# nLab Hamiltonian action

### Context

#### Symplectic geometry

symplectic geometry

higher symplectic geometry

# Contents

## Definition

### Integrated

Given a symplectic manifold $(X,\omega)$, there is the group of Hamiltonian symplectomorphisms $HamSympl(X,\omega)$ acting on $X$. If $(X,\omega)$ is prequantizable this lifts to the group of quantomorphisms, both of them covering the diffeomorphisms of $X$:

quantomorphisms $\to$ Hamiltonian symplectomorphisms $\to$ diffeomorphisms .

A Hamiltonian action of a Lie group $G$ on $(X,\omega)$ is an action by quantomorphisms, hence a Lie group homomorphism $\hat \phi : G \to Quant(X, \omega)$

$\array{ && Quant(X, \omega) \\ & {}^{\mathllap{\hat \phi}}\nearrow & \downarrow \\ G &\stackrel{\phi}{\to}& HamSympl(X, \omega) \\ & {}_{\mathllap{}}\searrow & \downarrow \\ && Diff(X) } \,.$

See (Brylinski, prop. 2.4.10).

### Differentially

In the literature this is usually discussed at the infinitesimal level, hence for the corresponding Lie algebras:

smooth functions+Poisson bracket $\to$ Hamiltonian vector fields $\to$ vector fields

Now an (infinitesimal) Hamiltonian action is a Lie algebra homomorphism $\mu : \mathfrak{g} \to (C^\infty(X), \{-,-\})$

$\array{ && (C^\infty(X),\{-,-\}) \\ & {}^{\mathllap{\mu}}\nearrow & \downarrow \\ \mathfrak{g} &\stackrel{}{\to}& HamVect(X, \omega) \\ & {}_{\mathllap{}}\searrow & \downarrow \\ && Vect(X) } \,.$

Dualizing, the homomorphism $\mu$ is equivalently a linear map

$\tilde \mu : X \to \mathfrak{g}^*$

which is a homomorphism of Poisson manifolds. This is called the moment map of the (infinitesimal) Hamiltonian $G$-action.

Warning The lift from $\phi$ to $\hat \phi$ above, hence from the existence of Hamiltonians to an actual choice of Hamiltonians is in general indeed a choice. There may be different choices. In the literature the difference between $\hat \phi$ and $\phi$ (or of their Lie theoretic analogs) is not always clearly made.

## Properties

### Characterization

By (Atiyah-Bott), the action of a Lie algebra on a symplectic manifold is Hamiltonian if and only if the symplectic form has a (basic, closed) extension to equivariant de Rham cohomology.

## References

A comprehensive account is in (see around section 2.1)

The perspective on Hamiltonian actions in terms of maps to extensions, infinitesimally and integrally, is made explicit in prop. 2.4.10 of

• Jean-Luc Brylinski, Loop spaces, characteristic classes and geometric quantization, Birkhäuser (1993)

The characterization in equivariant cohomology is due to

Generalization to Hamiltonian actions by a Lie algebroid (instead of just a Lie algebra) is discussed in

• Rogier Bos, Geometric quantization of Hamiltonian actions of Lie algebroids and Lie groupoids (arXiv:math.SG/0604027)

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Revised on September 13, 2013 02:08:21 by Urs Schreiber (77.251.114.72)