An extreme counterexample to the Lubotzky--Weiss conjecture

Abstract

In 1993, Lubotzky and Weiss conjectured that if a compact group admits two finitely generated dense subgroups, one of which is amenable and the other has Kazhdan's property (T), then it would be finite. This conjecture was resolved in the negative by Ershov and Jaikin-Zapirain, and by Kassabov around 2010. In the present paper, we provide an extreme counterexample to this conjecture. More precisely, the latter dense group with property (T) may contain a given countable residually finite group; in particular, it can be non-exact by a result of Osajda. We may construct these counterexamples with a compact group common for all countable residually finite groups.

17 pages, new results (Theorems 1.5 and 5.9) on common compact groups (v4); 13 pages, major update, new title and new main theorem (v3); 4 pages, some references and exposition added (v2); 3 pages (v1)