On the growth and integral (co)homology of free regular star-monoids

Abstract

The free regular $\star$-monoid of rank $r$ is the freest $r$-generated regular monoid $\mathbf{F}_r^\star$ in which every element $m$ has a distinguished pseudo-inverse $m^\star$ satisfying $mm^\star m = m$ and $(m^\star)^\star = m$. We study the growth rate of the monogenic regular $\star$-monoid $\mathbf{F}_1^\star$, and prove that this growth rate is intermediate. In particular, we deduce that $\mathbf{F}_r^\star$ is not rational or automatic for any $r \geq 1$, yielding the analogue of a result of Cutting & Solomon for free inverse monoids. Next, for all ranks $r \geq 1$ we determine the integral homology groups $H_\ast(\mathbf{F}_r^\star, \mathbb{Z})$, and by constructing a collapsing scheme prove that they vanish in dimension $3$ and above. As a corollary, we deduce that the free regular $\star$-monoid $\mathbf{F}_r^\star$ of rank $r \geq 1$ does not have the homological finiteness property $\operatorname{FP}_2$, yielding the analogue of a result of Gray & Steinberg for free inverse monoids.