arXiv subjects
Publications and source records attributed to Vladimir Tolstykh.
Let F be a relatively free algebra of infinite rank. We say that F has the SMALL INDEX PROPERTY if any subgroup of Gamma=Aut(F) of index at most rank(F) contains the pointwise stabilizer Gamma_(U) of a subset U of F of cardinality less than rank(F). We prove that every infinitely generated free nilpotent/abelian group has the small index property, and discuss a number of applications.
Let D be a division ring such that the number of conjugacy classes in the multiplicative group D^* is equal to the power of D^*. Suppose that H(V) is the group GL(V) or PGL(V), where V is an infinite-dimensional vector space over D. We prove, in particular, that, uniformly in dim(V) and D, the first-order theory of H(V) is mutually syntactically interpretable with the theory of the two-sorted structure (whose only relations are the division ring operations on D) in the second-order logic with quantification over arbitrary relations of power <= dim(V). A certain analogue of this results is proved for the groups the collinear groups GammaL(V) and PGammaL(V). These results imply criteria of elementary equivalence for infinite-dimensional classical groups of types H=GammaL, PGammaL, GL, PGL over division rings, and solve, for these groups, a problem posed by Felgner. It follows from the criteria that if H(V_1), H(V_2) are elementarily equivalent, then the cardinals dim(V_1) and dim(V_2) are second order equivalent as sets.
Considering a particular case of a problem posed by Saharon Shelah, we prove that the automorphism group of an infinitely generated free nilpotent group N first-order interprets the full second-order theory of the set rank(N) in the empty language.
Let S be a generating set of a group G. We say that G has FINITE WIDTH relative to S if G=(S\cup S^{-1})^k for a suitable natural number k. We say that a group G is a group of FINITE C-WIDTH if G has finite width with respect to all conjugation-invariant generating sets. We give a number of examples of groups of finite C-width, and, in particular, we prove that the commutator subgroup F' of Thompson's group F is a group of finite C-width. We also study the behaviour of the class of all groups of finite C-width under some group-theoretic constructions; it is established, for instance, that this class is closed under formation of group extensions.
Let F be an infinitely generated free group and R a fully invariant subgroup of F such that (a) R is contained in the commutator subgroup F' of F and (b) the quotient group F/R is residually torsion-free nilpotent. Then the automorphism group Aut(F/R') of the group F/R' is complete. In particular, the automorphism group of any infinitely generated free solvalbe group of derived length at least two is complete. This extends a result by Dyer and Formanek (1977) on finitely generated groups F_n/R' where F_n is a free group of finite rank n at least two and R a characteristic subgroup of F_n.
Baumslag conjectured in the 1970s that the automorphism tower of a finitely generated free nilpotent group must be very short. Let F_{n,c} denote a free nilpotent group of finite rank n at least two and of nilpotency class c at least two. In 1976 Dyer and Formanek proved that the automorphism group of F_{n,2} is even complete (and hence the height of the aumorphism tower of F_{n,2} is two) provided that n is not three; in the case when n=3, the height of the automorphism tower of F_{n,2} is three. The author proved in 2001 that the automorphism group of any infinitely generated free nilpotent of class two is complete. In his Ph. D. thesis (2003) Kassabov found an upper bound u(n,c) (a natural number) for the height of the automorphism tower of F_{n,c} in terms of n and c, thereby finally proving Baumslag's conjecture. By analyzing the function u(n,c), one can conclude that if c is small compared to n, then the height of the automorphism tower of F_{n,c} is at most three. The main result of the present paper states that the automorphism group of any infinitely generated free nilpotent group of nilpotency class at least two is complete. Thus the automorphism tower of any free nilpotent group terminates after finitely many steps.
A survey article that presents some recent algebraic and model-theoretic results on the automorphism groups of relatively free groups of infinite rank. The topics include topological aspects, generating sets, descripition of automorpisms and expressive power of the first-order theories.
Dyer and Formanek (1976) proved that if N is a free nilpotent group of class two and of finite rank which is not equal to 1, or to 3, then the automorphism group Aut(N) of N is complete. The main result of the present paper states that the automorphism group of any infinitely generated free nilpotent group of class two is also complete.
We prove that the elementary theory of Thompson's group $F$ is hereditarily undecidable.
Let $A$ be an infinitely generated free abelian group. We prove that the automorphism group $\aut A$ first-order interprets the full second-order theory of the set $|A|$ with no structure. In particular, this implies that the automorphism groups of two infinitely generated free abelian groups $A_1,A_2$ are elementarily equivalent if and only if the sets $|A_1|,|A_2|$ are second-order equivalent.
We say that a group $G$ has Bergman's property (the property of universality of finite width) if for every generating set $X$ of $G$ with $X=X^{-1}$ we have that $G=X^k$ for some natural number $k.$ The property is named after George Bergman who have proved recently that the infinite symmetric groups are groups of universally finite width. The first example of an infinite group with Bergman's property is due to Shelah (1980s). Lately some other examples have been found: the automorphism groups of doubly transitive chains (Droste-Holland), the automorphism group of reals as a Borel space (Droste-Göbel) and the infinite-dimensional general linear groups over division rings. In the present paper we prove that the automorphism group $Aut(N)$ of any infinitely generated free nilpotent group $N$ has Bergman's property, is generated by involutions, perfect and has confinality greater than $rank(N).$ Also, we obtain a partial answer to a question posed by Bergman establishing that the automorphism group of a free group of countably infinite rank is a group of universally finite width.
Recently George Bergman proved that the symmetric group of an infinite set possesses the following property which we call by the {\it universality of finite width}: given any generating set $X$ of the symmetric group of an infinite set $Ω,$ there is a uniform bound $k \in \N$ such that any permutation $σ\in \text{Sym}(Ω)$ is a product of at most $k$ elements of $X \cup X^{-1},$ or, in other words, $\text{Sym}(Ω)=(X^{\pm 1})^k.$ Bergman also formulated a sort of general conjecture stating that `the automorphism groups of structures that can be put together out of many isomorphic copies of themselves' might be groups of universally finite width and particularly mentioned, in this respect, infinite-dimensional linear groups. In this note we confirm Bergman's conjecture for infinite-dimensional linear groups over division rings.
Palindromes are those reduced words of free products of groups that coincide with their reverse words. We prove that a free product of groups $G$ has infinite palindromic width, provided that $G$ is not the free product of two cyclic groups of order two. This means that there is no a uniform bound $k$ such that every element of $G$ is a product of at most $k$ palindromes. Earlier the similar fact established for non-abelian free groups.
Let G be a group and S a subset of G that generates G. For each x in G define the length l_S(x) of x relative to S to be the minimal k such that x is a product of k elements of S. The supremum of the values l_S(x), x \in G, is called the width of G with respect to S. Here we focus on a free group F. The width of F relative to the set of all primitive (respectively palindromic) elements is called the primitive (respectively palindromic) width of F. We prove that for a free group F_n of finite rank n, both widths are infinite. A result of independent interest is that every primitive element of F_2 is a product of at most two palindromes.
In 1976 S. Shelah posed the following problem: for which variety V of algebras the automorphism group of any free algebra F from V of "large" infinite rank interprets by means of first-order logic set theory (according to his results, for every variety V the endomorphism semi-group of F interprets set theory if rank(F) is an infinite cardinal greater than the power of the language of V). There are examples of varieties for which the answer is negative; one such an example, the variety of all algebras in empty language, is due to Shelah (1973). The author earlier showed that the answer is positive for any variety of vector spaces over a fixed division ring. In the present paper it is proved that the same holds for the variety of all groups: the automorphism group of any infinitely generated free group F interprets set theory. It follows, in particular, that the group Aut(F) is as undecidable as possible.
We prove that the automorphism group of an arbitrary non-abelian free group is complete. It generalizes the result by J.Dyer and E.Formanek (1975) stating the completeness of automorphism group of finitely generated free groups. Using the description of involutions in automorphism groups of free groups (J. Dyer, P. Scott, 1975) we obtain a group-theoretic characterization of inner automorphisms determined by primitive elements in the automorphism group of any non-abelian free group F. It follows that the subgroup Inn(F) is characteristic in Aut(F), and hence the latter one is complete.