Weight-space decomposition ($V={⨁}_{\mu }{V}_{\mu }$)

A weight-space decomposition expresses a Lie algebra representation as a direct sum of simultaneous eigenspaces for a Cartan subalgebra. Each summand is the weight space for its weight. In complex finite-dimensional representations of a semisimple Lie algebra, the simple coroots act as commuting diagonalizable operators, giving this decomposition.

A diagram placing the weights of a representation in the real span of its weight lattice, recording each weight multiplicity. Coincident weights must retain their multiplicity: for an Adjoint representation the zero-weight multiplicity is the dimension of its Cartan subalgebra.

For a tensor product of Lie algebra representations with weight-space decompositions, the weights add and their weight multiplicities convolve. A diagram must sum contributions landing at the same weight, rather than treating coincident points as distinct positions.