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LieAlgebras[RootToCartanSubalgebraElementH] - associate to each positive root of a simple Lie algebra a vector in the Cartan subalgebra.
Calling Sequences
RootToCartanSubalgebraElementH()
Parameters
alpha - a vector, defining a positive (or negative) root of a simple Lie algebra
RSD - a table, defining the root space decomposition of a simple Lie algebra
Description
Let g be a simple Lie algebra, h a Cartan subalgebra, and the root space decomposition of g with respect to h. For each root , there are vectors and such that and These conditions uniquely determine The procedure RootToCartanSubalgebraElementH() calculates the vector
Note that the vectors define the 3-dimensional Lie algebra .
The assignment is used to calculate the Cartan matrix for the Lie algebra .
Examples
Example 1.
We consider the Lie algebra This is the 24-dimensional real Lie algebra of 6×6 complex matrices which are trace-free and skew-Hermitian with respect to the quadratic form . We use the command SimpleLieAlgebraData to initialize this Lie algebra.
We use the command SimpleLieAlgebraProperties to obtain the Cartan subalgebra, the root space decomposition, and the simple roots.
The result is a table. Here is the Cartan subalgebra for
Here is the root space decomposition for
Here are the positive roots.
Let us find where is the first root (2.4)
We check that is in the Cartan subalgebra.
Here are the root spaces for and
We check that defines a Lie subalgebra.
If we scale the vectors X and Y then the structure equations take the standard form for .
Example 2.
We illustrute how to use RootToCartanSubalgebraElementH to calculate the Cartan matrix for We first calculate the for the simple roots .
Then we calculate the Killing form , restricted to subspace [
The Cartan matrix is given by normalizing the entries of
The Lie algebra is a rank 5 simple Lie algebra of type "A". The matrix in (2.15) is therefore correct.
See Also
DifferentialGeometry, CartanMatrix, Killing, LieAlgebraData, RootSpace, SimpleLieAlgebraData, SimpleLieAlgebraProperties
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