Hamiltonian circle actions with minimal isolated fixed points

Abstract

Let the circle act in a Hamiltonian fashion on a compact symplectic manifold $(M, \omega)$ of dimension $2n$. Then the $S^1$-action has at least $n+1$ fixed points. We study the case when the fixed point set consists of precisely $n+1$ isolated points. We first show certain equivalence on the first Chern class of $M$ and some particular weight of the $S^1$-action at some fixed point. Then we show that the particular weight can completely determine the integral cohomology ring of $M$, the total Chern class of $M$, and the sets of weights of the $S^1$-action at all the fixed points. We will see that all these data are isomorphic to those of known examples, $\CP^n$, or $\Gt_2(\R^{n+2})$ with $n\geq 3$ odd, equipped with standard circle actions.