Frobenius groups of automorphisms and their fixed points E. I. Khukhro Sobolev Institute of Mathematics, Novosibirsk, 630 090, Russia [email protected] N. Yu. Makarenko Sobolev Institute of Mathematics, Novosibirsk, 630 090, Russia, and Universit´ede Haute Alsace, Mulhouse, 68093, France natalia [email protected] P. Shumyatsky Department of Mathematics, University of Brasilia, DF 70910-900, Brazil [email protected] Abstract Suppose that a finite group G admits a Frobenius group of automorphisms FH with kernel F and complement H such that the fixed-point subgroup of F is trivial: CG(F ) = 1. In this situation various properties of G are shown to be close to the corresponding properties of CG(H). By using Clifford’s theorem it is proved that the order |G| is bounded in terms of |H| and |CG(H)|, the rank of G is bounded in terms of |H| and the rank of CG(H), and that G is nilpotent if CG(H) is nilpotent. Lie ring arXiv:1010.0343v1 [math.GR] 2 Oct 2010 methods are used for bounding the exponent and the nilpotency class of G in the case of metacyclic FH. The exponent of G is bounded in terms of |FH| and the exponent of CG(H) by using Lazard’s Lie algebra associated with the Jennings–Zassenhaus filtration and its connection with powerful subgroups. The nilpotency class of G is bounded in terms of |H| and the nilpotency class of CG(H) by considering Lie rings with a finite cyclic grading satisfying a certain ‘selective nilpotency’ condition. The latter technique also yields similar results bounding the nilpotency class of Lie rings and algebras with a metacyclic Frobenius group of automorphisms, with corollaries for connected Lie groups and torsion-free locally nilpotent groups with such groups of automorphisms. Examples show that such nilpotency results are no longer true for non-metacyclic Frobenius groups of automorphisms. Keywords. Frobenius group, automorphism, finite group, exponent, Lie ring, Lie alge- bra, Lie group, graded, solvable, nilpotent Mathematics Subject Classification (2010): Primary 17B40, 20D45; Secondary 17B70, 20D15, 20E36, 20F40, 22E25 1 1 Introduction Suppose that a finite group G admits a Frobenius group of automorphisms FH with kernel F and complement H such that the fixed-point subgroup (which we call the centralizer) of F is trivial: CG(F ) = 1. Experience shows that many properties of G must be close to the corresponding properties of CG(H). For example, when GF is also a Frobenius group with kernel G and complement F (so that GFH is a double Frobenius group), the second and third authors [19] proved that the nilpotency class of G is bounded in terms of |H| and the nilpotency class of CG(H). This result solved in the affirmative Mazurov’s problem 17.72(a) in Kourovka Notebook [25]. In this paper we derive properties of G from the corresponding properties of CG(H) in more general settings, no longer assuming that GF is also a Frobenius group. Note also that the order of G is not assumed to be coprime to the order of FH. By using variations on Clifford’s theorem it is shown that the order |G| is bounded in terms of |H| and |CG(H)|, the rank of G is bounded in terms of |H| and the rank of CG(H), and that G is nilpotent if CG(H) is nilpotent. By using various Lie ring methods bounds for the exponent and the nilpotency class of G are obtained in the case of metacyclic FH. For bounding the exponent, we use Lazard’s Lie algebra associated with the Jennings–Zassenhaus filtration and its connection with powerful subgroups. For bounding the nilpotency class we consider Lie rings with a finite cyclic grading satisfying a certain condition of ‘selective nilpotency’. The latter technique yields similar results giving nilpotency of bounded nilpotency class (also known as nilpotency index) of Lie rings and algebras with a metacyclic Frobenius group of automorphisms, with corollaries for connected Lie groups and torsion-free locally nilpotent groups with such groups of automorphisms. Examples show that such nilpotency results are no longer true for non- metacyclic Frobenius groups of automorphisms. We now describe the results and the structure of the paper in more detail. Recall that we consider a finite group G admitting a Frobenius group of automorphisms FH with kernel F and complement H such that CG(F ) = 1. It is worth noting from the outset that since F is nilpotent being a Frobenius kernel, the condition CG(F ) = 1 implies the solvability of G by a theorem of Belyaev and Hartley [1] based on the classification of finite simple groups. In § 2 we begin establishing the connection between the properties of G and CG(H) by |H| proving that the orders satisfy the equation |G| = |CG(H)| , the rank of G is bounded in terms of |H| and the rank of CG(H), and that G is nilpotent if CG(H) is nilpotent (Theorem 2.7). These results are proved by using Clifford’s theorem on the basis of information about the fixed points of H in FH-invariant sections of G. In particular, we prove that these fixed points are the images of elements of CG(H) (Theorem 2.3), which is a non-trivial fact since the orders of G and H are not assumed to be coprime. In § 3 we deal with bounding the exponent. First we develop the requisite Lie ring technique, some of which was used earlier by the first and third authors in [11]. We define Lazard’s Lie algebra Lp(P ) associated with the Jennings–Zassenhaus filtration of a finite p-group P and recall Lazard’s observation that elements of P of order pk give rise to k elements of Lp(P ) that are ad-nilpotent of index p . A useful property is the existence of a powerful subgroup of P of index bounded in terms of the number of generators of P 2 and the nilpotency class of Lp(P ). A key lemma gives a bound for the nilpotency class of a finitely generated solvable Lie algebra ‘saturated’ with ad-nilpotent elements. The main result of the section is Theorem 3.4: if a finite group G admits a Frobenius group of automorphisms FH with cyclic kernel F and complement H such that CG(F ) = 1, then the exponent of G is bounded in terms of |FH| and the exponent of CG(H). At present it is unclear, even in the case where GFH is a double Frobenius group, if the bound can be made independent of |F |, which would give an affirmative answer to part (b) of Mazurov’s problem 17.72 in [25]. In the proof of Theorem 3.4 a reduction to finite p-groups is given by Dade’s theorem [3]. Then nilpotency of Lazard’s Lie algebra gives a reduction to powerful p-groups, to which a lemma from § 2 about fixed points of H is applied. In § 4 a different Lie ring theory is developed, which is used later in § 5 for bounding the nilpotency class of groups and Lie rings with a metacyclic Frobenius group FH of automorphisms. This theory is stated in terms of a Lie ring L with a finite cyclic grading (which naturally arises from the ‘eigenspaces’ for F ). The condition of the fixed-point subring CL(H) being nilpotent of class c implies certain restrictions on the commutation of the grading components, which we nickname ‘selective nilpotency’. For example, in [9] it was shown that if CL(F ) = 0, c = 1, and |F | is a prime, then each component commutes with all but at most (c, |H|)-boundedly many components, which in turn implies a strong bound for the nilpotency class of L. For greater values of c more complicated ‘selective nilpotency’ conditions naturally arise; similar conditions were exploited earlier in the paper [19] on double Frobenius groups. In § 5 we obtain bounds for the nilpotency class of groups and Lie rings with metacyclic Frobenius groups of automorphisms. (Examples show that such results are no longer true for non-metacyclic Frobenius groups of automorphism.) The main result for finite groups is Theorem 5.8: if a finite group G admits a Frobenius group of automorphisms FH with cyclic kernel F and complement H such that CG(F ) = 1 and CG(H) is nilpotent of class c, then G is nilpotent of class bounded in terms of c and |H| only. The proof is based on the analogous result for Lie rings (Theorem 5.6). We state separately the result for Lie algebras as Theorem 5.1: if a Lie algebra L over any field admits a Frobenius group of automorphisms FH with cyclic kernel F such that CL(F ) = 0 and CL(H) is nilpotent of class c, then L is nilpotent of class bounded in terms of c and |H| only. (Here CL(F ) and CL(H) denote the fixed-point subalgebras for F and H.) One corollary of this theorem is for connected Lie groups with metacyclic Frobenius groups of automorphisms satisfying similar conditions (Theorem 5.4). Another application is for torsion-free locally nilpotent groups with such groups of automorphisms (Theorem 5.5). The induced group of automorphisms of an invariant section is often denoted by the same letter (which is a slight abuse of notation as the action may become non-faithful). We use the abbreviation, say, “(m, n)-bounded” for “bounded above in terms of m, n only”. 3 2 Fixed points of Frobenius complements We begin with a theorem of Belyaev and Hartley [1] based on the classification of finite simple groups (see also [14]).