Realizing Non-Archimedean Polish Groups as Outer Automorphism Groups

Abstract

We show that every non-Archimedean Polish group $P$ is the outer automorphism group of a countable discrete group $G_P$. Moreover, our construction provides a Borel map $f$ from the Effros space of closed subgroups of the permutation group $S_\infty$ to the space of normal subgroups of the countably-generated free group $F_\infty$ such that $G_P = F_\infty/f(P)$. The proof relies on small cancellation theory.