In mathematics, specifically in symplectic geometry, the momentum map (or, by false etymology, moment map) is a tool associated with a Hamiltonian action of a Lie group on a symplectic manifold, used to construct conserved quantities for the action. The momentum map generalizes the classical notions of linear and angular momentum. It is an essential ingredient in various constructions of symplectic manifolds, including symplectic (Marsden–Weinstein) quotients, discussed below, and symplectic cuts and sums. Formal definition Let M be a manifold with symplectic form ω. Suppose that a Lie group G acts on M via symplectomorphisms (that is, the action of each g in G preserves ω). Let be the Lie algebra of G, its dual, and the pairing between the two. Any ξ in induces a vector field ρ(ξ) on M describing the infinitesimal action of ξ. To be precise, at a point x in M the vector is where is the exponential map and denotes the G-action on M. Let denote the contraction of this vector field with ω. Because G acts by symplectomorphisms, it follows that is closed (for all ξ in ). Suppose that is not just closed but also exact, so that for some function . Suppose also that the map sending is a Lie algebra homomorphism. Then a momentum map for the G-action on (M, ω) is a map such that for all ξ in . Here is the function from M to R defined by . The momentum map is uniquely defined up to an additive constant of integration. A momentum map is often also required to be G-equivariant, where G acts on via the coadjoint action. If the group is compact or semisimple, then the constant of integration can always be chosen to make the momentum map coadjoint equivariant. However, in general the coadjoint action must be modified to make the map equivariant (this is the case for example for the Euclidean group). The modification is by a 1-cocycle on the group with values in , as first described by Souriau (1970). Hamiltonian group actions The definition of the momentum map requires to be closed. In practice it is useful to make an even stronger assumption. The G-action is said to be Hamiltonian if and only if the following conditions hold. First, for every ξ in the one-form is exact, meaning that it equals for some smooth function If this holds, then one