Edgar Bering (Temple University)

Title: Subgroups of Out(F_r) generated by Dehn (multi-)twists.

Abstract: Pick two (multi)-curves on a surface. What can be said about the subgroup generated by powers of Dehn twists about the two curves? For twists about single curves a complete classification up to isomorphism is known and is determined by intersection number and twist power. For multi-curves with more components, the fourth powers of the twists generate a free group. I will present analogous results for groups generated by two Dehn (multi-)twists in Out(F_r), where the analysis is complicated by the absence of a standard topological model.