Infinite presentability of groups and condensation

Compact presentability of tree almost automorphism groups

We establish compact presentability, i.e. the locally compact version of finite presentability, for an infinite family of tree almost automorphism groups. Examples covered by our results include Neretin's group of spheromorphisms, as well as the topologically simple group containing the profinite completion of the Grigorchuk group constructed by Barnea, Ershov and Weigel. We additionally obtain an upper bound on the Dehn function of these groups in terms of the Dehn function of an embedded Higman-Thompson group. This, combined with a result of Guba, implies that the Dehn function of the Nereti…

关于某些部分Torelli子群在Aut(F_n)中有限可现性的研究

On finite presentability of some partial Torelli subgroups of Aut(F_n)

本文研究了某些部分Torelli子群在Aut(F_n)中的有限可现性问题，证明了包含IA_n的特定子群的有限可现性。

绝对投射有限刚性、直积与有限展示性

Absolute profinite rigidity, direct products, and finite presentability

本文构造了有限展示的剩余有限群，证明其在有限展示群类中投射有限刚性但在有限生成群类中不刚性，并刻画了与刚性3-流形群直积投射有限等价的所有有限生成群。

Presentability by products for some classes of groups

In various classes of infinite groups, we identify groups that are presentable by products, i.e. groups having finite index subgroups which are quotients of products of two commuting infinite subgroups. The classes we discuss here include groups of small virtual cohomological dimension and irreducible Zariski dense subgroups of appropriate algebraic groups. This leads to applications to groups of positive deficiency, to fundamental groups of three-manifolds and to Coxeter groups. For finitely generated groups presentable by products we discuss the problem of whether the factors in a presentati…

Finitely presentable, non-Hopfian groups with Kazhdan's Property (T) and infinite outer automorphism group

We give simple examples of Kazhdan groups with infinite outer automorphism groups. This answers a question of Paulin, independently answered by Ollivier and Wise by completely different methods. As arithmetic lattices in (non-semisimple) Lie groups, our examples are in addition finitely presented. We also use results of Abels about compact presentability of p-adic groups to exhibit a finitely presented non-Hopfian Kazhdan group. This answers a question of Ollivier and Wise.

Groups not presentable by products

In this paper we study obstructions to presentability by products for finitely generated groups. Along the way we develop both the concept of acentral subgroups, and the relations between presentability by products on the one hand, and certain geometric and measure or orbit equivalence invariants of groups on the other. This leads to many new examples of groups not presentable by products, including all groups with infinitely many ends, the (outer) automorphism groups of free groups, Thompson's groups, and even some elementary amenable groups.