= Weight projection matrix
{title2=$a'=Pa$}
A coordinate matrix expressing restriction of <weights> to the <Cartan subalgebra> of a <Lie subalgebra>. With <Dynkin labels> as column <vectors>, its rows encode the subalgebra's coroots in terms of the original coroots. It need not be an idempotent <projection operator>: an equal-rank embedding can give an invertible matrix. Applying it to the full <weight system>, including <weight multiplicities>, permits reconstruction of the restricted <irreducible representations>.
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