HAMILTONIAN CIRCLE ACTIONS WITH MINIMAL FIXED SETS

Consider an effective Hamiltonian circle action on a compact symplectic 2n-dimensional manifold (M, ω). Assume that the fixed set MS1 is minimal, in two senses: It has exactly two components, X and Y, and dim (X) + dim (Y) = dim (M) - 2. We prove that the integral cohomology ring and Chern classes of M are isomorphic to either those of ℂℙn or (if n ≠ 1 is odd) to those of [Formula: see text], the Grassmannian of oriented two-planes in ℝn+2. In particular, Hi(M;ℤ) = Hi(ℂℙn; ℤ) for all i, and the Chern classes of M are determined by the integral cohomology ring. We also prove that the fixed set data of M agrees exactly with the fixed set data for one of the standard circle actions on one of these two manifolds. In particular, we show that there are no points with stabilizer ℤk for any k > 2. The same conclusions hold when MS1 has exactly two components and the even Betti numbers of M are minimal, that is, b2i(M) = 1 for all i ∈ {0, …, ½ dim (M)}. This provides additional evidence that very few symplectic manifolds with minimal even Betti numbers admit Hamiltonian actions.