Subgroups generated by two pseudo-Anosov elements in a mapping class group. II. Uniform bound on exponents

Abstract

Let $S$ be a compact orientable surface, and $\Mod(S)$ its mapping class group. Then there exists a constant $M(S)$, which depends on $S$, with the following property. Suppose $a,b \in \Mod(S)$ are independent (i.e., $[a^n,b^m]\ne 1$ for any $n,m\ne 0$) pseudo-Anosov elements. Then for any $n,m\ge M$, the subgroup $<a^n,b^m>$ is free of rank two, and convex-cocompact in the sense of Farb-Mosher. In particular all non-trivial elements in $<a^n,b^m>$ are pseudo-Anosov. We also show that there exists a constant $N$, which depends on $a,b$, such that $<a^n,b^m>$ is free of rank two and convex-cocompact if $|n|+|m|\ge N$ and $nm\ne 0$.

33 pages, 11 figures

References in corpus (1)

• Two-generator subgroups of the pure braid group