Alphabeta Math

Moment Maps and Symplectic Reduction

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1 · Prerequisites
Abstract Algebra
Category Theory
Combinatorics
Coxeter Groups
Differential Geometry
Functional Analysis
Homological Algebra
Lie Theory
Linear Algebra
Measure Theory
Number Theory
Real Analysis
Representation Theory of Groups
Set Theory
Topology
2 · Logical flowchart

3 · Items

Definition

The coadjoint representation, action and orbits

Definition

Symplectic and Hamiltonian Lie-group actions

Definition

Moment map, component Hamiltonians and infinitesimal moment maps

Proposition

The infinitesimal generator of a symplectic action is symplectic

Proposition

Moment map components generate the negative infinitesimal action

Proposition

For connected groups, equivariance is equivalent to the moment-map Poisson bracket identity

Lemma

The nonequivariance defect of an infinitesimal moment map is a constant Lie-algebra two-cocycle

Proposition

Moment maps for one action form an affine space over coadjoint-fixed covectors

Corollary

Semisimple Hamiltonian actions have a unique equivariant moment map when one exists

Proposition

Whitehead's second lemma removes the infinitesimal equivariance obstruction for semisimple actions

Theorem

Noether's conservation law for Hamiltonian actions

Proposition

Equivariant symplectomorphisms preserve moment maps up to a coadjoint-fixed covector

Proposition

The cotangent lift of an action is Hamiltonian with the tautological moment map

Lemma

The tautological cotangent moment map is equivariant

Definition

The Kirillov--Kostant--Souriau form on a coadjoint orbit

Lemma

The KKS formula is independent of the Lie-algebra representatives

Theorem

Coadjoint orbits are symplectic manifolds

Proposition

The coadjoint-orbit inclusion is an equivariant moment map

Proposition

Products and opposites of symplectic moment maps

Lemma

The differential of the moment map and the orbit-orthogonal identity

Proposition

Regularity of a moment map is equivalent to local freeness

Proposition

The moment level is invariant under the coadjoint stabilizer

Lemma

The characteristic kernel on a regular moment level

Lemma

An invariant horizontal form on a free proper quotient descends uniquely

Theorem

Marsden--Weinstein--Meyer symplectic reduction

Corollary

Zero-level symplectic reduction and the dimension formula

Proposition

The dimension of a regular reduced space at a nonzero value

Proposition

Invariant Hamiltonians descend to reduced Hamiltonians

Proposition

Reduction commutes with products

Theorem

Reduction in stages for free proper regular actions

Proposition

The shifting trick identifies reduction at a value with a zero reduction

Proposition

Compact-group symplectic actions admit an invariant compatible almost-complex structure

Proposition

A compact-group moment map can be averaged to an equivariant one when the affine obstruction vanishes

Remark

Nonregular or nonfree symplectic quotients need not be manifolds

Remark

Convexity and toric classification for Hamiltonian torus actions

False statement

Every symplectic action is Hamiltonian

False statement

An infinitesimal moment map is automatically equivariant

False statement

Moment maps are unique without normalization

False statement

The cotangent-lift moment map has a plus sign under the library fundamental-field convention

False statement

Every value of a moment map gives a smooth symplectic quotient

False statement

The general reduced dimension is dim M minus two dim G

Sources