Extending automorphisms of the genus-2 surface over the 3-sphere

Abstract

An automorphism $f$ of a closed orientable surface $Σ$ is said to be extendable over the 3-sphere $S^3$ if $f$ extends to an automorphism of the pair $(S^3, Σ)$ with respect to some embedding $Σ\hookrightarrow S^3$. We prove that if an automorphism of a genus-2 surface $Σ$ is extendable over $S^3$, then $f$ extends to an automorphism of the pair $(S^3, Σ)$ with respect to an embedding $Σ\hookrightarrow S^3$ such that $Σ$ bounds genus-2 handlebodies on both sides. The classification of essential annuli in the exterior of genus-2 handlebodies embedded in $S^3$ due to Ozawa and the second author plays a key role.