Finiteness and quasi-simplicity for symmetric k3-surfaces

Abstract. We compare the smooth and deformation equivalence of actions of finite groups on K3-surfaces by holomorphic and anti-holomorphic transformations. We prove that the number of deformation classes is finite and, in a number of cases, establish the expected coincidence of the two equivalence relations. More precisely, in these cases we show that an action is determined by the induced action in the homology. On the other hand, we construct two examples to show that, first, in general the homological type of an action does not even determine its topological type, and second, that K3-surfaces X and X ̄ with the same Klein action do not need to be equivariantly deformation equivalent even if the induced action on H2,0(X) is real, i.e., reduces to multiplication by ±1. 1