Groups of positive weighted deficiency and their applications

Abstract

In this paper we introduce the concept of weighted deficiency for abstract and pro-p groups and study groups of positive weighted deficiency which generalize Golod–Shafarevich groups. In order to study weighted deficiency, we introduce weighted versions of the notions of rank for groups and index for subgroups and establish weighted analogues of several classical results in combinatorial group theory, including the Schreier index formula.

Two main applications of groups of positive weighted deficiency are given. First we construct infinite finitely generated residually finite p-torsion groups in which every finitely generated subgroup is either finite or of finite index—these groups can be thought of as residually finite analogues of Tarski monsters. Second we develop a new method for constructing just-infinite groups (abstract or pro-p) with prescribed properties; in particular, we show that graded group algebras of just-infinite groups can have exponential growth. We also prove that every group of positive weighted deficiency has a hereditarily just-infinite quotient. This disproves a conjecture of Boston on the structure of quotients of certain Galois groups and solves Problem 15.18 from the Kourovka notebook.