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The total multiplicity in the decomposition into irreducibles of the tensor product i x j of two irreducible representations of a simple Lie algebra is invariant under conjugation of one of them sum_k N_{i j}^{k}= sum_k N_{ibar j}^{k}. This also applies to the fusion multiplicities of affine algebras in conformal WZW theories. In that context, the statement is equivalent to a property of the modular S matrix, Sigma(k)= sum_j S_{j k}=0 if k is a complex representation. Curiously, this vanishing of Sigma(k) also holds when k is a quaternionic representation. We provide proofs of all these statements. These proofs rely on a case-by-case analysis, maybe overlooking some hidden symmetry principle. We also give various illustrations of these properties in the contexts of boundary conformal field theories, integrable quantum field theories and topological field theories.Comment: 28 pages, 1 figure, LaTeX; corrected typos, added references, shortened appendix A and section 3.3, added comments in section 8.

Topics:
Mathematical Physics, High Energy Physics - Theory, Mathematics - Quantum Algebra, Mathematics - Representation Theory

Year: 2011

DOI identifier: 10.1088/1751-8113/44/29/295208

OAI identifier:
oai:arXiv.org:1103.2943

Provided by:
arXiv.org e-Print Archive

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