The Collection Process and Basic Commutators

Brian Hartley asked me whether a free (nilpotent of class 2 and exponent p2)-group of countable rank has a faithful linear representation of finite degree, p here being a prime of course. The answer is yes. The point is that this then yields via work of F. Leinen and M. J. Tomkinson, see [3,3.6] an image of a linear p-group, which is not even finitary linear. The question of which relatively free groups have faithful linear representations dates back at least to work of W. Magnus in the 1930's, see [4, pp. 33, 34 and the final comment on p. 40] for a discussion of this. Our construction, which works more generally, is a further contribution. We write ℜc for the variety of nilpotent groups of class at most c and (ℭ9 for the variety of groups of exponent dividing q.

An automorphism θ of a group G is pointwise inner if θ (t) is conjugate to t for any t ∈ G. It is known that in a free group, each pointwise inner automorphism is inner. We prove that this property remains true in a free nilpotent group. In fact, more generally, an automorphism of a non‐cyclic free group fixing each normal subgroup is inner. We show that in a non‐abelian free nilpotent group of class c, this last property is true if c = 2, but false if c > 2.

Let G be a group and C = [G, G] be its commutator subgroup. Denote by c(G) the minimal number such that every element of G′ can be expressed as a product of at most c(G) commutators. The exact values of c{G) are computed when G is a free nilpotent group or a free abelian-by-nilpotent group. If G is a free nilpotent group of rank n>2 and class c>2, c(G) = [n/2] if c = 2 and c(G) = n if c>2. If G is a free abelian-by-nilpotent group of rank n > 2 then c(G) = n.

We study the computational complexity of the Word Problem (WP) in free solvable groups S r , d S_{r,d} , where r ≥ 2 r \geq 2 is the rank and d ≥ 2 d \geq 2 is the solvability class of the group. It is known that the Magnus embedding of S r , d S_{r,d} into matrices provides a polynomial time decision algorithm for WP in a fixed group S r , d S_{r,d} . Unfortunately, the degree of the polynomial grows together with d d , so the uniform algorithm is not polynomial in d d . In this paper we show that WP has time complexity O ( r n log 2 ⁡ n ) O(r n \log _2 n) in S r , 2 S_{r,2} , and O ( n 3 r d ) O(n^3 r d) in S r , d S_{r,d} for d ≥ 3 d \geq 3 . However, it turns out, that a seemingly close problem of computing the geodesic length of elements in S r , 2 S_{r,2} is N P NP -complete. We prove also that one can compute Fox derivatives of elements from S r , d S_{r,d} in time O ( n 3 r d ) O(n^3 r d) ; in particular, one can use efficiently the Magnus embedding in computations with free solvable groups. Our approach is based on such classical tools as the Magnus embedding and Fox calculus, as well as on relatively new geometric ideas; in particular, we establish a direct link between Fox derivatives and geometric flows on Cayley graphs.

Euler estimates for rough differential equations

The paper discusses modules over free nilpotent groups and demonstrates that faithful modules are more restricted than might appear at first glance. Some discussion is also made of applying the techniques more generally.

Charles Sims has asked whether or not the lower central subgroup γ n (F) of a free group F coincides with the normal closure in F of the basic commutators of weight n. This question has a positive answer in weights at most 5, but remains an open question in general. In earlier work with Gaglione and Spellman, it was shown that γ n (F) is the normal closure in F of the basic commutators of weights n through 2n − 4. Here, we specialize to the case where F has rank 2 and show that γ6(F) is the normal closure in F of the basic commutators of weights 6 and 7.

A Levi class L(ℳ) generated by a class ℳ of groups is a class of all groups in which the normal closure of each element belongs to ℳ. It is stated that there exist finite groups G such that a Levi class L(qG), where qG is a quasivariety generated by a group G, has infinite axiomatic rank. This is a solution for [The Kourovka Notebook, Quest. 15.36]. Moreover, it is proved that a Levi class L(ℳ), where ℳ is a quasivariety generated by a relatively free 2-step nilpotent group of exponent ps with a commutator subgroup of order p, p is a prime, p ≠ 2, s ≥ 2, is finitely axiomatizable.

The classic Magnus embedding is a very effective tool in the study of abelian extensions of a finitely generated group \(G\), allowing us to see the extension as a subgroup of a wreath product of a free abelian group with \(G\). In particular, the embedding has proved to be useful when studying free solvable groups. An equivalent geometric definition of the Magnus embedding is constructed and it is used to show that it is \(2\)-bi-Lipschitz, with respect to an obvious choice of generating sets. This is then applied to obtain a non-zero lower bound on \(L_p\) compression exponents in free solvable groups.

Some results obtained by the calculations of the integral homology of free nilpotent Lie algebras Hi (L(x1, x2,. . . , xr)/γN+1) in the system for computational discrete algebra GAP are presented. The attention is focused on the occurrence of unexpected torsion in this homology, similar to one that arises for a 4-generated free nilpotent group of class 2. The main result is that even for the case of two generators, the torsion occurs in the fourth integral homology when the nilpotency class is 5. Moreover, only 7-torsion occurs, and no others. Namely, there is an isomorphism H4(L(x1, x2)/γ6) ≅ ℤ85 ⨁ ℤ /7ℤ. We also give an explicit mathematical proof of the fact that H4(L(x1, x2, x3, x4)/γ3) has nontrivial 3-torsion. Bibliography: 6 titles.

We prove twisted homological stability with polynomial coefficients for automorphism groups of free nilpotent groups of any given class. These groups interpolate between two extremes for which homological stability was known before, the general linear groups over the integers and the automorphism groups of free groups. The proof presented here uses a general result that applies to arbitrary extensions of groups, and that has other applications as well.

We prove that if a (ℤ/nℤ)-graded Lie algebra L = ↕i=0n−1Li has d nontrivial components Li and the null component L0 has finite dimension m, then L has a homogeneous solvable ideal of derived length bounded by a function of d and of codimension bounded by a function of m and d. An analogous result holds also for the (ℤ/nℤ)-graded Lie rings L = ↕i=0n−1 with few nontrivial components Li if the null component L0 has finite order m. These results generalize Kreknin’s theorem on the solvability of the (ℤ/nℤ)-graded Lie rings L = ↕i=0n−1Li with trivial component L0 and Shalev’s theorem on the solvability of such Lie rings with few nontrivial components Li. The proof is based on the method of generalized centralizers which was created by E. I. Khukhro for Lie rings and nilpotent groups with almost regular automorphisms of prime order [1], as well as on the technique developed in the work of N. Yu. Makarenko and E. I. Khukhro on the almost solvability of Lie algebras with an almost regular automorphism of finite order [2].

We show that, even under very favourable hypotheses, a polygonal product of finitely generated torsion free nilpotent groups amalgamating infinite cyclic subgroups is, in general, not residually finite, thus answering negatively a question of C. Y. Tang. A second example shows similar kinds of limitations apply even when the factors of the product are free abelian groups.

Let [Formula: see text] be a division ring with center [Formula: see text] and multiplicative group [Formula: see text], of char [Formula: see text], and with an involution *. Let [Formula: see text] be the group of unitary units of [Formula: see text], namely [Formula: see text]. We investigate various instances where the dimension [Formula: see text], and in which every non-central normal subgroup [Formula: see text] contains a free non cyclic subgroup. Among them, we consider the cases where [Formula: see text] is either generated over its center of characteristic 0 by a torsion free nilpotent group, or [Formula: see text] is the field of fractions of a group algebra [Formula: see text] of the residually torsion free nilpotent group [Formula: see text] over the field [Formula: see text] of characteristic 0 , or the field of fractions of the enveloping algebra of a locally solvable residually nilpotent Lie [Formula: see text]-algebra [Formula: see text].

Finite groups and Lie rings with a metacyclic Frobenius group of automorphisms