Regular genus and gem-complexity of some mapping tori

Abstract: In this article, we construct a crystallization of the mapping torus of some (PL) homeomorphisms $f:M \to M$ for a certain class of PL-manifolds $M$. These yield upper bounds for gem-complexity and regular genus of a large class of PL-manifolds. The bound for the regular genus is sharp for the mapping torus of some (PL) homeomorphisms $f:M \to M$, where $M$ is $\mathbb{RP}^2$, $\mathbb{RP}^2#\mathbb{RP}^2$, $\mathbb{S}^1\times \mathbb{S}^1$, $\mathbb{RP}^3$, $\mathbb{S}^{2} \times \mathbb{S}^1$, $\mathbb{S}^{\hspace{.2mm}2} \mbox{$\times \hspace{-2.6mm}{-}$} \, \mathbb{S}^{\hspace{.1mm}1}$ or $\mathbb{S}^d$. In particular, for $M=\mathbb{S}^{d-1} \times \mathbb{S}^1$ or $\mathbb{S}^{\hspace{.2mm}d-1} \mbox{$\times\hspace{-2.6mm}{-}$} \, \mathbb{S}^{\hspace{.1mm}1}$, our construction gives a crystallization of a mapping torus of a (PL) homeomorphism $f:M \to M$ with regular genus $d^2-d$. As a consequence, we prove the existence of an orientable mapping torus of a (PL) homeomorphism $f:(\mathbb{S}^{2} \times \mathbb{S}^1)\to (\mathbb{S}^{2} \times \mathbb{S}^1)$ with regular genus 6. This disproves a conjecture of Spaggiari which states that regular genus six characterizes the topological product $\mathbb{RP}^3 \times \mathbb{S}^1$ among closed connected prime orientable PL $4$-manifolds.