Averaging formula for Nielsen numbers

Abstract

We prove that the averaging formula for Nielsen numbers holds for continuous maps on infra-nilmanifolds: Let $M$ be an infra-nilmanifold and $f : M \to M$ be a continuous map. Suppose $M_{K}$ is a regular covering of $M$ which is a compact nilmanifold with $\pi_{1}(M_{K}) = K$. Assume that $f_{*}(K) \subset K$. Then $f$ has a lifting $\bar{f} : M_{K} \to M_{K}$ on $M_{K}$. We prove a question raised by McCord, which is for an $\alpha \in \pi_{1}(M)$ with $p$(Fix$(\alpha\tilde{f}))$ an essential fixed point class, Fix$(\tau_{\alpha}\varphi) = 1$. As a consequence, we obtain the following averaging formula for Nielsen numbers $$ N(f) = \frac{1}{[\pi_{1}(M):K]} \sum_{\bar\alpha \in \pi_{1}(M)/K} N(\bar\alpha\bar{f}).$$