4 (1)x2+4 (η)x1−2−3+4 (1)x1−2 (η)
x29 = x1−2 (1)x2−3 (1)x3−4 (1)x4 (1)
x30 = x1−2 (1)x2−3 (1)x3−4 (1)x4 (1)x3+4 (η)
x31 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)
x32 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)x3+4 (η)
x33 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)x1−2 (η)
x34 = x2−3 (1)x3−4 (1)x4 (1)x1−2−3−4 (1)x3+4 (η)x1−2 (η)

Finally, we deal with the xi ’s, i ∈ I2,6 . Let L1 be the standard Levi
subgroup (of type B3 ) generated by the root subgroups Uγ , for γ = ±α1 ,
±α2 , ±α3 . We claim that all xi , i ∈ I2,6 , are conjugated to elements in M1 ;
then the result follows by Proposition 4.3. Indeed, x2 , x3 lie in UF1−2 UF1+2 ;

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

15

Table 6. Oxi with the αβ-property
i
α
β
7
2
1-2+3+4
8
1-2
2-3
9,10
2-3
3-4
11
1+2-3-4 1-2+3+4
12,13
4
1-4
14,15
2-4
1-2
16
2-4
1-2+3+4
17,21,22
2-4
1-2-3+4
18
2
1-2
19,23,24,25,(26,27)
2-3
3-4
20
2
1-2-3-4

thus conjugating by s1−3 s2−4 , we get a representative in UF3−4 UF3+4 . Also
x5 , x6 lie in UF2−3 UF2+3 UF4 , and x4 = x2 (1)x3+4 (1), so they all lie in M1 . 
Lemma 4.5. If q is even, then O is not kthulhu.
Proof. The representative x1 , respectively x2 , is equal to xγ (1) for a short,
respectively long, root γ. By the discussion in §2.2.2, Ox1 intersects UFα3
and Ox2 intersects UFα1 . Let M = hUF±α3 , UF±α4 i and M ′ = hUF±α1 , UF±α2 i,
both of type A2 . Then Ox1 ∩ M , respectively Ox2 ∩ M ′ , is a unipotent class
corresponding to the partition (2, 1) in M , respectively M ′ . By Theorem
3.3, these classes are not kthulhu.
We consider now the classes labelled by i ∈ I20,34 . Let P1 be the standard
parabolic subgroup with standard Levi L1 as in the proof of Lemma 4.4. Set
yi = π1 (xi ). Then the class OyMi 1 satisfies the αβ-property; we list in Table 7
the roots α and β for each representative. Since ΦΠ1 is of type B3 , the group
[L1 , L1 ] is isogenous to Sp6 (k). By Remark 3.4, OyMi 1 is not kthulhu, hence
neither is O.
We consider now the classes labelled by i ∈ I′ = {3, 4, 7, 8, 12} ∪ I14,19 .
Let P2 be the standard parabolic subgroup with standard Levi L2 (of type
C3 ) associated with Π2 = {α2 , α3 , α4 }; here Φ+
Π2 consists of the roots 1 − 2,
3, 4, 3 ± 4, 1 − 2 ± 3 ± 4. Let β1 = α4 , β2 = α3 , β3 = α2 be the simple
M2
roots of Φ+
Π2 . Set zi = π2 (xi ). Now Ozi is a unipotent class in Sp6 (q). Let
I′′ = I′ − {3, 4}. Table 8 lists the index i ∈ I′′ , the support of zi and the

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Table 7. OyMi 1 with the αβ-property.
i
α
β
i ∈ I20,21
α1
α2 + α3
i ∈ I22,23
α3
α1 + α2
i ∈ I24,28 α1 + α2 α2 + 2α3
i ∈ I29,34
α1
α2
partition associated to OzMi 2 , obtained from the Jordan form of zi in Sp6 (k).
Since the partition is always different from (2, 14 ), the label of the class
in Sp6 (q) is never W (1) ⊕ V (2), whence OzMi 2 is not kthulhu by Theorem
3.3. The remaining classes in I′ are represented by x3 = x1 (1)x1+2 (1) and
x4 = x2+3 (1)x1 (1). Let x = (ṡ1−3 ṡ2−4 ) ⊲ x3 ∈ OxG3 and y = (ṡ2−3 ṡ1−2 ṡ3 ) ⊲
x4 ∈ OxG4 . Then x ∈ U3 U3+4 , so x ∈ UFβ2 +β3 UF2β2 +β3 ⊂ M2 , y ∈ U1−2 U3 , so
y ∈ UF2β1 +2β2 +β3 UFβ2 +β3 ⊂ M2 . The partition associated to x, respectively
y, as unipotent element in Sp6 (q) is (2, 2, 1, 1), respectively (2, 2, 2). Hence,
neither OxG3 nor OxG4 is kthulhu by Theorem 3.3.
Table 8. supp zi and its partition
i
supp zi
7,8,12,15
β1 + 2β2 + β3
14
2β1 + 2β2 + β3 , 2β2 + β3 ,
16
2β1 + 2β2 + β3 , β1 + 2β2 + β3 ,
17,18
2β1 + 2β2 + β3 , β1 + β2 ,
19
2β1 + 2β2 + β3 , 2β2 + β3 , β1 + β2 + β3

partition
(2, 2, 1, 1)
(2, 2, 1, 1)
(2, 2, 1, 1)
(2, 2, 1, 1)
(2, 2, 2)

The xi ’s for i ∈ I′′′ = {5, 6, 9, 10, 11, 13} lie in the subgroup K of type
B4 generated by the subgroups U±α , α ∈ {1 − 2, 2 − 3, 3 − 4, 4}. If i ∈ I′′′ ,
F
OxKi has the αβ-property, see Table 9. Since SO9 (k) is isogenous to Sp8 (k),
Remark 3.4 applies.

4.4. Unipotent classes in G2 (q). Here we deal with unipotent classes in
G2 (q), q > 2. As for F4 (q), we shall use explicit representatives of the
classes, the parabolics being too small. The list of representatives can be
found in [6] when p > 3 and in [9] otherwise; see (4.2), (4.3), (4.9).
Lemma 4.6. If q is odd, then O is not kthulhu.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

17

F

Table 9. OxKi with the αβ-property, i ∈ I′′ = {5, 6, 9, 10, 11, 13}.
xi
α
β
5, 6
2+3 1-3
9, 10, 13
2
1-2
11
1-4 3+4

Proof. Assume first p > 3. By [6, Theorems 3.1, 3.2, 3.9] every non-trivial
class of p-elements in G is either regular or can be represented by an element
of the following form, for suitable a, b, c ∈ F×
q :
(4.2)

xα2 (1),

xα2 (1)x3α1 +α2 (b),

xα2 (1)x2α1 +α2 (−1)x3α1 +α2 (c),

xα1 +α2 (1), xα2 (1)x2α1 +α2 (a).

The regular classes are covered by Proposition 3.6 (1). The elements
xα2 (1) and xα2 (1)x3α1 +α2 (b) lie in the subgroup of type A2 generated by
UF±α2 and UF±(3α2 +α2 ) and we apply Theorem 3.3. The classes represented
by xα2 (1)x2α1 +α2 (−1)x3α1 +α2 (c) enjoy the αβ-property, so we invoke Proposition 2.7. We prove now that the class of r = xα1 +α2 (1) is of type D.
We observe that there is an element σ = ṡα2 ∈ G ∩ NG (T) such that
s := σ ⊲ r = xα1 (ξ), ξ ∈ F×
q . Then sr 6= rs by the Chevalley commutator formula (2.1) and, as rs, sr ∈ UF and p is odd, we have (rs)2 6= (sr)2 .
In addition, r, s ∈ PF1 , for P1 the standard parabolic subgroup with Levi L1
associated with α1 . Since r lies in the unipotent radical V1 of P1 and s lies
hr,si
hr,si
6= Or .
in L1 , we have Os
Let r = xα2 (1)x2α1 +α2 (a); it lies in hUF±α2 i × hUF±(2α1 +α2 ) i. We argue as
in §3.3. As q > 3, Lemmata 3.5 and 2.1 apply whence Or is of type D.
Assume now p = 3. By [9, 6.4] the non-trivial classes of p-elements in G
are either regular or are represented by an element of the following form:
(4.3)

x3α1 +2α2 (1),

xα1 +α2 (1)x3α1 +α2 (a),

x2α1 +α2 (1)x3α1 +2α2 (1), x2α1 +α2 (1),

for suitable a ∈ F×
q . The regular classes are covered by Proposition 3.6 (1).
The element x3α1 +2α2 (1) lies in the subgroup of type A2 generated by UF±α2
and UF±(3α1 +2α2 ) and Theorem 3.3 applies.
We show that if r = xα1 +α2 (1)x3α1 +α2 (a) ∈ O, then it is of type D.
Indeed, let s := ṡα2 ⊲ r ∈ UFα1 UF3α1 +2α2 . Then sr 6= rs; since sr, rs ∈ UF , we

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N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

have (sr)2 6= (rs)2 . Moreover, r, s ∈ PF1 with s ∈ L1 , r ∈ V1 , with notation
hr,si
hr,si
as for p > 3. Thus, Os
6= Or
and O is of type D.
Assume that u = x2α1 +α2 (1)x3α1 +2α2 (1) ∈ O. Conjugating by suitable
elements in NG (T) we find r ∈ O ∩ Uα1 U3α1 +α2 ⊂ P1 , r 6∈ V1 and s ∈
O ∩ Uα1 +α2 Uα2 ⊂ V1 . By repeated use of (2.1), we see that the coefficient
of xα1 +α2 in srs−1 is 6= 0, hence rs 6= sr, (rs)2 6= (sr)2 and O is of type D.
Assume finally that u = x2α1 +α2 (1) ∈ O. Let r = ṡα1 ⊲ u ∈ OuG ∩ Uα1 +α2
and s = ṡα1 +α2 ⊲ u ∈ OuG ∩ Uα1 . Then rs, sr ∈ U, (rs)2 6= (sr)2 , and
hr,si
hr,si
6= Or , as s ∈ L1 and r ∈ V1 , so O is of type D.

Os
In order to deal with some unipotent classes in G2 (4) we will need a precise
version of (2.1) for all pairs of positive roots. We shall use the relations from
[9, II.2], that we write for convenience. They hold in general for q even, and
we shall use them recalling that a3 = 1 for every a ∈ F×
4.
(4.4)

xα1 (a)xα2 (b) = xα2 (b)xα1 (a)xα1 +α2 (ab)x2α1 +α2 (a2 b)x3α1 +α2 (a3 b)

(4.5)

xα1 (a)xα1 +α2 (b) = xα1 +α2 (b)xα1 (a)x3α1 +α2 (a2 b)x3α1 +2α2 (ab2 )

(4.6)

xα1 (a)x2α1 +α2 (b) = x2α1 +α2 (b)xα1 (a)x3α1 +α2 (ab)

(4.7)

xα2 (a)x3α1 +α2 (b) = x3α1 +α2 (b)xα2 (a)x3α1 +2α2 (ab)

(4.8)

xα1 +α2 (a)x2α1 +α2 (b) = x2α1 +α2 (b)xα1 +α2 (a)x3α1 +2α2 (ab)

For all other pairs of positive roots the corresponding subgroups commute.
Lemma 4.7. If q > 2 is even, then O is not kthulhu.
Proof. By [9, 2.6] all non-trivial classes of 2-elements in G can be represented
by an element of the following form, for suitable a, b, c ∈ Fq :
(4.9)

x2α1 +α2 (1),

x3α1 +2α2 (1),

xα1 (1)xα2 (1)x2α1 +α2 (a),

xα1 +α2 (1)x2α1 +α2 (1)x3α1 +α2 (b), xα2 (1)x2α1 +α2 (1)x3α1 +α2 (c).

Assume that r = x2α1 +α2 (1) ∈ O. It is enough to prove that O is of
type C for G2 (2), which is a non-simple subgroup of G2 (q). We consider
G (2)
ṡα1 +α2 ⊲ r = xα1 (1) ∈ O and s := x−α1 (1) ⊲ xα1 (1) = ṡα1 ∈ Or 2 . Let
H := hr, s, z = xα1 +α2 (1)i ≤ P1 (the parabolic subgroup associated with
α1 ), with r ∈ V1 , s ∈ L1 . Hence, OrH 6= OsH . By a direct computation,
s ⊲ r = xα1 +α2 (1) = z 6= r,
r ⊲ s = szr,

z ⊲ r = rx3α1 +2α2 (1),
z ⊲ (szr) = sx3α1 +2α2 (1).

So H ≤ hOrH , OsH i ≤ H; {r, z, z ⊲ r} ⊂ OrH and {s, szr, sx3α1 +2α2 (1)} ⊂ OsH
G (2)
hence Or 2 is of type C by [3, Lemma 2.8].

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

19

Assume that r = x3α1 +2α2 (1) ∈ O. Now r ∈ M = hU±α2 , U±(3α1 +α2 ) i, of
type A2 . Since OrM has partition (2, 1), O is not kthulhu by Theorem 3.3
and [16, Theorem 24.15].
Assume q > 4. The classes represented by the xα1 (1)xα2 (1)x2α1 +α2 (a) for
a ∈ Fq are regular, thus they are not kthulhu by Proposition 3.6. The classes
of xα1 +α2 (1)x2α1 +α2 (1)x3α1 +α2 (b) and xα2 (1)x2α1 +α2 (1)x3α1 +α2 (c) enjoy the
αβ-property. By [2, Proposition 3.6], these classes are of type F.
2
3
Let now q = 4 and let ζ be a generator of F×
4 so ζ + ζ + 1 = 0 and ζ = 1.
By [9] there are 2 regular unipotent classes, one represented by xα1 (1)xα2 (1)
and the other by xα1 (1)xα2 (1)x2α1 +α2 (ζ). We shall apply Lemma 2.3 in
order to show that these classes are of type C. For this, we need the following
formula which can be retrieved applying (4.4) and (4.7).

(4.10)

xα1 (a)xα2 (b)xα1 (c)xα2 (d) = xα1 (a + c)xα2 (b + d)
×x3α1 +α2 (b)x3α1 +2α2 (bd)x2α1 +α2 (c2 b)xα1 +α2 (bc),

2
a, b, c, d ∈ Fq . Let r = xα1 (1)xα2 (1), t := α∨
1 (ζ), s := t ⊲ r = xα1 (ζ )xα2 (1) ∈
OrG . By direct computation using (4.10) we see that

r 2 = x3α1 +α2 (1)x3α1 +2α2 (1)x2α1 +α2 (1)xα1 +α2 (1)
s2 = x3α1 +α2 (1)x3α1 +2α2 (1)x2α1 +α2 (ζ)xα1 +α2 (ζ 2 ).
Using (4.8) and that ξ 2 6= ξ, we see r 2 s2 6= s2 r 2 , hence r 2 s 6= sr 2 and
s2 r 6= rs2 . In addition, hr, si ⊆ UF and UF ⊲ r ⊂ rhUγ | γ ∈ Φ+ − ∆i and
hr,si
hr,si
UF ⊲ s ⊂ shUγ | γ ∈ Φ+ − ∆i, so Or
6= Os , whence OrG is of type C.
Similarly, we consider now r = xα1 (1)xα2 (1)x2α1 +α2 (ζ), t := α∨
1 (ζ) and
2
2
G
s := t ⊲ r = xα1 (ζ )xα2 (1)x2α1 +α2 (ζ ) ∈ Or . In this case
r 2 = x3α1 +α2 (ζ 2 )x3α1 +2α2 (ζ 2 )x2α1 +α2 (1)xα1 +α2 (1)
s2 = x3α1 +α2 (ζ 2 )x3α1 +2α2 (ζ 2 )x2α1 +α2 (ζ)xα1 +α2 (ζ 2 ).
hr,si

hr,si

As above we verify that r 2 s 6= s2 r and s2 r 6= r 2 s and that Or
6= Os
so
G
Or is of type C.
We assume now that x := xα1 +α2 (1)x2α1 +α2 (1)x3α1 +α2 (b) ∈ O, with b 6=
0. By [9, Proposition 2.6, page 499] if q = 4 we can take b = ζ. We prove
that this class is of type C. Set rα := xα (1)x−α (1)xα (1) = ṡα , α ∈ Φ+ , see

20

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

[22, Lemma 19]. The elements
s = rα1 ⊲ x = x2α1 +α2 (1)xα1 +α2 (1)xα2 (ζ)
= xα1 +α2 (1)x2α1 +α2 (1)xα2 (ζ)x3α1 +2α2 (1),
r = rα2 rα1 ⊲ s = xα1 (1)x2α1 +α2 (1)x3α1 +2α2 (ζ)
hr,si

hr,si

belong to O. We claim that Or
6= Os . Indeed, r, s ∈ P1 with r 6∈ V1 ,
s ∈ V1 . A direct calculation shows that r 2 = x3α1 +α2 (1),
r ⊲ s = xα1 +α2 (1 + ζ)x2α1 +α2 (1 + ζ)xα2 (ζ)x3α1 +α2 (ζ),
r 2 ⊲ s = sx3α1 +2α2 (ζ),
r 3 ⊲ s = r ⊲ (r 2 ⊲ s) = r ⊲ (sx3α1 +2α2 (ζ)) = (r ⊲ s)x3α1 +2α2 (ζ),
s ⊲ (r ⊲ s) = xα1 +α2 (1 + ζ)x2α1 +α2 (1 + ζ)xα2 (ζ)x3α1 +α2 (ζ)x3α1 +2α2 (1 + ζ).
We see that all these are distinct, and different from s, by looking at the
unique expression as a product of elements in root subgroups in the order:
α1 < α1 + α2 < 2α1 + α2 < α2 < 3α1 + α2 < 3α1 + 2α2
hr,si

Hence, |Or | ≥ 5 and O is of type C, by [3, Lemma 2.8], with H = hr, si.
∨ 2
∨
∨
∨
Let now t2 := α∨
1 (ζ)α2 (ζ), t3 := α2 (ζ), t4 := α1 (ζ)α2 (ζ ) and set
x = xα1 +α2 (1)x2α1 +α2 (1),
x1 = rα2 ⊲ x = xα1 (1)x2α1 +α2 (1) ∈ Ox ;
x2 = t2 ⊲ x1 = xα1 (ζ)x2α1 +α2 (ζ),
x3 = t3 ⊲ x1 = xα1 (ζ 2 )x2α1 +α2 (1),
x4 = t4 ⊲ x1 = xα1 (1)x2α1 +α2 (ζ).
Let Yi = UF ⊲ xi , i ∈ I4 . A direct computation shows that
[
Y1 =
xα1 (1)xα1 +α2 (ℓ)x2α1 +α2 (ℓ + 1)x3α1 +2α2 (f 2 + f )UF3α1 +α2 ,
f,ℓ∈F4

Y2 =

[

xα1 (ζ)xα1 +α2 (ℓζ)x2α1 +α2 (ℓζ 2 + ζ)x3α1 +2α2 (f 2 ζ + f ζ)UF3α1 +α2 ,

f,ℓ∈F4

Y3 =

[

xα1 (ζ 2 )xα1 +α2 (ℓ2 ζ)x2α1 +α2 (ℓζ + 1)x3α1 +2α2 (f 2 ζ 2 + f )UF3α1 +α2 ,

f,ℓ∈F4

Y4 =

[

f,ℓ∈F4

xα1 (1)xα1 +α2 (ℓ)x2α1 +α2 (ℓ + ζ)x3α1 +2α2 (f 2 + f ζ)UF3α1 +α2 .

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

The union Y =
r1 = x1 ,

S

i∈I4

21

Yi is disjoint and a subrack of Ox . We take

r2 = xα1 (ζ)xα1 +α2 (ζ)x2α1 +α2 (1) ∈ UF ⊲ x2 ,

(ℓ = 1, f = 0),

r3 := xα1 (ζ 2 )x2α1 +α2 (1) ∈ UF ⊲ x3 ,
2

(ℓ = f = 0),
F

r4 := xα1 (1)xα1 +α2 (1)x2α1 +α2 (ζ ) ∈ U ⊲ x4 ,

(ℓ = 1, f = 0).

We claim that xα1 (a)xα1 +α2 (b)x2α1 +α2 (c) and xα1 (e
a)xα1 +α2 (eb)x2α1 +α2 (e
c) do
2
2
e
e
not commute, for a, b, c, e
a, b, e
c ∈ Fq such that ce
a +e
a b 6= e
ca+a b. This follows
from the formula:
xα1 (a)xα1 +α2 (b)x2α1 +α2 (c)xα1 (e
a)xα1 +α2 (eb)x2α1 +α2 (e
c) =

xα1 (a + e
a)xα1 +α2 (b + eb)x2α1 +α2 (c + e
c)x3α1 +α2 (ce
a+e
a2 b)x3α1 +2α2 (b2 e
a + ceb).

Hence, ri rj 6= rj ri for i 6= j, i, j ∈ I4 and the class Ox is of type F.
By [9], the remaining class can be represented by any of
r1 = xα2 (1)x2α1 +α2 (1)x3α1 +α2 (ζ),
r2 = rα1 ⊲ r1 = xα2 (ζ)xα1 +α2 (1)x3α1 +α2 (1)x3α1 +2α2 (ζ).
∨ 2
Let t := α∨
1 (ζ)α2 (ζ ) and

x := t ⊲ r1 = xα2 (ζ)x2α1 +α2 (ζ)x3α1 +α2 (ζ 2 ),
y := t ⊲ r2 = xα2 (ζ 2 )xα1 +α2 (ζ)x3α1 +α2 (ζ)x3α1 +2α2 (1).
It is easier now to work with a different ordering of the positive roots:
α1 < α2 < 2α1 + α2 < α1 + α2 < 3α1 + α2 < 3α1 + 2α2 .
Let Yi = UF ⊲ ri , i ∈ I2 , Y3 = UF ⊲ x, Y4 = UF ⊲ y. A direct computation
shows that
[
Y1 =
xα2 (1)x2α1 +α2 (1 + ℓ2 )xα1 +α2 (ℓ)x3α1 +α2 (ζ + ℓ3 + ℓ)UF3α1 +2α2 ,
ℓ∈F4

Y2 =

[

xα2 (ζ)x2α1 +α2 (ℓ2 ζ)xα1 +α2 (1 + ℓζ)x3α1 +α2 (ℓ2 + ℓ3 ζ + 1)UF3α1 +2α2 ,

ℓ∈F4

Y3 =

[

xα2 (ζ)x2α1 +α2 (ζ + ℓ2 ζ)xα1 +α2 (ℓζ)x3α1 +α2 (ℓ3 ζ + ℓζ + ζ 2 )UF3α1 +2α2 ,

ℓ∈F4

Y4 =

[

xα2 (ζ 2 )x2α1 +α2 (ℓ2 ζ 2 )xα1 +α2 (ℓζ 2 + ζ)x3α1 +α2 (ℓ3 ζ 2 + ℓ2 ζ + ζ)UF3α1 +2α2 .

ℓ∈F4

The union Y =

S

i∈I4

Yi is disjoint and a subrack of O. We take

r3 := xα2 (ζ)x2α1 +α2 (1)xα1 +α2 (1) ∈ Y3 ,
r4 := xα2 (ζ 2 )x2α1 +α2 (ζ 2 )xα1 +α2 (1)x3α1 +α2 (ζ 2 ) ∈ Y4 ,

(ℓ = ζ 2 ),
(ℓ = 1).

22

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

By looking at the coefficient of x3α1 +2α2 in the expression of each product,
we verify that ri ⊲ rj 6= rj ⊲ ri if i 6= j, hence O is of type F.

5. Unipotent classes in Steinberg groups
In this Section we deal with unipotent classes in Steinberg groups, i.e.
3
2
PSUn (q), n ≥ 3 ; PΩ−
2n (q), n ≥ 4; D4 (q) and E6 (q). In order to apply
inductive arguments as in Section 4, we first need information about the
unitary groups PSUn (q), including the non-simple group PSU3 (2).
5.1. Unipotent classes in unitary groups. Here G = PSUn (q), G =
SUn (q), n ≥ 3 and G = SLn (k), for n ≥ 2. For a clearer visibility of the
behaviour of the conjugacy classes, we use the language of matrices and
partitions. Here we choose B, U, as the subgroups of upper triangular, respectively unipotent upper triangular, matrices. We start by some notation
and basic
 facts.
1

⋄ Jn =

1

.
..

= J−1
n ∈ GLn (k).

⋄ Frq is the Frobenius endomorphism of GLn (k) raising all entries of the
matrix to the q-th power.
⋄ F : GLn (k) → GLn (k), F (X) = Jn t(Frq (X))−1 Jn , X ∈ GLn (k).

⋄ GUn (q) = GLn (k)F , SUn (q) = SLn (k)F ≤ SLn (q 2 ), [16, 21.14(2),
23.10(2)].
⋄ To every unipotent class in SUn (q) we assign the partition of n corresponding to the class in GLn (q) it is embedded into.
⋄ Every unipotent class in GLn (k) meets GUn (q) in exactly one class, since
CGLn (k) (x) is connected for every x [15, 8.5], [21, I.3.5]. In other words,
every partition comes from a class in SUn (q).
⋄ Since SUn (q) is normal in GUn (q), [1, Remark 2.1] says that all unipotent
class in SUn (q) with the same partition are isomorphic as racks.
n−d
⋄ For d ≤ n with d ≡ n mod 2 and
 h = 2 , we denote by Md ≤ G the
idh
A
subgroup of matrices
with A ∈ SLd (k). So MFd ≃ SUd (q).
idh

A
 
B
idn−2c
⋄ For c ≤ n2 we denote by H2c ≤ G the subgroup of matrices
C
D

A B ∈ SL (k). Then HF ≃ SU (q).
for C
2c
2c
2c
D

⋄ If q is odd, then GFrq = SOn (q). If q and n are even, then GFrq = Spn (q).
Here is the main result of this Subsection:

Proposition 5.1. Let O =
6 {e} be a unipotent class in G = PSUn (q) with
partition λ, where λ is different from (2, 1, . . .) if q is even. Then O is not
kthulhu.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

23

Proof. First, we reduce our analysis to G = SUn (q) by the isogeny argument
[1, Lemma 1.2]. Thus, from now on O is a unipotent class in G. Second,
we split the proof for q odd in §5.1.1 and for q even in §5.1.2. In each of
these, we distinguish several cases according to the partition λ = (λ1 , . . . , λn )
associated to O.
5.1.1. Proof of Proposition 5.1 when q is odd.
Lemma 5.2. If λ1 ≥ 3, but λ 6= (3, 1) in SU4 (3), then O is not kthulhu.
Proof. If λ1 is even, we may find u in O ∩ (HFλ1 × MFn−λ1 ) such that its
component in HFλ1 is regular. If λ1 and n are odd, then we may find u
in O ∩ (HFn−λ1 × MFλ1 ) whose component in MFλ1 is regular. In both cases
Proposition 3.6 applies.
It remains the case when λ1 is odd and n is even. Then there is i > 1 such
that λi is odd, and l = λ1 + λi ≥ 4. We take i minimal with this property.
Then, we may find u in O ∩ (HFl × MFn−l ), whose component in HFl has
Fr
partition (λ1 , λi ). We consider H := HF ∩ H q ∼
= SOl (q). Since (λ1 , λi ) is
l

l

an orthogonal partition, we may assume u ∈ H. If l ≥ 6 the class OuH is of
type D by Theorem 3.3 and Proposition 4.3, since SO6 (k) is SL4 (k) up to
isogeny.
Let l = 4 and q > 3. Now SOl (k) is SL2 (k) × SL2 (k) up to isogeny, the
class OuH is isomorphic as a rack to the product X × X for X the non-trivial
unipotent class in SL2 (q). By Lemma 2.1, OuH is of type D.
If q = 3 and n > 4, then the partition either contains the sub-partition
(3, 3) or (3, 1, 1, 1). Reducing to the subgroup M6 , we look at the classes
(3, 3) and (3, 1, 1, 1) in SU6 (q). Since SO6 (q) < SU6 (q) and SO6 (k) is
SL4 (k) up to isogeny, these racks contain a subrack isomorphic to a nontrivial unipotent class in SL4 (q). Then Theorem 3.3 applies.

Lemma 5.3. If λ = (3, 1), G = SU4 (3), then O is of type D.
Proof. Let ζ be a generator of
 6 2 
2ζ ζ
5 0
2
0 ζ6 1

O. Let t :=  02 ζζ 2

ζ
0 
ζ5
ζ7

F×
9.

We may assume that r :=

∈ SU4 (3) and let s = t ⊲ r =

1
0
0
0

ζ
1
0
0

ζ 1
0 −ζ 3
1 −ζ 3
0 1

0 1 0 1
2212
0002
0012

!

∈

∈ O. A

direct computation shows that (rs)2 6= (sr)2 . The subgroup H = hr, si ⊂


B
| A, D ∈ SL2 (9)}. If s ∈ OrH , then ( 02 12 ) and 10 1ζ would be
{ A0 D
conjugate in SL2 (9). But ( 12 01 ) ⊲ ( 02 12 ) = ( 10 11 ) which is not conjugate to

1ζ
because ζ is not a square. Hence, OrH 6= OsH and O is of type D. 
01

24

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

By Lemmata 5.2 and 5.3, there remain the partitions (2a , 1b ), a > 0.
Lemma 5.4. If q > 3 and λ1 = 2, then O is of type D.
Proof. Assume that n is odd. Then the partition contains (2, 1) and we
may find a representative whose component in MF3 has partition (2, 1). It
is therefore
to prove the statement for G = SU3 (q) and λ = (2, 1).
 1 0enough

a
q+1 |ξ ∈ F× } and
1 0
Let r =
, aq = −a. As F×
∈ O with a ∈ F×
q = {ξ
q2
q2
1

2
such that −a2 ξ q+1 ∈ F×
− ({2} ∪ (F×
q > 3, we may pick ξ ∈ F×
q ) ). Let
q2
 0 q0 1 
t ∈ G be the diagonal matrix (ξ, ξ q−1 , ξ −q ), σ = 0 −1 0 ∈ G and
1 0 0
 1

0
1
s := (σt) ⊲ r =
∈ O.
1+q
01

aξ

−a2 ξ q+1 ,

(rs)2

(sr)2 .

Since 2 6=
6=
Let η ∈ k be such that η 2 = a−1 .
Conjugating by the diagonal matrix (η, η −1 ) we have


  1 0
1 0
11 ,
.
≃
H := hr, si ≃ ( 10 a1 ) , aξ q+1
01
a2 ξ q+1 1
1

By [23, Theorem 6.21, page 409], H ≃ SL2 (q). Since −a2 ξ q+1 is not a
square, OrH 6= OsH . Thus O is of type D.
Assume that n is even. Then the partition contains either (2, 2) or (2, 1, 1)
and we may use M4 to reduce to λ = (2, 2) or (2, 1, 1) in G = SU4 (q). If
λ = (2, 2), which is an orthogonal partition, then we may assume that the
SO (q) ∼
representative u lies in SO4 (q), and Ou 4
= X × X, where X is the
non-trivial unipotent class in SL2 (q). Hence, 
it is of type D.
100a
100
10
1

If λ = (2, 1, 1), then we take r =

satisfying
∈ O for a ∈ F×
q2

, such
aq = −a. Let t ∈ G be the diagonal matrix (ξ, ξ −1 , ξ q , ξ −q ) for ξ ∈ F×
! q2

that

−a2 ξ q+1

∈

F×
q

is not a square in

F×
q ,

such that ζ q = −ζ. We consider
ζ ∈ F×
q2
 1
0
s := (σt) ⊲ r =
0

and let σ =

aξ 1+q

and we proceed as for n odd.

1
01
001



0
0
0
1

0 0
ζ 0
0 ζ −q
0 0

1
0
0
0

∈ G with

∈O


Lemma 5.5. If q = 3 and λ1 = λ2 = 2, then O is of type D.
Proof. By reducing to the subgroup H4 , it is enough to consider λ = (2, 2)
in G = SU4 (3). Let ζ be a generator of F×
9 . Let
!
!
!
2
3
r=

1ζ 0 0
10 0
1 −ζ 3
1

∈ O, σ =

ζ
0
0
0

0
0
1
0

0
1
0
0

0
0
0
ζ2

∈ G, s = σ ⊲ r =

10ζ 0
1 0 −ζ
1 0
1

.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

25
F

F

It is easy to see that (rs)2 6= (sr)2 . In addition, hr, si ⊂ UF and OrU 6= OsU ,
whence the statement.

Lemma 5.6. If λ = (2, 1, . . .) in G = SUn (3), n ≥ 3, then O is of type C.
Proof. Let F×
9 = hζi. Without loss of generality we may assume that
!
2
1

ζ

r=

..

.

= idn +ζ 2 e1,n ∈ O.

1

We consider, for n odd, respectively even, the following element of SUn (3):
 0
0
0 0
0
ζ
 0

0
0
0
ζ
0
0
0 id n−4 0 0
0 id n−3 0
0
0
2




2
0
0
ζ
0
0
0


0
0
−ζ 2
0
0
.
σ := 
τ
:=

−3
0
0ζ
0
0
 0

 00
0
0 id n−3 0 
0
0
0 0 id
n−4
2

2

ζ −3

0

0

0

0

ζ −3

0

0

0

0

0

1

..

Accordingly, we set s := σ ⊲ r or s := τ ⊲ r. In both cases, s =

.
1

ζ −2

!

=

idn +ζ −2 en,1 . Then rs 6= sr. Let H := hr, si. We have


 
2
H ≃ h 10 ζ1 , ζ12 10 i ≃ SL2 (3).

Conjugation by diag(ζ −1 , ζ) and [23, Theorem 6.21, page 409] give H ≃
h( 10 11 ) , ( 11 01 )i ≃ SL2 (3), so OrH 6= OsH . We conclude by [3, Lemma 2.7]. 
5.1.2. Proof of Proposition 5.1 when q is even.

Lemma 5.7. If O is a regular unipotent class, then it is not kthulhu.
Proof. By Proposition 3.6 it is enough to deal with regular unipotent classes
in SU3 (q) for q = 2, 8, SU4 (q) for q = 2, 4 and SUn (2) for n ≥ 5.
(i) Regular unipotent classes in SU3 (22h+1 ), h ∈ N0 , are of type D.
It suffices to prove the claim for G = SU3 (2) ≤ SU3 (22h+1). Let ζ be
11ζ
a generator of F×
0 1 1 . Let
4 . Consider the class O represented by r =
001




00 1
1 00
2
t= 01ζ
∈ SU3 (2) and s := t⊲r = 12 1 0 ∈ O. By direct verification,
1ζ ζ

(rs)2

6=

(rs)2 .

ζ 11

hr,si

A computation with GAP shows that Os

hr,si

6= Or

.

(ii) Regular unipotent classes in SUn (q), n ≥ 4 even, are not kthulhu.
Indeed, by the Jordan form theory, O is represented by an element of a
regular class in Spn (q) = SUn (q)Frq . We conclude invoking Theorem 3.3.
(iii) Regular unipotent classes in SUn (2), n ≥ 5 odd, are not kthulhu.

26

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

By projecting a representative in UF to M5 , we obtain a regular unipotent
class in the latter. Hence, it is enough to assume n = 5. Let P be the
standard F -stable parabolic subgroup of G with Levi factor L corresponding
to the simple roots α2 , α3 , and let π : PF → LF be the projection. Any
u ∈ O∩UF lies in P := PF and π(u)is regular in LF , which is the subgroup of
det A
matrices of the form
, where A ∈ GU3 (2), so LF ≃ GU3 (2).
A
F

det A

L
is not kthulhu. Then Lemma 2.5 applies.
By Proposition 3.6, Oπ(u)



Now we argue inductively starting from Lemma 5.7.
Lemma 5.8. If any of the following conditions holds, then O is not kthulhu.
(a)
(b)
(c)
(d)

There is j such that λj ≥ 4 is even.
n is odd and there is j such that λj ≥ 3 is odd.
There is i such that λi = λi+1 > 2.
n is even and there are i, j such that λj > λi ≥ 3 and are both odd.

Proof. (a), (b): Apply Lemma 5.7 either to Hλj or to Mλj .
(c): The class with partition (λi , λi ) in HF2λi has a representative in
(HF2λi )Frq ≃ Sp2λi (q). Its class has label W (λi ) or V (λi ) ⊕ V (λi ) hence
Theorem 3.3 applies.
(d): By considering the class with partition (λj , λi ) in Mλi +λj we may
assume n = λi + λj . Let d = λj − λi and let P be the parabolic subgroup
with standard Levi subgroup of type Aλi −1 × Aλi −1 associated with
{αk ∈ ∆ | k ∈ Iλi −1 ∪ Iλj +1,λj +λi −1 }.
Then M ≃ SLλi (q 2 ). By Lemma 3.2 and Theorem 3.3 it is enough to show
that O ∩ V = ∅. Now, if u ∈ V then it is of the form


idλi A1 A2


B A3  ,
 0
0
0 idλi

for some upper-triangular B ∈ SUd (q) and some matrices Ai , i ∈ I3 . Hence,
!
A1
A2
(5.1)
≤ d + λi .
rk(u − id) = rk
B − idd A3
On the other hand, if u ∈ O by Jordan form theory we have
(5.2)

rk(u − id) = λi + λj − 2 = 2λi + d − 2.

As λi ≥ 3, conditions (5.1) and (5.2) are not compatible, so O ∩ V = ∅.



Lemma 5.9. If there is i such that λi = λi+1 = 2, then O is not kthulhu.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

27

Proof. Looking at HF2λi , we reduce to the partition (2, 2) in SU4 (q). Let u
be a representative of a unipotent class with label V (2) ⊕ V (2) in Sp4 (q) =
SU4 (q)Frq ≤ SU4 (q). By Jordan form theory, we may assume that u ∈ O.
Sp (q)
By Theorem 3.3, Ou 4 is not kthulhu, whence the statement.

By Lemmata 5.7, 5.8 and 5.9 there remain the partitions: (2, 1a ) for all
n ≥ 3, and (λ1 , 2, 1a ), (λ1 , 1a ) for λ1 > 1 odd and n even.
Lemma 5.10. If n is even and λ = (λ1 , 2, 1a ), where 1 < λ1 is odd, then O
is of type D.
Proof. It is enough to consider λ = (λ1 , 2, 1). We pick a representative of
O lying in HF2 × MFλ1 +1 ≃ SL2 (q) × SUλ1 +1 (q). Then O contains a subrack
isomorphic to X × Y where X 6= {e} is a unipotent class in SL2 (q) and Y
is a unipotent class with partition (λ1 , 1) in SUλ1 +1 (q). The latter is not a
class of involutions because λ1 > 2. By [24, 1.4(ii)] and Remark 2.2 there are
y1 6= y2 ∈ Y such that y1 y2 = y2 y1 . Now Lemmata 3.7 and 2.1 apply.

Lemma 5.11. If n is even and λ = (λ1 , 1, . . .) for some 3 < λ1 odd, then
O is not kthulhu.
Proof. It is enough to deal with the partition (λ1 , 1). Set d := (λ1 +1)/2 > 2.
Let P be the parabolic subgroup with standard Levi subgroup associated
with ∆ − αd . Then M ≃ SLd (q 2 ). We claim that O ∩ V = ∅. Indeed, if
u ∈ V then it is of the form
!
idd A
,
0 idd
for some A, so rk(u − 1) ≤ d. If, in addition, u ∈ O, then rk(u − id) = λ1 − 1.
This is impossible because λ1 > 3. Lemma 3.2 and Theorem 3.3 apply. 
Lemma 5.12. If n ≥ 6 is even and λ = (3, 1, 1, 1, . . .), then O is of type D.
Proof. It is enough to deal
the 
partition (3, 1, 1, 1) in SU6 (q). Let
 1with
x001 x


x ∈ F×
− F×
q and let r =
q2
σ=

0 0 1 0 0 0
1
0
0
0
0

0
1
0
0
0

0
0
0
0
0

0
0
0
0
1

0
0
1
0
0

0
0
0
1
0

1 000
100
10
1

1
0
0
xq
1

 ∈ O,

= s1 s5 s2 s4 ∈ SU6 (q),

s =σ⊲r =

1 0



0 0 0 0
1x1 x 0
1 0 1 0 ∈

1 xq 0
1 0
1
hr,si
hr,si
Or
6= Os .

O.

Since r, s ∈ U, the discussion in [1, 3.1] shows that
By
2
2
looking at the (1, 5)-entry, we see that (rs) 6= (sr) and O is of type D. 

28

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Lemma 5.13. If q > 4 and λ = (3, 1) in SU4 (q), then O is of type F.
Proof. Let x, y ∈ Fq2 such that xq y + y q x 6= 0 and ζi ∈ F×
q , for i ∈ I4 ,
satisfying ζi 6= ζj if i 6= j. Let


!
!
1 xζi−1 yζi xy q
1
1 x y xy q
ζi
1
0 y q ζi 
1 0 yq
, ti =
, ri := ti ⊲ r = 
r=
.
−1
q
−1
ζi

1 x
1

1 xq ζi
1

1

Then r, ri ∈ O, since ti ∈ SU4 (q). Let H := hr1 , r2 , r3 , r4 i ⊂ UF . By the
discussion in [1, 3.1] that OrHi 6= OrHj if i 6= j. Then ri rj = rj ri if and only if
(xq y + xy q )(ζi ζj−1 + ζi−1 ζj ) = 0, if and only if ζi = ζj .

Lemma 5.14. The unipotent classes of type (3, 1) in SU4 (2) are of type D.
!

Proof. Let ζ be a generator of F×
4 . We may assume that r =
Let t =

0 0 1 0
0001
1000
0110

∈ SU4 (2) and s := t ⊲ r =
hr,si

(rs)2 6= (rs)2 . By GAP we see that Os

hr,si

6= Or

1
0
ζ2
0

1
1
ζ
ζ

0
0
1
0

0
0
1
1

1
0
0
!0

1
1
0
0

ζ
0
1
0

ζ
ζ2
1
1

∈ O.

∈ O. Then

and the claim follows.



Lemma 5.15. The unipotent classes of type (3, 1) in SU4 (4) are of type D.
!

Proof. Let ζ be a generator of
 11 2 5 14 
Let t =

ζ
11
ζ
0
0

ζ
ζ2
ζ 14
ζ 14

ζ
ζ8
ζ9
ζ9

check at once that

F×
16 .

We may assume that r =

ζ
ζ2 
∈ SU4 (4) and s := t ⊲ r =
ζ 11
ζ6
(rs)2 6= (rs)2 , and with GAP that

1
0
0
0

1
1
0
0

ζ
0
1
0

!

ζ
ζ4
1
1

∈ O.

0 1 0 ζ 12
1 0 ζ 3 ζ 10
∈ O.
00 0 1
00 1 0
hr,si
hr,si
Os
6= Or .

The Proposition is now proved.

We



The remaining classes could not be reached with our methods.
Lemma 5.16. If λ = (2, 1a ) in SUn (q), then O is austere, hence kthulhu.
Proof. We show that any subrack generated by two elements is either abelian
⋆

or indecomposable. Let r, s ∈ O, rs 6= sr. We may assume r = idn +ae1,n =
xβ (a) where β is the highest positive root in Φ and a ∈ F×
q . Let g ∈ G be
−1
F
such that s = grg . By [16, 24.1] there are u, v ∈ U , and σ ∈ G ∩ N (T)
such that g = uσv. As F (σ) = σ, the coset σ = σT ∈ W lies in W F ≃ SFn
which is the centralizer of the permutation
n
n
(1, n)(2, n − 1) · · · ([ ], n + 1 − [ ]);
2
2

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

29

hence, either σ({1, n}) = {1, n} or σ({1, n}) ∩ {1, n} = ∅. Since r is central
in UF , s = uσrσ −1 u−1 = uxσ(β) (a′ )u−1 for some a′ ∈ Fq . Since ru = ur and
rv = vr, ⋆ holds if and only if r 6= σrσ −1 . Thus, σ(1) = n and σ(n) = 1, so
σ is of the form


0 0 ξ
q−1
0
A
0
σ=
,
where A ∈ GUn−2 (q),
ξ ∈ F×
= det A.
q , ξ
−q
ξ

Then

σrσ −1

0 0

= idn +aξ −1−q en,1 , so
H := hr, si ≃ h( 10 a1 ) ,



1
ξ −q−1 a

0
1


i ⊂ SL2 (q).

Since the non-trivial unipotent class in SL2 (q) is sober [1, 3.5], OrH = OsH .

5.2. Unipotent classes in PΩ−
2n (q), n ≥ 4. In this subsection G =
−
PΩ2n (q), n ≥ 4. We shall use the knowledge of unipotent conjugacy classes
in PSLn (q) and PSUn (q) and apply inductive arguments.
Here G is assumed simply-connected. The root system of G is of type
Dn , and the Dynkin diagram automorphism ϑ interchanges αn−1 and αn ;
it fixes the basis vectors εj for j ∈ In−1 , and maps εn to −εn . Here is the
main result of this Subsection:
Proposition 5.17. Let O be a non-trivial unipotent class in PΩ−
2n (q) with
n ≥ 4. Then O is not kthulhu.
We split the proof for q odd in §5.2.1 and for q even in §5.2.2.
5.2.1. Proof of Proposition 5.17 when q is odd.
Proof. Let P1 and P2 be the standard F -stable parabolic subgroups with
F -stable Levi factors L1 and L2 associated respectively with Π1 := ∆ −
{αn−1 , αn } (of type An−2 ), and Π2 := {αn−2 , αn−1 , αn } (of type A3 ). Then
+
Φ+
Π1 = {εi − εj | i < j ∈ In−1 }, ΦΠ2 = {εi ± εj | i < j ∈ In−2,n },

Ψ Π1 = Φ + r Φ +
Π1 = {εi + εj , εk − εn | i < j ∈ In , k ∈ In−1 },
ΨΠ2 = {εi ± εj | i < j, i ∈ In−3 , j ∈ In }.
By Lemma 3.2, Theorem 3.3 and Proposition 5.1, it is enough to show that
O ∩ UF 6⊂ V1 ∩ V2 . Assume that there is u ∈ O ∩ V1 ∩ V2 ; then
supp u ⊂ ΨΠ1 ∩ ΨΠ2 = {εi + εj , εi − εn | i < j, i ∈ In−3 , j ∈ In }.
We consider various different cases.
(i) εi − εn ∈ supp u for some i ∈ In−3 .

30

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Then sεi −εn−2 (supp u) ⊆ Φ+ . Since sεi −εn−2 ∈ W F , it has a representative ṡεi −εn−2 ∈ NGF (T); hence ṡεi −εn−2 ⊲ u ∈ O ∩ UF and εn−2 − εn ∈
supp(ṡεi −εn−2 ⊲ u). Thus ṡεi −εn−2 ⊲ u ∈ UF ∩ O − V2 .
(ii) εi − εn 6∈ supp u for all i ∈ In−3 .
Then there exist k ∈ In−3 and j such that εk + εj ∈ supp u. Let
ℓ = max{j | εk + εj ∈ supp u for some k}.
If ℓ = n, then pick a representative σ ∈ NGF (T) ∩ L2 of sεn−1 −εn sεn−1 +εn ∈
W F . Thus σ ⊲ u ∈ O ∩ V2 and εk − εn ∈ supp(σ ⊲ u) for all k such that
εk + εn ∈ supp u. Therefore, either supp(σ ⊲ u) 6⊂ V1 , and we are done, or
supp(σ ⊲ u) ⊂ V1 and εk − εn ∈ supp(σ ⊲ u), and we fall in (i).
If ℓ = n − 1, then pick a representative σ ∈ NGF (T) ∩ L2 of sεn−2 +εn−1 ∈
W F . As above, σ ⊲ u ∈ O ∩ V2 , and εi − εn−2 ∈ supp(σ ⊲ u) ∩ ΦΠ1 for some
i < n − 2. That is, supp(σ ⊲ u) 6⊂ V1 .
Finally, if ℓ < n−1, then we pick a representative σ ∈ NGF (T) of sεℓ −εn−1 .
Then supp σ ⊲ u ⊂ V1 ∩ V2 , and we fall in the case ℓ = n − 1.

5.2.2. Proof of Proposition 5.17, q even. Here Lemma 3.2 does not apply
in its full strength because of the existence of kthulhu classes in PSU4 (q),
q even, and in PSL3 (2). We proceed by induction on n. The case n = 4,
Lemma 5.18 below, requires a special treatment.
Lemma 5.18. If G = PΩ−
8 (q) with q even, then O is not kthulhu.
Proof. Let us consider the F -stable standard parabolic subgroups P1 , P2
with standard Levi subgroups L1 and L2 associated with the sets Π1 =
{α1 , α2 } and Π2 = {α2 , α3 , α4 }, respectively. Let u ∈ O ∩ UF . We analyse different situations, according to ∆ ∩ supp u. Recall that, u being F invariant, the simple root α3 ∈ supp u if and only if α4 ∈ supp u.
(i) α2 , α3 , α4 ∈ supp u.
The projection π2 (u) ∈ L2 is regular, thus OπM22(u) is isomorphic as a rack to
a unipotent class in SU4 (q) of partition (4) and Proposition 5.1 applies.
(ii) ∆ ∩ supp u = {α1 , α3 , α4 } or ∆ ∩ supp u = {α3 , α4 }.
Then OπM22(u) has partition (2, 2) or (3, 1) and Proposition 5.1 applies.
(iii) ∆ ∩ supp u = {α1 } or ∆ ∩ supp u = {α2 }.
Here π1 (u) ∈ L1 is not regular, hence OπM11(u) is isomorphic as a rack to a
unipotent class in SL3 (q) with partition 6= (3); Theorem 3.3 applies.
(iv) ∆ ∩ supp u = {α1 , α2 }: either α2 + α3 + α4 ∈ supp u or not.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

31

We may assume that α2 + α3 6∈ supp u, by conjugating with a suitable
element in (Uα3 Uα4 )F and using (2.1). If α2 + α3 + α4 ∈ supp u, then
SU (q)
OπM22(u) ≃ Ov 4 , where rk(v − id) = 2 and (v − id)2 = 0, which is not
kthulhu since its partition is (2, 2). If α2 + α3 + α4 6∈ supp u, then pick
a representative σ ∈ NGF (T) of s3 s4 ∈ W . Then σ ⊲ u ∈ O ∩ UF and
∆ ∩ supp(σ ⊲ u) = {α1 } so we reduce to (iii).
(v) ∆ ∩ supp u = ∅ and α1 + α2 ∈ supp u or α2 + α3 ∈ supp u.
In the first case, OπM11(u) has type (2, 1), and Theorem 3.3 applies. In the
second, also α2 + α4 ∈ supp u and OπM22(u) has type (2, 2). Indeed, OπM22(u) ≃
SU4 (q)

Ov

, where rk(v − id) = 2 and (v − id)2 = 0. We invoke Proposition 5.1.

(vi) (∆ ∪ {α1 + α2 , α2 + α3 , α2 + α4 }) ∩ supp u = ∅.
Let ṡi ∈ NGF (T) be a representative of si , i ∈ I2 . If α1 + α2 + α3 ∈ supp u,
then also α1 + α2 + α4 ∈ supp u. Now ṡ1 ⊲ u ∈ UF ∩ O, ∆ ∩ supp(ṡ1 ⊲ u) = ∅
and α2 + α3 ∈ supp(ṡ1 ⊲ u), so we fall in (v). Let σ be as in (iv). If
α2 + α3 + α4 ∈ supp u, then σ ⊲ u ∈ O ∩ UF and α2 ∈ supp(σ ⊲ u) and we
are in case (iii).
(vii) supp u ⊂ {α1 + α2 + α3 + α4 , α1 + 2α2 + α3 + α4 }.
If α1 + α2 + α3 + α4 ∈ supp u, then ṡ1 ⊲ u is as in case (vi); while if
supp u = {α1 + 2α2 + α3 + α4 }, then supp(ṡ2 ⊲ u) = {α1 + α2 + α3 + α4 }. 
We now proceed with the recursive step and assume that all non-trivial
unipotent classes in a twisted group with root system Dn−1 are not kthulhu.
Let P1 and P2 be the standard parabolic subgroups with F -stable standard
Levi subgroups L1 and L2 associated with the sets Π1 = {αi | i ∈ In−2 } and
Π2 = {αi | i ∈ I2,n }, of type An−2 and Dn−1 respectively. By Lemma 3.2 in
order to prove the inductive step, it is enough to show that no non-trivial
unipotent class O in GF satisfies O ∩ UF ⊂ V1 ∩ V2 . As usual let
ΦΠ1 = {εi − εj | i < j ∈ In−1 },

ΦΠ2 = {εi ± εj | i < j ∈ I2,n },

ΨΠ1 = {εi − εn , εj + εk | i ∈ In−1 , j < k ∈ In }, ΨΠ2 = {ε1 ± εj | j ∈ I2,n }.
Let u ∈ O ∩ V1 ∩ V2 . Then supp u ⊂ ΨΠ1 ∩ ΨΠ2 = {ε1 − εn , ε1 + εj | j ∈ I2,n }.
Let ṡi ∈ NGF (T) be a representative of si ∈ W F , i ∈ I2 . If supp u 6= {ε1 +ε2 }
then ṡ1 ⊲ u ∈ O ∩ UF , but ṡ1 ⊲ u 6∈ V2 (look at its support). If, instead,
supp u = {ε1 + ε2 } then ṡ1 ṡ2 ⊲ u ∈ O ∩ UF ∩ Uε2 +ε3 , so ṡ1 ṡ2 ⊲ u 6∈ V1 .
This finishes the proof for q even and Proposition 5.17 is now proved.



5.3. Unipotent classes in 2 E6 (q). We deal now with the group 2 E6 (q).
Here the Dynkin diagram automorphism ϑ interchanges α1 with α6 and α3
with α5 . Here is the main result of this Subsection:

32

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Proposition 5.19. Let O =
6 {e} be a unipotent class in 2 E6 (q). Then O is
not kthulhu.
We give the proof for q odd in §5.3.1 and for q even in §5.3.2. Let P1
and P2 be the F -stable standard parabolic subgroups with standard Levi
subgroups L1 and L2 associated with Π1 = ∆ − {α2 } (of type A5 ) and
Π2 = {α2 , α3 , α4 , α5 } (of type D4 ). Then ΨΠ1 , respectively ΨΠ2 , consists of
all positive roots containing α2 , respectively at least one of α1 and α6 .
5.3.1. Proof of Proposition 5.19, q odd. Here Lemma 3.2 (c) applies softly
to the F -stable parabolic subgroups.
Proof. By Lemma 3.2, Propositions 5.1 and 5.17, it is enough to show that
P
P
O ∩ UF 6⊂ V1 ∩ V2 . Let β = 4i=1 αi , γ = 6i=1 αi ; thus ϑ(β) = α2 + α4 +
α5 + α6 . Let u ∈ O ∩ UF lying in V1 ∩ V2 . Then
supp u ⊂ ΨΠ1 ∩ ΨΠ2 = Ψ(β) ∪ Ψ(ϑ(β)) = Σ ∪ ϑ(Σ) ∪ Ψ(γ);
here Σ = {βj | j ∈ I0,3 } and Ψ(γ) = {γj | j ∈ I0,6 }, where
β0 = β,

β1 = s5 β0 ;

β2 = s4 β1 ;

β3 = s3 β2 ;

γ0 = γ,

γ1 = s4 γ0 ;

γ2 = s3 γ1 ;

γ3 = s 5 γ1 ;

γ4 = s 5 γ2 = s 3 γ3 ;

γ5 = s4 γ4 ;

γ6 = s2 γ5 .

Let ẇ ∈ NGF (T) be a representative of w ∈ W F . If either βj or ϑ(βj ) ∈
supp u for j ∈ I0,3 , then ẇj ⊲ u ∈ O ∩ UF − V1 , where w0 = w1 = s2 ,
w2 = s2 s4 , w3 = s2 s4 s5 s3 . Thus we may assume that supp u ⊂ Ψ(γ).
If γ0 ∈ supp u, then ṡ2 ⊲ u ∈ O ∩ UF , γ − α2 ∈ supp(ṡ2 ⊲ u) − ΨΠ1 .
Now we argue inductively. Suppose that γi ∈ supp u for some i ∈ I0,j−1
implies that O is not kthulhu. Assume that γi 6∈ supp u for i ∈ I0,j−1 and
γj ∈ supp u. We claim that there is ωj ∈ W F with ω̇j ⊲ u ∈ O ∩ UF and
either supp(ω̇j ⊲ u) 6⊂ Ψ(γ) (a case settled above), or γl ∈ supp(ω̇j ⊲ u) for
some l ∈ I0,j−1 , where the recursive hypothesis applies. The claim holds,
taking ω1 = ω5 = s4 , ω2 = ω3 = s1 s6 , ω4 = s3 s5 , w6 = s2 .

5.3.2. Proof of Proposition 5.19, q even. Here, the use of Lemma 3.2 is
hampered by the presence of kthulhu classes in PSU6 (q).
Proof. As we have shown in the odd case, §5.3.1, there is u ∈ O ∩ UF such
that u 6∈ V1 ∩ V2 . If u 6∈ V2 , then the result follows from Proposition 5.17.
Let us assume that u ∈ V2 − V1 . In particular, α3 , α4 , α5 , α3 + α4 , α4 + α5 ,
α3 + α4 + α5 6∈ supp u. Then OπM11(u) is non-trivial.

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES

33
SU (q)

If supp u ∩ ΦΠ1 6= {α1 + α3 + α4 + α5 + α6 }, then OπM11(u) ≃ Ov 6 where
rk(v − id) = 2, hence its associated partition is not (2, 1, 1, 1, 1). By Lemma
5.1, OvM1 and O are not kthulhu.
If supp u ∩ ΦΠ1 = {α1 + α3 + α4 + α5 + α6 }, then ẇ ⊲ u ∈ O ∩ UF where
w = s1 s6 ∈ W F . But α3 + α4 + α5 ∈ supp(ẇ ⊲ u), hence ẇ ⊲ u ∈
/ V2 and we
are done.

5.4. Unipotent classes in 3 D4 (q). We deal now with triality; F arises
from the graph automorphism ϑ of order 3 determined by ϑ(α1 ) = α3 . We
assume that G = Gsc . We fix and ordering of the ϑ-orbits in Φ+ . Let
2

yα (ξ) := xα (ξ)xϑα (ξ q )xϑ2 α (ξ q ),

α ∈ Φ, ϑ(α) 6= α,

ξ ∈ Fq 3 .

Every element in UF can be uniquely written as a product of elements yα (ξ),
ϑα 6= α, ξ ∈ Fq3 , and xβ (ζ), ϑβ = β, ζ ∈ Fq . Let
(5.3)

F
Υ = hx±γ (ξ), y±δ (ξ) | ϑ(γ) = γ, ϑ(δ) 6= δ, ξ ∈ F×
q i≤G .

The generators in (5.3) are the non-trivial elements in the root subgroups
with respect to TF ∩ TFrq . It is known that Υ ≃ G2 (q) ≃ GFrq .
Proposition 5.20. Every unipotent class O =
6 {e} in 3 D4 (q) is not kthulhu.
Proof. By the isogeny argument we work in G = GFsc [1, Lemma 1.2]. We
analyse different cases separately, according to q being odd, even and > 2,
or 2.
(i) q is odd.
The list of representatives of the unipotent classes in 3 D4 (q) appears in
[10, Table 3.1]; they all have one of the following forms:
xα1 +2α2 +α3 +α4 (1),

xα2 (1)yα1 +α2 +α3 (−1),

u =xα2 (1)yα1 +α2 +α3 (ζ),

yα1 +α2 +α3 (1),

yα1 (1)xα2 (1),

r =yα1 (1)yα1 +α2 (a),

where ζ ∈ Fq3 is not a square and a ∈ Fq3 − Fq . So all classes but those
of u and r have a representative in Υ ≃ G2 (q), hence they are not kthulhu
by Lemma 4.6. Now u ∈ H = hUF±α2 , y±(α1 +α2 +α3 ) (b) | b ∈ F×
i, which is
q3
3
H
isogeneous to SL2 (q) × SL2 (q ). Since Ou is the product of two non-trivial
racks and q 3 > 3, OuH is of type D by Lemmata 2.1 and 3.5.
Assume that r ∈ O. Let ξ be a generator of F×
,
q3
η = ξ q−1 ,

2

∨ q
∨ q
t = α∨
1 (η)α3 (η )α4 (η ),

s = t ⊲ r = yα1 (η 2 )yα1 +α2 (aη 2 ).

34

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

we have
By [10, Table 3.2], for every b, c ∈ F×
q3
yα1 +α2 (b)yα1 (c) = yα1 (c)yα1 +α2 (b)yα1 +α2 +α3 (bcq + cbq )
× xα1 +α2 +α3 +α4 (−(bcq

(5.4)

2 +q

× xα1 +2α2 +α3 +α4 (−(cbq

+ bq cq

2 +q

2 +1

+ cq bq

2

+ bq cq+1 ))

2 +1

2

+ cq bq+1 )).

Using (5.4) we verify that the coefficient of yα1 +α2 +α3 in the expression
of rs, respectively sr, equals aη 2q + aq η 2 , respectively aq η 2q + aη 2 . These
coefficients are equal if and only if (aq − a)(η 2q − η 2 ) = 0. As η 2(q−1) 6= 1
and aq 6= a, we have rs 6= sr, with rs, sr ∈ UF . Thus, (sr)2 6= (rs)2 , as q
is odd. Comparing the coefficients of xα1 in the expressions of r and s as
products of elements in root subgroups, we see that
UF ⊲ r ⊂ xα1 (1)hUβ |β ∈ Φ+ − {α1 }i, UF ⊲ s ⊂ xα1 (η 2 )hUβ |β ∈ Φ+ − {α1 }i.
hr,si

So Or

hr,si

6= Os

, whence Or is of type D.

(ii) q > 2 is even.
The list of representatives of the unipotent classes in G appears in [8], see
[11, Table A2]. For suitable ζ, ζ ′ ∈ Fq , the representatives are of the form
u1 = xα1 +2α2 +α3 +α4 (1), u2 = xα2 (1)xα1 +α2 +α3 +α4 (1),
u3 = yα1 +α2 +α3 (1),

u4 = yα1 +α2 (1)yα1 +α2 +α3 (1)xα1 +α2 +α3 +α4 (ζ),

u5 = yα1 (1)xα2 (1),

u6 = yα1 (1)xα2 (1)yα1 +α2 +α3 (ζ ′ ),
a ∈ Fq 3 − Fq .

u7 = yα1 (1)yα1 +α2 (a),

All classes except those like Ou7 are represented by v ∈ Υ ≃ G2 (q); thus,
P
these are not kthulhu by Lemma 4.7. We deal with Ou7 . Let γj = ji=1 αi
for shortness. We use (5.4) and the following relations from [10], cf. [11]:
2

2

yα1 (b)yγ3 (c) = yγ3 (c)yα1 (b)xγ4 (cq b + cq bq + cbq )
2

2

yγ2 (b)yγ3 (c) = yγ3 (c)yγ2 (b)xα1 +2α2 +α3 +α4 (cq b + cq bq + cbq ),
xα2 (d)xγ4 (e) = xγ4 (e)xα2 (d)xα1 +2α2 +α3 +α4 (de),
yα1 (b)xα2 (d) = xα2 (d)yα1 (b)yγ2 (bd)yγ3 (dbq+1 )xγ4 (dbq

2 +q+1

);

×
and d, e ∈ F×
here b, c ∈ F×
q . Let C ≤ Fq 3 be the cyclic subgroup of order
q3
q 2 +q+1 and D := C∩F×
q , a cyclic group of order (q−1, 3). Thus |C/D| ≥ 4.
Let ξi , i ∈ I4 , be representatives of 4 distinct cosets in C/D and let
2

ti : = α1 (ξi )α3 (ξiq )α4 (ξiq ), ri : = ti ⊲ u7 = yα1 (ξi2 )yα1 +α2 (aξi2 ) ∈ Or ∩ UF .

NICHOLS ALGEBRAS OVER UNIPOTENT CLASSES
hr ,r2 ,r3 ,r4 i

Since UF ⊲ri ⊂ yα1 (ξi2 )hUγ | γ ∈ Φ+ −∆i, we have Ori 1
for i 6= j. In addition by (5.4) we see that

35
hr ,r2 ,r3 ,r4 i

6= Orj 1

ri rj ∈ yα1 (ξi2 + ξj2 )yγ2 (a(ξi2 + ξj2 ))yγ3 (aξi2 ξj2q + aq ξi2q ξj2 ) UFγ4 UFα1 +2α2 +α3 +α4
The coefficients of yγ3 in the expressions of ri rj and rj ri are equal iff
(a + aq )(ξi2 ξj2q + ξj ξi2q ) = 0, iff (ξi ξj−1 )2(q−1) = 1 (since a 6∈ Fq ), iff i = j by
our choice of the ξi ’s. Hence, ri ⊲ rj 6= rj for i 6= j and Ou7 is of type F.
(iii) q = 2.
The description of the representatives is the same as in (ii) with ζ = 0
and ζ ′ = 1, see [11, §3], so that
u4 = yα1 +α2 (1)yα1 +α2 +α3 (1),

u6 = yα1 (1)xα2 (1)yα1 +α2 +α3 (1).

We do not have information on the unipotent classes of G2 (2) yet, so we
have to argue differently. However, the argument for u7 is exactly as for
q > 2. Now u1 ∈ hUF±α2 , UF±(α1 +2α2 +α3 +α4 ) i, a subgroup of type A2 , but it
is not regular there. Hence Ou1 is of not kthulhu by Theorem 3.3 and [16,
Theorem 24.15].
By [11, Tables A.8], we have r := yα1 (1)yγ3 (1)xα1 +2α2 +α3 +α4 (1) ∈ Ou2 .
×
3
Let ξ ∈ F×
8 such that ξ = ξ + 1. Then the roots in F8 of the polynomial
X 4 + X 2 + X are ξ, ξ 2 and ξ 4 . Their inverses, together with 1, are the roots
of the polynomial X 4 + X 2 + X + 1. Let P1 be the parabolic subgroup with
standard Levi subgroup associated with {α1 , α3 , α4 }, and, for i ∈ I4 , let
i ∨ 2i ∨ 4i
ti := α∨
1 (ξ )α3 (ξ )α4 (ξ ),

ri := ti ⊲ r = yα1 (ξ 2i )yγ3 (ξ 6i )xα1 +2α2 +α3 +α4 (1) ∈ Ou2 , so
UF ⊲ ra ⊂ yα1 (ξ 2i )V1 .
hr ,r2 ,r3 ,r4 i

Then, Ori 1

hr ,r2 ,r3 ,r4 i

6= Orj 1

for i 6= j. In addition,

ri rj = yα1 (ξ 2i + ξ 2j )yγ3 (ξ 6i + ξ 6j )xγ4 (ξ 4(j−i) + ξ 2(j−i) + ξ j−i ).
Let i 6= j. The coefficient of xγ4 in the expression of ri rj is 0 if and only if
ξ j−i ∈ {ξ, ξ 2 , ξ 4 } if and only if the coefficient of xγ4 in the expression of rj ri
is 1. Thus, Ou2 is of type F.
Let now r1 = u3 . Let σ and τ in Υ be representatives of s1 s3 s4 , s2 ∈ W F ,
respectively. Let P2 be the F -stable parabolic subgroup with standard Levi
subgroup associated with α2 . We consider the following elements in O ∩ V2 :
r2 = σ ⊲ r1 = yα1 +α2 (1),

r3 = τ ⊲ r2 = yα1 (1)

r4 = xα2 (1) ⊲ r3 = yα1 (1)yα1 +α2 (1)yα1 +α2 +α3 (1)xα1 +α2 +α3 +α4 (1).

36

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

Let Z = hUγ | γ ∈ Φ+ − {α1 , α2 , α1 + α2 }i. Then
V2 ⊲ r1 ⊂ yα1 +α2 +α3 (1)Z,

V2 ⊲ r2 ⊂ yα1 +α2 (1)Z,

V2 ⊲ r3 ⊂ yα1 (1)Z,

V2 ⊲ r4 ⊂ yα1 (1)yα1 +α2 (1)Z.

hr ,r ,r ,r i

Hence, the classes Ori 1 2 3 4 for i ∈ I4 are disjoint. A direct computation
shows that ri rj 6= rj ri for i 6= j, so Ou3 is of type F.
We deal now with u4 . Let ξ, P1 and P2 be as above and let
3 ∨ 6 ∨ 5
t1 := α∨
1 (ξ )α3 (ξ )α4 (ξ ),

∨ 2 ∨ 4
t2 := α∨
1 (ξ)α3 (ξ )α4 (ξ ),

r1 := t1 ⊲ u4 = yγ2 (ξ 6 )yγ3 (ξ 4 ),

r2 := xα2 (1)yγ3 (1)yγ4 (1),

r3 := yα1 (1)yγ3 (1),

r4 := t2 ⊲ r3 = yα1 (ξ 2 )yγ3 (ξ −1 ).

Then, ri ∈ Ou4 ∩ UF , [11, Tables A.2, A.4, A.8, A.12]. In addition,
UF ⊲ r1 ⊂ V1 ∩ V2 ,

UF ⊲ r2 ⊂ xα2 (1)V1 ∩ V2 ,

UF ⊲ r3 ⊂ yα1 (1)V1 ∩ V2 ,

UF ⊲ r4 ⊂ yα1 (ξ 2 )V1 ∩ V2 .

Hence, for H = hri | i ∈ I4 i we have OrHi 6= OrHj for i, j ∈ I4 , with i 6= j. A
direct computation shows that ri rj 6= rj ri , for i 6= j, so Ou4 is of type F.
Finally, we treat simultaneously the classes of u5 and u6 , that are of the
form x = yα1 (1)xα2 (1)yα1 +α2 +α3 (ǫ) with ǫ ∈ {0, 1} respectively. Let C be
as in the odd case and let (ξi )i∈I4 be a family of distinct elements in C. Set
2

∨ q
∨ q
ti : = α∨
1 (ξi )α3 (ξi )α4 (ξi ),
2

ri : = ti ⊲ x = yα1 (ξi2 )xα2 (1)yα1 +α2 +α3 (ǫξi1+q−q ) ∈ Ox ∩ UF .
Let Q = hr1 , r2 , r3 , r4 i. Since UF ⊲ ri ⊂ yα1 (ξi2 )xα2 (1)hUγ | γ ∈ Φ+ − ∆i, we
have OrQi 6= OrQj for i 6= j. The coefficient of yα1 +α2 in the expression of ri rj
equals ξi2 , hence ri rj 6= rj ri for i 6= j. Hence Ou5 and Ou6 are of type F. 
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N. A. : FaMAF, Universidad Nacional de Córdoba. CIEM – CONICET. Medina
Allende s/n (5000) Ciudad Universitaria, Córdoba, Argentina
E-mail address: andrus@famaf.unc.edu.ar

38

N. ANDRUSKIEWITSCH, G. CARNOVALE, G. A. GARCÍA

G. C.: Dipartimento di Matematica, Università degli Studi di Padova, via Trieste 63, 35121 Padova, Italia
E-mail address: carnoval@math.unipd.it
G. A. G.: Departamento de Matemática, Facultad de Ciencias Exactas, Universidad Nacional de La Plata. CONICET. C. C. 172, (1900) La Plata, Argentina.
E-mail address: ggarcia@mate.unlp.edu.ar

