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Cartan Subalgebra


Let g be a finite-dimensional Lie algebra over some field k. A subalgebra h of g is called a Cartan subalgebra if it is nilpotent and equal to its normalizer, which is the set of those elements x in g such that [x,h] subset h.

It follows from the definition that if g is nilpotent, then g itself is a Cartan subalgebra of g. On the other hand, let g be the Lie algebra of all endomorphisms of k^n (for some natural number n), with [f,g]=f degreesg-g degreesf. Then the set of all endomorphisms f of k^n of the form f(x_1,...,x_n)=(lambda_1x_1,...,lambda_nx_n) is a Cartan subalgebra of g.

It can be proved that:

1. If k is infinite, then g has Cartan subalgebras.

2. If the characteristic of k is equal to 0, then all Cartan subalgebras of g have the same dimension.

3. If k is algebraically closed and its characteristic is equal to 0, then, given two Cartan subalgebras h and h^' of g, there is an automorphism f of g such that f(h)=h^'.

4. If g is semisimple and k is an infinite field whose characteristic is equal to 0, then all Cartan subalgebras of g are Abelian.

Every Cartan subalgebra of a Lie algebra g is a maximal nilpotent subalgebra of g. However, a maximal nilpotent subalgebra of g doesn't have to be a Cartan subalgebra. For instance, if g is the Lie algebra of all endomorphisms of k^2 with [f,g]=f degreesg-g degreesf and if h is the subalgebra of all endomorphisms f of the form f(x,y)=(lambday,0), then h is a maximal nilpotent subalgebra of g, but not a Cartan subalgebra.


See also

Cartan Algebra, Cartan Subgroup, Subalgebra

This entry contributed by José Carlos Santos

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References

Bourbaki, N. Ch. 7-9 in Lie Groups and Lie Algebras. New York: Springer-Verlag, 2005.Jacobson, N. Lie Algebras. New York: Dover, 1979.

Referenced on Wolfram|Alpha

Cartan Subalgebra

Cite this as:

Santos, José Carlos. "Cartan Subalgebra." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/CartanSubalgebra.html

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