The mapping class group of a nonorientable surface is quasi-isometrically embedded in the mapping class group of the orientation double cover

Abstract

Let $N$ be a connected nonorientable surface with or without boundary and punctures, and $j:S\to N$ be the orientation double covering. It has previously been proved that $j$ induces an embedding $\iota :\mathrm{Mod}(N)\hookrightarrow \mathrm{Mod}(S)$ with one exception. In this paper, we prove that the injective homomorphism $\iota$ is a quasi-isometric embedding. The proof is based on the semihyperbolicity of $\mathrm{Mod}(S)$, which has already been established. We also prove that the embedding $\mathrm{Mod}(F^{\prime })\hookrightarrow \mathrm{Mod}(F)$ induced by an inclusion of a pair of possibly nonorientable surfaces $F^{\prime }\subset F$ is a quasi-isometric embedding.