On finite groups acting on a connected sum of 3-manifolds S2×S1

Abstract

Let H_g denote the closed 3-manifold obtained as the connected sum of g copies of S^2 times S^1, with free fundamental group of rank g. We prove that, for a finite group G acting on H_g which induces a faithful action on the fundamental group, there is an upper bound for the order of G which is quadratic in g, but that there does not exist a linear bound in g. This implies then a Jordan-type bound for arbitrary finite group actions on H_g which is quadratic in g. For the proofs we develop a calculus for finite group-actions on H_g, by codifying such actions by handle-orbifolds and finite graphs of finite groups.