Twisted conjugacy in residually finite groups of finite Prüfer rank

Abstract: Suppose, $G$ is a residually finite group of finite upper rank admitting an automorphism $\varphi$ with finite Reidemeister number $R(\varphi)$ (the number of $\varphi$-twisted conjugacy classes). We prove that such $G$ is soluble-by-finite (in other words, any residually finite group of finite upper rank, which is not soluble-by-finite, has the $R_\infty$ property). This reduction is the first step in the proof of the second main theorem of the paper: suppose, $G$ is a residually finite group of finite Pr\"ufer rank and $\varphi$ is its automorphism with $R(\varphi)<\infty$; then $R(\varphi)$ is equal to the number of equivalence classes of finite-dimensional irreducible unitary representations of $G$, which are fixed points of the dual map $\widehat{\varphi}:[\rho]\mapsto [\rho\circ \varphi]$ (i.e., we prove the TBFT$_f$, the finite version of the conjecture about the twisted Burnside-Frobenius theorem, for such groups).