Hamiltonian circle actions with minimal fixed sets

Hui Li, Susan Tolman

Research output: Contribution to journalArticlepeer-review


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 G2(Rn+2), the Grassmannian of oriented two-planes in Rn+2. In particular, Hi(M; Z) = H i(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, . . . , 1/2 dim(M)}. This provides additional evidence that very few symplectic manifolds with minimal even Betti numbers admit Hamiltonian actions.

Original languageEnglish (US)
JournalInternational Journal of Mathematics
Issue number8
StatePublished - 2012


  • Chern classes
  • Equivariant cohomology
  • Hamiltonian circle action
  • Moment map
  • Symplectic manifold
  • Symplectic quotient

ASJC Scopus subject areas

  • Mathematics(all)


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