Baumslag-Solitar relations in abstract commensurators of free groups

Abstract

Any non-residually finite Baumslag-Solitar group has a non-residually finite image in the abstract commensuration of a nonabelian free group. This gives a new proof (avoiding Britton's Lemma) of the classification of residually finite Baumslag-Solitar groups.

9 pages, corrected the main theorem and corollary statements. We mistakenly thought we were addressing whether the BS groups are Hopfian