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momentum map (Definition)

Let $ (M,\omega)$ be a symplectic manifold, $ G$ a Lie group acting on that manifold, $ \mathfrak{g}$ its Lie algebra, and $ \mathfrak{g}^*$ the dual of the Lie algebra. This action induces a map $ \alpha:\mathfrak{g}\to\mathfrak{X}(M)$ where $ \mathfrak{X}(M)$ is the Lie algebra of vector fields on $ M$, such that $ \exp(tX)(m)=\rho_t(m)$ where $ \rho$ is the flow of $ \alpha(X)$. Then a moment map $ \mu:M\to\mathfrak{g}^*$ for the action of $ G$ is a map such that

$\displaystyle H_{\mu(X)}=\alpha(X).$
Here $ \mu(X)(m)=\mu(m)(X)$, that is, $ \mu(m)$ is a covector, so we apply it to the vector $ X$ and get a scalar function $ \mu(X)$, and $ H_{\mu(X)}$ is its Hamiltonian vector field.

Generally, the moment maps we are interested in are equivariant with respect to the coadjoint action, that is, they satisfy

$\displaystyle \mathrm{Ad}^*_g\circ\mu=\mu\circ g.$



"momentum map" is owned by bwebste.
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Other names:  moment map
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Cross-references: equivariant, Hamiltonian vector field, function, scalar, vector, covector, flow, vector fields, map, induces, action, Lie algebra, manifold, Lie group, symplectic manifold
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This is version 1 of momentum map, born on 2002-12-10.
Object id is 3718, canonical name is MomentumMap.
Accessed 3851 times total.

Classification:
AMS MSC53D20 (Differential geometry :: Symplectic geometry, contact geometry :: Momentum maps; symplectic reduction)

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