A branch group with unsolvable conjugacy problem

Abstract

We prove that every finitely generated residually finite group $G$ can be embedded in a finitely generated branch group $\hat{I}“$ such that two elements in $G$ are conjugate in $G$ if and only if they are conjugate in $\hat{I}“$. As an application we construct a finitely generated branch group with solvable word problem and unsolvable conjugacy problem and thereby answer a question of Bartholdi, Grigorchuk, and Å unik.