On the Chern numbers for pseudo-free circle actions

Abstract

Let $(M,\psi)$ be a $(2n+1)$-dimensional oriented closed manifold equipped with a pseudo-free $S^1$-action $\psi : S^1 \times M \rightarrow M$. We first define a \textit{local data} $\mathcal{L}(M,\psi)$ of the action $\psi$ which consists of pairs $(C, (p(C) ; \overrightarrow{q}(C)))$ where $C$ is an exceptional orbit, $p(C)$ is the order of isotropy subgroup of $C$, and $\overrightarrow{q}(C) \in (\mathbb{Z}_{p(C)}^{\times})^n$ is a vector whose entries are the weights of the slice representation of $C$. In this paper, we give an explicit formula of the Chern number $\langle c_1(E)^n, [M/S^1] \rangle$ modulo $\mathbb{Z}$ in terms of the local data, where $E = M \times_{S^1} \mathbb{C}$ is the associated complex line orbibundle over $M/S^1$. Also, we illustrate several applications to various problems arising in equivariant symplectic topology.