compact Lie group



Group Theory


topology (point-set topology, point-free topology)

see also differential topology, algebraic topology, functional analysis and topological homotopy theory


Basic concepts

Universal constructions

Extra stuff, structure, properties


Basic statements


Analysis Theorems

topological homotopy theory



A real Lie group is compact if its underlying topological group is a compact topological group.


Maximal tori

All maximal tori of a compact Lie group are conjugate by inner automorphisms. The dimension of a maximal torus TT of a compact Lie group is called the rank of GG.

The normalizer N(T)N(T) of a maximal torus TT determines GG.

The Weyl group W(G)=W(G,T)W(G)=W(G,T) of GG with respect to a choice of a maximal torus TT is the group of automorphisms of TT which are restrictions of inner automorphisms of GG.

The maximal torus is of finite index in its normalizer; the quotient N(T)/TN(T)/T is isomorphic to W(G)W(G).

The cardinality of W(G)W(G) for a compact connected GG, equals the Euler characteristic of the homogeneous space G/TG/T (“flag variety”).

See also at relation between compact Lie groups and reductive algebraic groups

Invariant metric


(compact Lie groups admit bi-invariant Riemannian metrics)
Every compact Lie group admits a bi-invariant Riemannian metric.

(Milnor 76, Cor. 1.4)

Smooth actions


Let XX be a smooth manifold and let GG be a compact Lie group. Then every smooth action of GG on XX is proper.

(e.g. Lee 12, Corollary 21.6)

Equivariant triangulation theorem

The equivariant triangulation theorem (Illman 78, Illman 83) says that for GG a compact Lie group and XX a compact smooth manifold equipped with a smooth GG-action, there exists a GG-equivariant triangulation of XX.

Spaces of homomorphisms



In equivariant homotopy theory

Compact Lie groups make a somewhat unexpected appearance as equivariance groups in equivariant homotopy theory, where the compact Lie condition on the equivariance group is needed in order for (the available proofs of) the equivariant Whitehead theorem to hold.

(Namely, the equivariant triangulation theorem above is used in these proofs to guaratee that Cartesian products of coset spaces G/HG/H are themselves G-CW-complexes.)

In gauge theory

In gauge theory (Yang-Mills theory/Chern-Simons theory, …) …


Textbook accounts in the broader context of smooth manifolds:

Dedicated lecture notes:

Discussion in the context of representation theory:

For more see also the references at equivariant homotopy theory.

Last revised on September 6, 2021 at 10:29:03. See the history of this page for a list of all contributions to it.