Smooth action of a Lie group on a manifold

A smooth map defining a group action of a Lie group on a smooth manifold.
Smooth action of a Lie group on a manifold

Let GG and MM be given.

A smooth left action of GG on MM is a

Φ:G×MM \Phi: G\times M \longrightarrow M

such that, writing gx:=Φ(g,x)g\cdot x:=\Phi(g,x), the following hold for all g,hGg,h\in G and xMx\in M:

  1. (Identity) ex=xe\cdot x = x.
  2. (Compatibility) (gh)x=g(hx)(gh)\cdot x = g\cdot(h\cdot x).

For each gGg\in G, the map Φg:MM\Phi_g:M\to M, Φg(x)=gx\Phi_g(x)=g\cdot x, is a with inverse Φg1\Phi_{g^{-1}}.

A smooth right action is defined similarly, using a smooth map M×GMM\times G\to M, (x,g)xg(x,g)\mapsto x\cdot g, with xe=xx\cdot e=x and (xg)h=x(gh)(x\cdot g)\cdot h = x\cdot(gh).

Examples

  1. Left translation on the group. GG acts on itself by gh:=ghg\cdot h := gh.
  2. Rotations of the plane. SO(2)SO(2) acts on R2\mathbb{R}^2 by matrix multiplication.
  3. Change of frame. The general linear group GL(n,R)GL(n,\mathbb{R}) acts on the frame bundle of a manifold by post-composition of frames (a standard example of a right action in principal bundle theory).