-systems of finite simple groups
Tarski's problem about the elementary theory of free groups has a positive solution.
Test elements and test rank of a free metabelian group.
Test elements and the retract theorem in hyperbolic groups.
Testing Cayley graph densities
We present a computer-assisted analysis of combinatorial properties of the Cayley graphs of certain finitely generated groups: given a group with a finite set of generators, we study the density of the corresponding Cayley graph, that is, the least upper bound for the average vertex degree (= number of adjacent edges) of any finite subgraph. It is known that an -generated group is amenable if and only if the density of the corresponding Cayley graph equals to . We test amenable and non-amenable...
The 27-dimensional module for E6. I.
The 4-string braid group has property RD and exponential mesoscopic rank
We prove that the braid group on 4 strings, its central quotient , and the automorphism group of the free group on 2 generators, have the property RD of Haagerup–Jolissaint. We also prove that the braid group is a group of intermediate mesoscopic rank (of dimension 3). More precisely, we show that the above three groups have exponential mesoscopic rank, i.e., that they contain exponentially many large flat balls which are not included in flats.
The abelianization of hypercyclic groups
Let G be a hypercyclic group. The most substantial results of this paper are the following. a) If G/G′ is 2-divisible, then G is 2-divisible. b) If G/G′ is a 2′-group, then G is a 2′-group. c) If G/G′ is divisible by finite-of-odd-order, then G/V is divisible by finite-of-odd-order, where V is the intersection of the lower central series (continued transfinitely) of O 2′ (G).
The abelianization of the Johnson kernel
We prove that the first complex homology of the Johnson subgroup of the Torelli group is a non-trivial, unipotent -module for all and give an explicit presentation of it as a -module when . We do this by proving that, for a finitely generated group satisfying an assumption close to formality, the triviality of the restricted characteristic variety implies that the first homology of its Johnson kernel is a nilpotent module over the corresponding Laurent polynomial ring, isomorphic to the...
The absence of efficient dual pairs of spanning trees in planar graphs.
The accessibility of finitely presented groups.
The Action of Nilpotent Groups on Infinite Graphs.
The Algebraic Braid Groups are Torsion-Free: an Algebraic Proof.
The asymptotic dimension of the first Grigorchuk group is infinity.
We describe a sufficient condition for a finitely generated group to have infinite asymptotic dimension. As an application, we conclude that the first Grigorchuk group has infinite asymptotic dimension.
The automorphism group of HNN extensions with finite base group
The automorphism group of unary algebras.
The averaged Dehn function relative to a given probability measure.
The Bass conjecture and growth in groups
We discuss Bass's conjecture on the vanishing of the Hattori-Stallings rank from the viewpoint of geometric group theory. It is noted that groups without u-elements satisfy this conjecture. This leads in particular to a simple proof of the conjecture in the case of groups of subexponential growth.
The braid groups of the projective plane.
The Burau representation is not faithful for .