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LieAlgebras[CartanSubalgebra] - find a Cartan subalgebra of a Lie algebra
Calling Sequences
CartanSubalgebra()
CartanSubalgebra(alg)
CartanSubalgebra(N)
Parameters
alg - name or string, the name of an intialized Lie algebra
N - a list of vectors, defining a nilpotent subalgebra
Description
Let be a Lie algebra. A Cartan subalgebra h is nilpotent subalgebra whose normalizer in g is itself, that is, .
If g is a semi-simple Lie algebra, then any Cartan subalgebra h is Abelian and (see Adjoint) is a semi-simple linear transformation for every (that is, is diagonalizable over C).
Cartan subalgebras are not unique. However, if g is a semi-simple Lie algebra, then any two Cartan subalgebras of g are related by an automorphism of g.
Let n be a nilpotent subalgebra of g and let be the generalized null space of n. Then there always exists a Cartan subalgebra
If is a regular element, then the generalized null space of is a Cartan subalgebra.
The procedure CartanSubalgebra implements the algorithm for calculating Cartan subalgebras presented in W. A. De Graaf: Lie Algebras: Theory and Algorithms.
Examples
Example 1
We calculate the Cartan subalgebra for the 8-dimensional Lie algebra of 3x3 trace-free matrices. The structure equations are obtained using the SimpleLieAlgebraData command.
Initialized the Lie algebra.
Find a Cartan subalgebra.
We can check that this subalgebra is Abelian (and hence nilpotent) and self-normalizing.
These properties can also be checked with the Query command
For the split real forms of the simple Lie algebras, a Cartans subalgebra can always be found consisting of diagonal matrices in the standard representation.
Example 2
Other Cartan subalgebras for can be found with the second calling sequence.
Example 3
The Cartan subalgebra of a nilpotent Lie algebra g is g itself. Retrieve the stucture equations for a nilpotent Lie algebra from the DifferentialGeometry library.
Check that the algebra is nilpotent.
Example 4
We find the Cartan subalgebra for a solvable Lie algebra. Retrieve the structure equations for a solvable Lie algebra from the DifferentialGeometry library.
Check that the algebra is solvable.
Example 5.
We find the Cartan subalgebra for a Lie algebra with a non-trivial Levi decomposition. Retrieve the structure equations for such a Lie algebra from the DifferentialGeometry library.
Check that the Levi decomposition is non-trivial.
Calculate the Cartan subalgebra.
See Also
DifferentialGeometry, LieAlgebras, CartanMatrix, Query, RootSpaceDecomposition
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