Lie algebras admitting a metacyclic Frobenius group of automorphisms

0.25 MB·English

by  N. Yu. Makarenko

Lie algebras admitting a metacyclic Frobenius group
of automorphisms
N.Yu. Makarenko
3 Sobolev Institute of Mathematics, Novosibirsk, 630090, Russia and
1
0 Universit´e de Haute Alsace, Mulhouse,68093, France
2 [email protected]

n E.I. Khukhro
a
J
Sobolev Institute of Mathematics, Novosibirsk, 630090, Russia

6
[email protected]
1

]
A
UDC 512.5
R
.
h
to Victor Danilovich Mazurov on the occasion of his 70th birthday
t
a
m
Аннотация
[
Suppose that a Lie algebra L admits a finite Frobenius group of automorphisms

1
FH with cyclic kernel F and complement H such that the characteristic of the
v
7 ground field does not divide H . It is proved that if the subalgebra CL(F) of fixed
| |
4 points of the kernel has finite dimension m and the subalgebra C (H) of fixed
L
6
points of the complement is nilpotent of class c, then L has a nilpotent subalgebra
3
. of finite codimension bounded in terms of m, c, H , and F whose nilpotency class
1 | | | |
0 is bounded in terms of only H and c. Examples show that the condition of the
| |
3 kernel F being cyclic is essential.
1
:
v
Key words. Frobenius groups, automorphism, Lie algebras, nilpotency class
i
X
r
a
1 Introduction
Recall that a finite Frobenius group FH with kernel F and complement H is a semidirect
product of a normal subgroup F and a subgroup H in which every element of H acts
without non-trivial fixed points on F, that is, C (h) = 1 for all h H 1 . The
F
∈ \ { }
structure of Frobenius groups is well known. In particular, all abelian subgroup of H are
cyclic, and if F is a cyclic group, then H is also cyclic.
Mazurov’s problem 17.72 in “Kourovka Notebook” [1] gave rise to a number of recent
papers,wheregroupsGareconsideredadmittingaFrobeniusgroupofautomorphismsFH
with kernel F and complement H such that F acts without fixed points, C (F) = 1. The
G
goal of these papers [2, 3, 4, 5, 6, 7, 8, 9, 10] are restrictions on the order, rank, nilpotent
length, nilpotency class, and exponent of the group G in terms of the corresponding
1
properties and parameters of the centralizer C (H) and order H . For estimating the
G
| |
nilpotency class of the group G in the case of nilpotent centralizer of the complement
C (H), Lie ring methods are used. The corresponding theorems on Lie rings and algebras
G
L with a Frobenius group of automorphisms FH such that C (F) = 0 are also important
L
in their own right.
A natural and important generalization of this situation is consideration of groups
and Lie rings with a Frobenius group of automorphisms FH such that its kernel F has
bounded cardinality or dimension of the set of fixed points. Then the goal is obtaining
similar restrictions ona subgroupor a subalgebra of bounded index orcodimension. Inthe
present paper we consider the case of Lie algebras, for which strong bounds are obtained
for the nilpotency class of a subalgebra of bounded codimension.
Suppose that a Lie algebra L of arbitrary, not necessarily finite, dimension admits a
finite Frobenius group of automorphisms FH with cyclic kernel F and complement H
such that the subalgebra C (H) of fixed points of the complement is nilpotent of class
L
c. If C (F) = 0, that is, the Frobenius kernel F acts regularly (without non-trivial fixed
L
points) onL, then by theMakarenko–Khukhro–Shumyatsky theorem [5,6] theLie algebra
L is nilpotent of class bounded by some function depending only on H and c. In this
| |
paper we generalize the Makarenko–Khukhro–Shumyatsky theorem to the case where the
Frobenius kernel F acts “almost regularly” on L. We prove that if the dimension of C (F)
L
is finite and the characteristic L does not divide H , then L is almost nilpotent with
| |
estimates for the codimension of a nilpotent subalgebra and for its nilpotency class.
Theorem 1.1. Let FH be a Frobenius group with cyclic kernel F of order n and
complement H of order q. Suppose that FH acts by automorphisms on a Lie algebra
L of characteristic that does not divide q in such a manner that the fixed point subalgebra
C (F) of the kernel has finite dimension m and the fixed point subalgebra C (H) of the
L L
complement is nilpotent of class c. Then L has a nilpotent subalgebra of finite codimension
bounded by some function depending only on m, n, q, and c, whose nilpotency class is
bounded by some function depending only on q and c.
There are examples showing that the result is not true if the kernel F is not cyclic
(see examples in [5]). The functions of m, n, q, c and of q and c in Theorem 1.1 can be
estimated from above explicitly, although we do not write out these estimates here.
The proof of Theorem 1.1 uses the method of generalized, or graded, centralizers,
which was originally created in [11] for almost regular automorphisms of prime order, see
also [12, 13, 14, 15] and Ch. 4 in [16]. This method consists in the following. In the proof
of Theorem 1.1 we can assume that the ground field contains a primitive nth root of
unity ω. Let F = ϕ . Then L decomposes into the direct sum of eigenspaces L = a
j
h i { ∈
L aϕ = ωja , which are also components of a (Z/nZ)-grading: [L , L ] L , where
s t s+t
| } ⊆
s+t is calculate modulo n. In each of the L , i = 0, certain subspaces L (k) of bounded
i i
6
codimension — “graded centralizers” — of increasing levels k are successively constructed,
and simultaneously certain elements (representatives) x (k) are fixed, all this up to a
i
certain (c,q)-bounded level. Elements of L (k) have a centralizer property with respect
j
to the fixed elements of lower levels: if a commutator (of bounded weight) that involves
exactly oneelement y (k) L (k) of level k and some fixed elements x (s) L (s) oflower
j j i i
∈ ∈
levelss < k belongstoL ,thenthiscommutatorisequalto0.Thesought-forsubalgebrais
0
2
thesubalgebra Z generatedby alltheL (T),i = 0,ofhighest level T.The proofofthefact
i
6
that the subalgebra Z is nilpotent of bounded class is based on Proposition 3.3, which
is a combinatorial consequence of the Makarenko–Khukhro–Shumyatsky theorem [5, 6]
and reduces the question of nilpotency to consideration of commutators of a special form.
Various collecting processes applied here and other arguments are based precisely on the
aforementioned centralizer property.
Results on Lie algebras (rings) with Frobenius groups of automorphisms are applicable
to various classes of groups. In particular, it follows from the Makarenko–Khukhro–
Shumyatsky theorem [5] that if a finite group (or a locally nilpotent group, or a Lie
group) G admits a Frobenius group of automorphisms FH with cyclic kernel F of order n
and complement H of order q such that C (F) = 1 and C (H) is nilpotent of class c, then
G G
G is nilpotent of (c,q)-bounded class. Theorem 1.1 is also applicable to locally nilpotent
torsion-free groups with a metacyclic Frobenius group of automorphisms (Theorem 7.2).
We briefly describe the plan of the paper. After recalling definitions and introducing
notation in §2 we firstly prove in §3 combinatorial consequences of the Makarenko–
Khukhro–Shumyatsky theorem (Theorem 3.2 and Proposition 3.3), which are key in the
proof of the theorem. Then in §4 and §5 generalized centralizers and fixed elements
are constructed and their basic properties are proved. This is based on the original
construction in [11], which, however, had to be considerably modified in accordance with
the hypotheses of the problem. In §6 the sought-for subalgebra is constructed and the
nilpotency ofthis subalgebra isproved. In§7 Theorem 7.2 onlocallynilpotent torsion-free
groups is proved.
2 Preliminaries
We recall some definitions and notions. For brevity we say that a certain quantity is (c,q)-
bounded (or, say, (m,n,q,c)-bounded) if it is bounded above by some function depending
only c and q (respectively, only on m, n, q, and c).
Products in a Lie algebra are called “commutators”. We denote by S the Lie
h i
subalgebra generated by a subset S.
Terms ofthelower centralseriesofaLiealgebraLaredefined byinduction:γ (L) = L;
1
γ (L) = [γ (L),L]. By definition a Lie algebra L is nilpotent of class h if γ (L) = 0.
i+1 i h+1
A simple commutator [a ,a ,...,a ] of weight (length) s is by definition the
1 2 s
commutator[...[[a ,a ],a ],...,a ].BytheJacobiidentity[a,[b,c]] = [a,b,c] [a,c,b] any
1 2 3 s
−
(complex, repeated) commutator in some elements in any Lie algebra can be expressed
as a linear combination of simple commutators of the same weight in the same elements.
Using also the anticommutativity [a,b] = [b,a], one can make sure that in this linear
−
combination all simple commutators begin with some pre-assigned element occurring in
the originalcommutator. Inparticular,if L = S ,then thespace Lisgenerated by simple
h i
commutators in elements of S.
Let A be an additively written abelian group. A Lie algebra L is A-graded if
L = L and [L ,L ] L , a,b A,
M a a b ⊆ a+b ∈
a A
∈
3
where L are subspaces of L. Elements of the subspaces L are called homogeneous, and
a a
commutators in homogeneous elements homogeneous commutators. A subspace H of the
spaceLissaidtobehomogeneous ifH = (H L );thenwesetH = H L .Obviously,
La ∩ a a ∩ a
any subalgebra or an ideal generated by homogeneous subspaces is homogeneous. A
homogeneoussubalgebraandthequotientalgebrabyahomogeneousidealcanberegarded
as A-graded algebras with induced grading.
Suppose that a Frobenius group FH with cyclic kernel F = ϕ of order n and
h i
complement H of order q acts on a Lie algebra L in such a way that the subalgebra
of fixed points C (F) has finite dimension dimC (F) = m, and the subalgebra of fixed
L L
points C (H) is nilpotent of class c.
L
Let ω be a primitive nth root of unity. We extend the ground field by ω and denote
by L the algebra over the extended field. The group FH naturally acts on L, and the
subaelgebra of fixed points Ce(H) is nilpotent of class c, while the subalgebera of fixed
L
points Ce(F) has dimension m.
L
Definition. We define ϕ-homogeneous components L for k = 0, 1, ...,n 1 as the
k
−
eigensubspaces
L = a L aϕ = ωka .
k
(cid:8) ∈ | (cid:9)
It is known that if the characteristic of the field does not divide n, then
L = L L L
0 1 n 1
⊕ ⊕···⊕ −
(see, for example, Ch. 10 in the book [17]). This decomposition is a (Z/nZ)-grading due
to the obvious inclusions
[L , L ] L ,
s t s+t(modn)
⊆
where s+t is calculated modulo n.
Index Convention. Henceforth a small letter with index i will denote an element of
the ϕ-homogeneous component L , here the index will only indicate the ϕ-homogeneous
i
component to which this element belongs: x L . To lighten the notation we will not
i i
∈
use numbering indices for elements in L , so that different elements can be denoted by
j
the same symbol when it only matters to which ϕ-homogeneous component these elements
belong. For example, x and x can be different elements of L , so that [x , x ] can be a
1 1 1 1 1
nonzero element of L . These indices will be usually considered modulo n; for example,
2
a L = L .
i i n i
− ∈ − −
Note that in the framework of the Index Convention a ϕ-homogeneous commutator
belongs to the ϕ-homogeneous component L , where s is the sum modulo n of the indices
s
of all the elements occurring in this commutator.
3 Combinatorial theorem
In this section we prove a certain combinatorial fact that follows from the following
Makarenko–Khukhro–Shumyatsky theorem [5].
Theorem 3.1 (Makarenko–Khukhro–Shumyatsky [5]). Let FH be a Frobenius group
with cyclic kernel F of order n and complement H of order q. Suppose that FH acts
4
by automorphisms on a Lie algebra L in such a way that C (F) = 0 and the subalgebra of
L
fixed points C (H) is nilpotent of class c. Then for some (q,c)-bounded number f = f(q,c)
L
the algebra L is nilpotent of class at most f.
We consider a Frobenius group FH with cyclic kernel F = ϕ of order n and
h i
complement H of order q that acts on a Lie algebra L in such a way that the subalgebra
of fixed points C (F) has finite dimension m and the subalgebra of fixed points C (H) is
L L
nilpotent of class c. Since the kernel F of the Frobenius group FH is a cyclic subgroup,
the subgroup H is also cyclic. Let H = h and ϕh−1 = ϕr for some 1 6 r 6 n 1. Then r
h i −
is a primitive qth root of unity in the ring Z/nZ and. moreover, the image of the element
r in Z/dZ is a primitive qth root of unity for every divisor d of the number n, since h acts
without non-trivial fixed points on every subgroup of the group F.
The group H permutes the homogeneous components L as follows: L h = L for all
i i ri
i Z/nZ. Indeed, if x L , then (xh)ϕ = xhϕh−1h = (xϕr)h = ωirxh.
∈ i ∈ i i i i i
Inwhat follows, foragiven u L we denote theelement uhi by u intheframework
k ∈ k k rik
the Index Convention, since L hi = L . Since the sum over any H-orbit belongs to the
k rik
centralizer CL(H), we have uk +urk + +urq−1k CL(H).
··· ∈
Theorem 3.2. Let FH be a Frobenius group with cyclic kernel F = ϕ of order n and
complement H = h of order q and let ϕh−1 = ϕr for some positive inthegeir 1 6 r 6 n 1.
h i −
Let f(q,c) be the function in Theorem 3.1, let F be a field containing a primitive nth root
of unity the characteristic of which does not divide q and n, and let L be a Lie algebra
over F. Suppose that FH acts by automorphisms on L in such a way that the subalgebra
of fixed points CL(H) is nilpotent of class c and L = Lin=−01Li, where Li = {x ∈ L | xϕ =
ωix are ϕ-homogeneous components (eigensubspaces for eigenvalues ωi). Then any ϕ-
}
homogeneous commutator [x ,x ,...,x ] with non-zero indices of weight T = f(q,c)+1
i1 i2 iT
can be represented as a linear combination of ϕ-homogeneous commutators of the same
weight T each of which, for every s = 1,...,T, includes exactly the same number of
elements of the orbit
O(xis) = {xis, xhis = xris, ..., xihsq−1 = xrq−1is}
as the original commutator, and contains a subcommutator with zero sum of indices
modulo n.
Доказательство. The idea of the proof consists in application of Theorem 3.1 to a free
Lie algebra with operators FH. Let F be a field containing a primitive nth root of unity
the characteristic of which does not divide q and n, and let n,q,r,T be the numbers in the
hypothesis of Theorem 3.2.In thering Z/nZ we choose arbitrarynon-zero (not necessarily
distinct) elements i ,i , ,i Z/nZ. We consider a free Lie algebra K over the field
1 2 T
··· ∈
F with qT free generators in the set
Y = {yi1,yri1,...,yrq−1i1, yi2,yri2,...,yrq−1i2,...,yiT,yriT,...,yrq−1iT},
| O({yzi1) } | O({yzi2) } | O({yziT) }
where the subsets O(yis) = {yis,yris,...,yrq−1is} are called the r-orbits of the elements
y ,y ,...,y . Here, as in the Index Convention, we do not use numbering indices, that
i1 i2 iT
5
is, all elements y are by definition different free generators, even if indices coincide.
rkij
(The Index Convention will come into force in a moment.) For every i = 0, 1, ...,n 1 we
−
define the subspace K of the algebra K generated by all commutators in the generators
i
y in which the sum of indices of the elements occurring in them is equal to i modulo n.
js
Then K = K K K . It is also obvious that [K ,K ] K ; therefore
0 1 n 1 i j i+j(modn)
⊕ ⊕···⊕ − ⊆
this is a (Z/nZ)-grading. The Lie algebra K also has the natural N-grading with respect
to the generating set Y:
K = G (Y),
M i
i
where G (Y) is the subspace generated by all commutators of weight i in elements of the
i
generating set Y.
We define an action of the Frobenius group FH on K. We set kϕ = ωik for k K
i i i ∈ i
and extend this action to K by linearity. Since K is the direct sum of homogeneous
ϕ-components and the characteristic of the ground field does not divide n, we have
K = k K kϕ = ωik ,
i
{ ∈ | }
that is, K is the eigensubspace for the eigenvalue ωi. An action of the subgroup H is
i
defined on the generating set Y as follows: H cyclically permutes the elements of the
r-orbits O(y ), s = 1,...,T:
is
(yrkis)h = yrk+1is, k = 0,...,q −2; (yrq−1is)h = yis.
Thus, the r-orbit of an element y is also the H-orbitof this element. Clearly, H permutes
is
the components K according to the following rule: K h = K for all i Z/nZ.
i i ri
∈
Let J = K be the ideal generated by the ϕ-homogeneous component K . By
id 0 0
h i
definition the ideal J consists of all linear combinations of commutators in elements of Y
each of which contains a subcommutator with zero sum of indices modulo n. Clearly, the
idealJ is generatedby homogeneous elements with respect tothe gradingsK = G (Y)
and K = Lin=−01Ki and, consequently, is homogeneous with respect to both gradLingis,ithat
is,
n 1
−
J = J G (Y) = J K .
M ∩ i M ∩ i
i i=0
Note also that the ideal J is obviously FH-invariant.
F
Let I = γ (C (H)) be the smallest F-invariant ideal containing the subalgebra
id c+1 K
h i
γ (C (H)) (this ideal can be called the F-closure of the ideal generated by this
c+1 K
subalgebra). We claim that the ideal I is homogeneous with respect to the grading
hKav=e tLheine=−q01uKalii.tySCince(Hq)is=noat+daivhis+ible +byahtqh−e1 chaaracKteri.stIticisoefatshyetogrsoeuentdhafiteltdheFi,dewael
K
{ ··· | ∈ }
I consists of linear combinations of all possible elements of the form
ϕi
(u +uh + +uhq−1), (v + +vhq−1),...,(w + +whq−1), y , y ,... , (1)
h a a ··· a b ··· b d ··· d j1 j2 i
c+1
| {z }
where u ,v ,...,w are ϕ-homogeneous commutators (possibly, of different weights) in
a b d
elements of Y and y ,y ,... Y.
j1 j2 ∈
6
We continue using the fact that H permutes the components K by the rule K h = K
i i ri
for all i Z/nZ, and denote ahi by a (under the Index Convention). It is important
∈ k rik
that then the image of a commutator in elements of the generating set Y under the action
of the automorphism h is again a commutator in elements of Y. Rewriting (1) in the new
notation we obtain that the ideal I consists of linear combinations of all possible elements
of the form
ϕi
h(ua +···+urq−1a), (vb +···+vrq−1b),...,(wd +···+wrq−1d), yj1, yj2,...i , (2)
c+1
| {z }
where u ,v ...,w are homogeneous commutators (possibly, of different weights) in
a b d
elements of the set Y and y ,y ,... Y.
j1 j2 ∈
We denote the element (2) by z and represent it as a sum of ϕ-homogeneous elements
of the form k + k + + k , where k K . For every i = 0,...,n 1 we set z =
0 1 n 1 i i i
ωPi,sn=t−h01aωt−isis,ziϕns.KIit.iFsueraths·ye·r·tmoovreer,−infyzt=haPt zjni=−0b1ez∈lio.nSginscteotthheecheiagreancstuebrissptiacceoffoth−rethfieeldeigdeonevsanluoet
divide n, the element n is invertible in the field F, that is, z = 1/nPjn=−01zi. By comparing
ethleemtewnot r(e1p/rne)sePntsna=−t01ioωn−sisozfϕsz,wbeeinobgtaainlintehaartckoim=bi(n1a/tnio)zni o=f (t1h/ene)lePmsne=−n01tsω−zi,szzϕϕ,s...B.,uztϕtnh−e1
in I, also belongs to I. Consequently, k I, that is, the ideal I is homogeneous with
i
respect to the grading K = Lin=−01Ki. ∈
Note that I is also homogeneous with respect to the grading K = G (Y) and is
Li i
FH-invariant.
We consider the quotient Lie algebra M = K/(J + I). Since the ideals J and I are
homogeneouswithrespect tothegradingsK = LiGi(Y)andK = Lin=−01Ki,thequotient
algebra M has the corresponding induced gradings. The group FH acts on M in such
a way that C (F) = 0 and γ (C (H)) = 0. By Theorem 3.1 the quotient algebra
M c+1 M
K/(J +I) is nilpotent of (q,c)-bounded class f = f(q,c). Consequently,
F
[y ,y ,...,y ] J +I = K + γ (C (H)) .
i1 i2 iT ∈ idh 0i idh c+1 K i
This means that the commutator [y ,y ,...,y ] can be represented modulo the ideal I
i1 i2 iT
as a linear combination of commutators of weight T in elements of Y belonging to the
ϕ-homogeneous component K that contain a subcommutator with zero sum of
i1+i2+ +iT
···
indices modulo n. It is claimed that for every s = 1,...,T any such commutator includes
exactly one element of the orbit
O(y ) = y ,yh,...,yhq−1 .
is { is is is }
For every s = 1,...,T we consider the homomorphism θ extending the mapping
s
O(y ) 0; y y if k = s.
is → ik → ik 6
Clearly, the kernel Kerθ is equal to the ideal generated by the orbit O(y ). Furthermore,
s is
clearly, the ideal I is invariant under θ (as any homogeneous ideal). We apply the
s
homomorphism θ to the commutator [y ,y ,...,y ] and its representation modulo
s i1 i2 iT
7
I as a linear combination of commutators in elements of Y of weight T that contain
a subcommutator with zero sum of indices modulo n. We obtain that the image of
θ ([y ,y ,...,y ]) is equal to 0, as well as the image of any commutator containing
j i1 i2 iT
elements of the orbit O(y ). Hence the sum of allthose commutators in the representation
is
of the element [y ,y ,...,y ] that do not contain elements of the orbit O(y ) is equal
i1 i2 iT is
to zero, and we can exclude all these commutators from our consideration. By applying
consecutively θ , s = 1,...,T, and excluding commutators not containing elements of
s
O(y ), s = 1,...,t, in the end we obtain modulo I a linear combination of commutators
is
each of which contains at least one element from every orbit O(y ), s = 1,...,T. Since
is
under these transformations the weight of commutators remains the same and is equal to
T, no other elements can appear, and every commutator will contain exactly one element
in every orbit O(y ), s = 1,...,T.
is
Thus, we proved that in the free Lie algebra K generated by elements of the set
Y the commutator [y ,y ,...,y ] can be represented modulo the ideal I as a linear
i1 i2 iT
combination of commutators of weight T in elements of Y, and for every s = 1,...,T
any of commutators of this linear combination contains exactly one element in the orbit
O(y ) and has a subcommutator with zero sum of indices modulo n.
is
Now suppose that L is an arbitrary Lie algebra satisfying the hypothesis of
Theorem 3.2. Let x ,x ,...,x be arbitrary ϕ-homogeneous elements with non-
i1 i2 iT
zero indices in the subspaces L ,L ,...,L , respectively. We define the following
i1 i2 iT
homomorphism δ from the free Lie algebra K into L:
δ(y ) = x , δ(y ) = xhk for s = 1,...,T; k = 1,...,q 1.
is is rkis is −
Then
δ[y ,y ,...,y ] = [x ,x ,...,x ]; δ(I) = 0; δ(J) = L ; δ(O(y )) = O(x ).
i1 i2 iT i1 i2 iT idh 0i is is
Byapplying δ totherepresentationofthecommutator[y ,y ,...,y ]constructed above,
i1 i2 iT
as the image we obtain a representation of the commutator [x ,x ,...,x ] as a linear
i1 i2 iT
combination of commutators in elements of the set X = O(x ) O(x ) O(x ).
i1 ∪ i2 ∪···∪ iT
Since δ(I) = 0, every commutator in this linear combination has weight T, contains
exactly the same number of elements from every orbit O(x ), s = 1,...,T, as the original
is
commutator, and has a subcommutator with zero sum of indices modulo n. The theorem
is proved.
WedefineaMKhSh-transformationofacommutator[x ,x ,...,x ]itsrepresentation
i1 i2 il
according to Theorem 3.2 as linear combination of simple commutators in elements of
X = O(x ) O(x ) O(x ) that contain exactly the same number of elements
i1 ∪ i2 ∪ ··· ∪ iT
from every orbit O(x ), s = 1,...,T, as the originalcommutator and have initial segment
is
from L of weight 6 T = f(q,c)+1, that is, commutators of the form
0
[c ,y ,...,y ],
0 jw+1 jv
where
c = [y ,...,y ] L , w 6 T, j +j + +j = 0(modn), y X,
0 j1 jw ∈ 0 1 2 ··· w jk ∈
8
with subsequent re-denoting
z = [c ,y ], z = y for s > 1.
i1 − 0 jw+1 is jw+s
The following assertion is obtained by repeated application of the MKhSh-
transformation.
Proposition 3.3. Let FH be a Frobenius group with cyclic kernel F = ϕ of order n
and complement H = h of order q, and let ϕh−1 = ϕr for some 1 6 r 6 nh i1. Let F be
h i −
a field containing a primitive nth root of unity the characteristic of which does not divide
q and n, and let L be a Lie algebra over F. Suppose that FH acts by automorphisms
on L in such a way that the subalgebra of fixed points C (H) is nilpotent of class c and
L
L = Lin=−01Li, where Li = {x ∈ L | xϕ = ωix} are eigensubspace for eigenvalues ωi of
the automorphism ϕ. Then for any positive integers t and t there exists a (t ,t ,q,c)-
1 2 1 2
bounded positive integer V = V(t ,t ,q,c) such that any commutator in ϕ-homogeneous
1 2
elements [x ,x ,...,x ] with non-zero indices of weight V can be represented as a linear
i1 i2 iV
V
combination of ϕ-homogeneous commutators in elements of the set X = O(x ),
Ss=1 is
where
O(xis) = {xis, xhis = xris, ... xihsq−1 = xrq−1is},
and every such commutator either has a subcommutator of the form
[u ,...,u ], (3)
k1 ks
where there are t different initial segments with zero sum of indices modulo n, that is,
1
k +k + +k 0 (modn), i = 1,2,...,t ,
1 2 ··· ri ≡ 1
1 < r < r < < r = s,
1 2 ··· t1
or has a subcommutator of the form
[u ,c ,...,c ], (4)
k0 1 t2
where u X, every c belongs to L , i = 1,...,t , and has the form
k0 ∈ i 0 2
[x ,...,x ], x X
k1 ki kj ∈
with zero sum of indices modulo n
k + +k 0 (modn).
1 i
··· ≡
Here we can set V(t ,t ,c,q) = t1 ((f(q,c)+1)2t )i +1.
1 2 Pi=1 2
Доказательство. The proof practically word-for-word repeats the proof of a
proposition in [11] (see also Proposition 4.4.2 in [16]), but instead of the HKK-
transformation one should repeatedly apply the MKhSh-transformation. In contrast to
the HKK-transformation, which always produces commutator in the original elements
x ,x ,...,x , in our case after the MKhSh-transformation in commutators there may
i1 i2 iV
appear some images of the elements x ,x ,...,x under the actionof the automorphism
i1 i2 iV
h. This detail does not affect the course of the proof, but it precisely why the conclusion
of Proposition 3.3 involves commutators in elements of the h-orbits of the original
elements.
9
4 Representatives and generalized centralizers
Let FH be a Frobenius group with kernel F = ϕ of order n and complement H = h
of order q, and let ϕh−1 = ϕr for some 1 6 rh6i n 1. Suppose that the group FhHi
−
acts by automorphisms on a Lie algebra L, and the subalgebra C (H) of fixed points
L
of the complement is nilpotent of class c, while the subalgebra L = C (ϕ) of fixed
0 L
points of the kernel has finite dimension m. First suppose that L = Lin=−01Li, where
L = x L xϕ = ωix are eigensubspace for eigenvalues ωi of the automorphism ϕ
i
{ ∈ | }
(which is actually the main case).
We begin construction of generalized centralizers by induction on the level — a
parameter taking integer values from 0 T, where the number T = T(q,c) = f(q,c) + 1
is define in Theorem 3.2. A generalized centralizer L (s) of level s is a certain subspace
j
of the ϕ-homogeneous component L . Simultaneously with construction of generalized
j
centralizers we fix certain elements of them — representatives of various levels, — the
total number of which is (m,n,q,c)-bounded.
Definition. Thepattern ofacommutator inϕ-homogeneouselements (inL )isdefinedas
i
itsbracketstructuretogetherwiththearrangementofindicesundertheIndexConvention.
The weight of a pattern is the weight of the commutator. The commutator itself is called
the value of its pattern on given elements.
Definition. Let ~x = (x ,...,x ) be some ordered tuple of elements x L , i =
i1 ik is ∈ is s
1,...,n 1, such that i + + i 0(modn). We set j = i i (modn) and
1 k 1 k
− ··· 6≡ − −···−
define the mappings
ϑ : y [y ,x ,...,x ]. (5)
~x j → j i1 ik
Bylinearitytheyallarehomomorphisms ofthesubspace L intoL .SincedimL = m,
j 0 0
we have dim(L /Kerϑ ) 6 m.
j ~x
Notation. Let U = U(q,c) denote the number V(T,T 1,q,c), where V is the function
−
in the conclusion of Proposition 3.3.
Definition of level 0. At level 0 we only fix representatives of level 0. First, for
every pattern P of a simple commutator of weight 6 U with indices i = 0 and zero
sum of indices, among all values of this pattern P on ϕ-homogeneous el6ements in L ,
i
i = 0 we choose commutators c that form a basis of the subspace spanned by all values
6
of this pattern on ϕ-homogeneous elements in L , i = 0. The elements of L , j = 0,
i j
6 6
occurring in these fixed representations of the commutators c are called representatives
of level 0. Representatives of level 0 are denoted by x (0) under the Index Convention
j
(Recall that the same symbol can denote different elements). Furthermore, together with
every representative x (0) L , j = 0, we also fix all elements of the orbit O(x (0)) of
j j j
∈ 6
this element under the action of the automorphism h
O(x (0)) = x (0),x (0)h, ...,x (0)hq−1 ,
j j j j
{ }
and also call them representatives of level 0. Elements of these orbits are denoted by
x (0) := x (0)hs under the Index Convention (since Lh 6 L ).
rsj j i ri
10