Cartan decomposition

The Cartan decomposition is a decomposition of a semisimple Lie group or Lie algebra, which plays an important role in their structure theory and representation theory. It generalizes the polar decomposition or singular value decomposition of matrices. Its history can be traced to the 1880s work of Élie Cartan and Wilhelm Killing.

Cartan involutions on Lie algebras

Let be a real semisimple Lie algebra and let be its Killing form. An involution on is a Lie algebra automorphism of whose square is equal to the identity. Such an involution is called a Cartan involution on if is a positive definite bilinear form.

Two involutions and are considered equivalent if they differ only by an inner automorphism.

Any real semisimple Lie algebra has a Cartan involution, and any two Cartan involutions are equivalent.

Examples

Cartan pairs

Let be an involution on a Lie algebra . Since , the linear map has the two eigenvalues . Let and be the corresponding eigenspaces, then . Since is a Lie algebra automorphism, eigenvalues are multiplicative. It follows that

, , and .

Thus is a Lie subalgebra, while any subalgebra of is commutative.

Conversely, a decomposition with these extra properties determines an involution on that is on and on .

Such a pair is also called a Cartan pair of , and is called a symmetric pair. This notion of "Cartan pair" is not to be confused with a distinct notion involving the relative Lie algebra cohomology .

The decomposition associated to a Cartan involution is called a Cartan decomposition of . The special feature of a Cartan decomposition is that the Killing form is negative definite on and positive definite on . Furthermore, and are orthogonal complements of each other with respect to the Killing form on .

Cartan decomposition on the Lie group level

Let be a semisimple Lie group and its Lie algebra. Let be a Cartan involution on and let be the resulting Cartan pair. Let be the analytic subgroup of with Lie algebra . Then:

The automorphism is also called global Cartan involution, and the diffeomorphism is called global Cartan decomposition.

For the general linear group, we get as the Cartan involution.

A refinement of the Cartan decomposition for symmetric spaces of compact or noncompact type states that the maximal Abelian subalgebras in are unique up to conjugation by K. Moreover,

In the compact and noncompact case this Lie algebraic result implies the decomposition

where A = exp . Geometrically the image of the subgroup A in G / K is a totally geodesic submanifold.

Relation to polar decomposition

Consider with the Cartan involution . Then is the real Lie algebra of skew-symmetric matrices, so that , while is the subspace of symmetric matrices. Thus the exponential map is a diffeomorphism from onto the space of positive definite matrices. Up to this exponential map, the global Cartan decomposition is the polar decomposition of a matrix. Notice that the polar decomposition of an invertible matrix is unique.

See also

References

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