Cyclic Group Actions on Contractible 4-Manifolds
  There are known infinite families of Brieskorn homology 3-spheres which can be realized as boundaries of smooth contractible 4-manifolds. In this paper we show that free periodic actions on these Brieskorn spheres do not extend smoothly over a contractible 4-manifold. We give a new infinite family of examples in which the actions extend locally linearly but not smoothly. 
Nima Anvari and Ian Hambleton
math.GT math.AT