Semifree symplectic circle actions on 4-orbifolds

Abstract

A theorem of Tolman and Weitsman states that all symplectic semifree circle actions with isolated fixed points on compact symplectic manifolds must be Hamiltonian and have the same equivariant cohomology and Chern classes of ( C P 1 ) n (\mathbb {C}P^1)^n equipped with the standard diagonal circle action. In this paper, we show that the situation is much different when we consider compact symplectic orbifolds. Focusing on 4 4 -orbifolds with isolated cone singularities, we show that such actions, besides being Hamiltonian, can now be obtained from either S 2 × S 2 S^2\times S^2 or a weighted projective space, or a quotient of one of these spaces by a finite cyclic group, by a sequence of special weighted blow-ups at fixed points. In particular, they can have any number of fixed points.