Finiteness properties of some groups of piecewise projective homeomorphisms

TL;DR

This paper investigates the finiteness properties of the Lodha-Moore group, showing it has type F_infinity, and extends these results to related groups using inverse semigroup techniques.

Contribution

It provides a new proof that the Lodha-Moore group and similar groups have type F_infinity, utilizing inverse semigroup descriptions and general procedures.

Findings

Lodha-Moore group has type F_infinity.

Groups acting on line, circle, and Cantor set also have type F_infinity.

Analogous results hold for groups defined by different hyperbolic translations.

Abstract

The Lodha-Moore group $G$ first arose as a finitely presented counterexample to von Neumann's conjecture. The group $G$ acts on the unit interval via piecewise projective homemorphisms. A result of Lodha shows that $G$ in fact has type $F_{\infty}$. Here we will describe $G$ as a group that is "locally determined" by an inverse semigroup $S_{2}$, in the sense of the author's joint work with Hughes. The semigroup $S_{2}$ is generated by three linear fractional transformations $A$, $B$, and $C_{2}$, where $A$ and $B$ are elliptical transformations of the hyperbolic plane and $C_{2}$ is a hyperbolic translation. Following a general procedure delineated by Farley and Hughes, we offer a new proof that $G$ has type $F_{\infty}$. Our proof simultaneously shows that various groups acting on the line, the circle, and the Cantor set have type $F_{\infty}$. We also prove analogous results for the groups that are locally determined by an inverse semigroup $S_{3}$, which shares the generators $A$ and $B$ with $S_{2}$, but replaces $C_{2}$ with a different hyperbolic translation $C_{3}$.