On normal subgroups in automorphism groups

PROOF. For i =1, 2, * * *, n let hi be the elements of H. Then h7'bhi are all the conjugates of b; for bh-lbhb= h-lbh by virtue of the hypothesis [ [h, b ]b] = e. It follows from the fact that a finite set of conjugates generate a finite normal subgroup (cf. [1]) that b is contained in a finite normal subgroup K of G. But H is finite and hence so also is G/K; and then finally G is finite. Furthermore since b is in the center of K, b is in the nil radical of G as was asserted. We can deduce another result from the fact that [[g, b ]b] =e for every gGG implies that b is in the center of a normal subgroup of G.

We say that an element g of a group G is almost right Engel if there is a finite set R(g) such that for every x∈G, there is a positive integer n(x,g) such that […[[g,x],x],…,x]∈R(g) if x is repeated at least n(x,g) times. Thus, g is a right Engel element precisely when we can choose R(g)={1}. We prove that if all elements of a compact (Hausdorff) group G are almost right Engel, then G has a finite normal subgroup N such that G/N is locally nilpotent. If in addition there is a uniform bound ∣R(g)∣⩽m for the orders of the corresponding sets, then the subgroup N can be chosen of order bounded in terms of m. The proofs use the Wilson–Zelmanov theorem saying that profinite Engel groups are locally nilpotent and previous results of the authors about compact groups in which all elements are almost left Engel.

This article began as a study of the structure of infinite permutation groups $G$ in which point stabilisers are finite and all infinite normal subgroups are transitive. That led to two variations. One is the generalisation in which point stabilisers are merely assumed to satisfy min-n, the minimal condition on normal subgroups. The groups $G$ are then of two kinds. Either they have a maximal finite normal subgroup, modulo which they have either one or two minimal nontrivial normal subgroups, or they have a regular normal subgroup $M$ which is a divisible abelian $p$-group of finite rank. In the latter case the point stabilisers are finite and act irreducibly on a $p$-adic vector space associated with $M$. This leads to our second variation, which is a study of the finite linear groups that can arise.

We correct an erroneous statement about induced morphisms of Mislin genera and give the correct statement, even under more general hypotheses.

n this paper we present a new algebraic characterization of (almost-)crys-tallographic groups. In fact we prove that a virtually abelian (resp. nilpotent) group E is crystallographic (resp. almost-crystallographic) if and only if E has no finite normal subgroups. Thereafter, we investigate more general groups sharing this property and refer to them as the almost-torsion free groups. We show that a virtually polycyclic group Y is almost-torsion free if and only if T is (almost-crystallographic)-by-crystallographic. Thereafter, we take a closer look at the topological properties of almost-torsion free groups.

We show that if all proper subgroups of a locally graded group G are finite-by-abelian-by-finite, then G contains a finite normal subgroup N such that all proper subgroups of G/N are abelian-by-finite. Then we apply this result to the study of groups which are minimal-non- $${\mathcal {P}}$$ also for related group properties $${\mathcal {P}}$$ . Finally we see how for locally (soluble-by-finite) groups of infinite rank, it is enough to restrict attention to the proper subgroups with infinite rank.

Let A be a complex unital Banach algebra. Since the set of invertible elements is open, there is an open ball around every invertible element. In this article, we investigate the Banach algebras for which the radius given by the Neumann series is optimal.

Results are obtained concerning normal subgroups of the automorphism groups of certain infinite trees. These structures are mostly ℵo-categorical, and are trees in a poset-theoretic but not graph-theoretic sense. It is shown that the automorphism group has a smallest non-trivial normal subgroup, a largest proper normal subgroup, and at least 22ο normal subgroups between these two. We also obtain and use some results on groups of automorphisms of chains.

We initiate a study of quasi-Jordan normed algebras. It is demonstrated that any quasi-Jordan Banach algebra with a norm1unit can be given an equivalent norm making the algebra isometrically isomorphic to a closed right ideal of a unital split quasi-Jordan Banach algebra; the set of invertible elements may not be open; the spectrum of any element is nonempty, but it may be neither bounded nor closed and hence not compact. Some characterizations of the unbounded spectrum of an element in a split quasi-Jordan Banach algebra with certain examples are given in the end.

We prove that if a countable discrete group Γ is w-rigid, i.e. it contains an infinite normal subgroup H with the relative property (T) (e.g. $\Gamma=SL(2,\mathbb{Z})\ltimes\mathbb{Z}^2$ , or Γ=H×H’ with H an infinite Kazhdan group and H’ arbitrary), and $\mathcal{V}$ is a closed subgroup of the group of unitaries of a finite separable von Neumann algebra (e.g. $\mathcal{V}$ countable discrete, or separable compact), then any $\mathcal{V}$ -valued measurable cocycle for a measure preserving action $\Gamma\curvearrowright X$ of Γ on a probability space (X,μ) which is weak mixing on H and s-malleable (e.g. the Bernoulli action $\Gamma\curvearrowright[0,1]^{\Gamma}$ ) is cohomologous to a group morphism of Γ into $\mathcal{V}$ . We use the case $\mathcal{V}$ discrete of this result to prove that if in addition Γ has no non-trivial finite normal subgroups then any orbit equivalence between $\Gamma\curvearrowright X$ and a free ergodic measure preserving action of a countable group Λ is implemented by a conjugacy of the actions, with respect to some group isomorphism Γ≃Λ.

Product graphs for given subgroups of the wreath product of two groups, II

Let Fr denote the free group of rank r and Out Fr: = AutFr/Inn Fr the outer automorphism group of Fr (automorphisms modulo inner automorphisms). In [10] we determined the maximal order 2rr! (for r > 2) for finite subgroups of Out Fr as well as the finite subgroup of that order which, for r > 3, is unique up to conjugation. In the present paper we determine all maximal finite subgroups (that is not contained in a larger finite subgroup) of Out F3, up to conjugation (Theorem 2 in Section 3). Here the considered case r = 3 serves as a model case: our method can be applied for other small values of r (in principle for any value of r) but the computations become considerably longer and are more apt for a computer then; the method can also be applied to determine the maximal finite subgroups of the automorphism group Aut Fr of Fr. Note that the canonical projection Aut Fr ⃗ Out Fr is injective on finite subgroups of Aut Fr; however, not all finite subgroups of Out Fr lift to finite subgroups of Aut Fr.

A density theorem of semisimple analytic groups acting on locally compact groups is presented.Let G and H be locally compact groups with G acting continuously on H as a group of automorphisms.An element h of H is said to be G-bounded if the orbit Gh = \g(h): g G] has compact closure in H.We write F AH) fot the set of all G-bounded elements in H.It is very easy to verify that F AH) is a G-invariant subgroup of H. However in general, EAH) is not closed in H.In this paper, we shall study the group EAH) fot certain topological groups G. Our main result is the following Theorem.Let G be a semisimple analytic group without compact actors acting on a locally compact group H continuously as a group of automorphisms.If the set F AH) is dense in H, then G acts trivially on H.The theorem generalizes some results in [2], [4] and is closely related to the density property of certain subgroups in semisimple analytic groups without compact factors.The result of Corollaries 4.1 and 4.2 is contained in [2], [4].In the sequel, we shall use the term "G acts on H" fot "G acts on H as a group of automorphisms".1. Minimally almost periodic groups.Let G be a locally compact group.We recall that G is minimally almost periodic if there are no nontrivial continuous homomorphisms /: G -* G of locally compact groups such that the closure C1(/(G)) of (G) in G is compact.Minimally almost periodic groups have been widely studied.Yet for our need, we shall establish some lemmas concerning minimally almost periodic groups.Lemma 1.1.Let G be a minimally almost periodic group acting continuously on a locally compact group H, and N be a closed G-invariant normal subgroup of H.

The notion ofgenus, applied to finitely generated nilpotent groups or to nilpotent spaces of finite type, was introduced by Mislin; he and the author showed how to introduce the structure of a finite abelian group into the genus if the groupN has finite commutator subgroup. An example is given of a complete genusN0,N1,...,Ns−1, which constitute a cyclic group generated byN1, with the additional property that eachNi embeds in its successor as a normal subgroup with quotient cyclic of orderl; of course,Ns−1 embeds inN0. The construction leads to the description of a family of nilpotent spacesX0,X1,...,Xs−1, all in the same genus, no two of the same homotopy type, such that eachXi covers its successor as a cyclicl-sheeted regular covering; of course,Xs−1 coversX0. Herep is a prime,n ≧ 1, ands=pn−1(p−1)/2, whilel is semiprimitive modulepn.

LetG be a finite group andA andB solvable subgroups ofG, such thatG=AB and 2 is the only common prime divisor ofA andB. Under suitable restrictions of the 2-structure ofA andB, it is shown that eitherG is solvable orG contains a solvable normal subgroupN, such thatG/N contains a normal subgroup, which is isomorphic to PGL(2,q),q odd.