Surface mapping class group actions on 3-manifolds

Abstract

For each circle bundle $S^1\to X\toΣ_g$ over a surface with genus $g\ge2$, there is a natural surjection $π:Homeo^+(X)\to Mod(Σ_g)$. When $X$ is the unit tangent bundle $UΣ_g$, it is well-known that $π$ splits. On the other hand $π$ does not split when the Euler number $e(X)$ is not divisible by the Euler characteristic $χ(Σ_g)$ by work of the second two authors. In this paper we show that this homomorphism does not split in many cases where $χ(Σ_g)$ divides $e(X)$.