Lie group action

In differential geometry, a Lie group action on a manifold M is a group action by a Lie group G on M that is a differentiable map; in particular, it is a continuous group action. Together with a Lie group action by G, M is called a G-manifold. The orbit types of G form a stratification of M and this can be used to understand the geometry of M.

Let $\sigma :G\times M\to M,(g,x)\to g\cdot x$ be a group action. It is a Lie group action if it is differentiable. Thus, in particular, the orbit map $\sigma _{x}:G\to M,g\cdot x$ is differentiable and one can compute its differential at the identity element of G:

${\mathfrak {g}}\to T_{x}M$.

If X is in ${\mathfrak {g}}$, then its image under the above is a tangent vector at x and, varying x, one obtains a vector field on M; the minus of this vector field is called the fundamental vector field associated with X and is denoted by $X^{\#}$. (The "minus" ensures that ${\mathfrak {g}}\to \Gamma (TM)$ is a Lie algebra homomorphism.) The kernel of the map can be easily shown to be the Lie algebra ${\mathfrak {g}}_{x}$ of the stabilizer $G_{x}$ (which is closed and thus a Lie subgroup of G.)

Let $P\to M$ be a principal G-bundle. Since G has trivial stabilizers in P, for u in P, $a\mapsto a_{u}^{\#}:{\mathfrak {g}}\to T_{u}P$ is an isomorphism onto a subspace; this subspace is called the vertical subspace.

In general, the orbit space $M/G$ does not admit a manifold structure since, for example, it may not be Hausdorff. However, if G is compact, then $M/G$ is Hausdorff and if, moreover, the action is free, then $M/G$ is a manifold (in fact, a principal G-bundle.) This is a consequence of the slice theorem. If the "free action" is relaxed to "finite stabilizer", one instead obtains an orbifold (or quotient stack.)

A substitute for the construction of the quotient is the Borel construction from algebraic topology: assume G is compact and let $EG$ denote the universal bundle, which we can assume to be a manifold since G is compact, and let G act on $EG\times M$ diagonally; the action is free since it is so on the first factor. Thus, one can form the quotient manifold $M_{G}=(EG\times M)/G$. The constriction in particular allows one to define the equivariant cohomology of M; namely, one sets

$H_{G}^{*}(M)=H_{\text{dr}}^{*}(M_{G})$,

where the right-hand side denotes the de Rham cohomology, which makes sense since $M_{G}$ has a structure of manifold (thus there is the notion of differential forms.)

If G is compact, then any G-manifold admits an invariant metric; i.e., a Riemannian metric with respect to which G acts on M as isometries.

See also

• Hamiltonian group action
• Equivariant differential form

References

• Michele Audin, Torus actions on symplectic manifolds, Birkhauser, 2004