Momentum maps and morita equivalence

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We introduce quasi-symplectic groupoids and explain their relation with momentum map theories. This approach enables us to unify into a single framework various momentum map theories, including ordinary Hamiltonian G-spaces, Lu’s momentum maps of Poisson group actions, and the group-valued momentum maps of Alekseev-Malkin-Meinrenken. More precisely, we carry out the following program: (1) We define and study properties of quasi-symplectic groupoids. (2) We study the momentum map theory defined by a quasisymplectic groupoid γ ⇒ P. In particular, we study the reduction theory and prove that J−1(𝒪)/γ is a symplectic manifold for any Hamiltonian γ-space (X →J P, ωX) (even though ωX ∈ ω2(X) may be degenerate), where 𝒪 ⊂ P is a groupoid orbit. More generally, we prove that the intertwiner space (X1×P X¯ 2)/γ between two Hamiltonian γ-spaces X1 and X2 is a symplectic manifold (whenever it is a smooth manifold). (3)We study Morita equivalence of quasi-symplectic groupoids. In particular, we prove that Morita equivalent quasi-symplectic groupoids give rise to equivalent momentum map theories. Moreover the intertwiner space (X1 ×P X2)/γ depends only on the Morita equivalence class. As a result, we recover various wellknown results concerning equivalence of momentum maps including the Alekseev-Ginzburg-Weinstein linearization theorem and the Alekseev-Malkin-Meinrenken equivalence theorem between quasi-Hamiltonian spaces and Hamiltonian loop group spaces.

Original languageEnglish (US)
Pages (from-to)289-333
Number of pages45
JournalJournal of Differential Geometry
Issue number2
StatePublished - 2004

All Science Journal Classification (ASJC) codes

  • Analysis
  • Algebra and Number Theory
  • Geometry and Topology


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