Journal of Symplectic Geometry

Volume 11 (2013)

Number 2

New techniques for obtaining Schubert-type formulas for Hamiltonian manifolds

Pages: 179 – 230

DOI: http://dx.doi.org/10.4310/JSG.2013.v11.n2.a3

Authors

Silvia Sabatini (Ecole Polytechnique Fédérale de Lausanne, Switzerland)

Susan Tolman (University of Illinois at Urbana-Champaign, Urbana, Il., U.S.A.)

Abstract

In [GT], Goldin and Tolman extend some ideas from Schubert calculus to the more general setting of Hamiltonian torus actions on compact symplectic manifolds with isolated fixed points. The main goal of this paper is to build on this work by finding more effective formulas. More explicitly, given a generic component of the moment map, they define a canonical class $\alpha_p$ in the equivariant cohomology of the manifold $M$ for each fixed point $p \in M$. When they exist, canonical classes form a natural basis of the equivariant cohomology of $M$. In particular, when $M$ is a flag variety, these classes are the equivariant Schubert classes. It is a long-standing problem in combinatorics to find positive integral formulas for the equivariant structure constants associated to this basis. Since computing the restriction of the canonical classes to the fixed points determines these structure constants, it is important to find effective formulas for these restrictions. In this paper, we introduce new techniques for calculating the restrictions of a canonical class $\alpha_p$ to a fixed point $q$. Our formulas are nearly always simpler, in the sense that they count the contributions over fewer paths. Moreover, our formula is manifestly positive and integral in certain important special cases.

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