A complementary group technique for a resolution of the outer multiplicity problem of SU(n): (I) Littlewood rule and a complementary group of SU(n)

A complementary group to SU(n) is found that realizes all features of the Littlewood rule for Kronecker products of SU(n) representations. This is accomplished by considering a state of SU(n) to be a special Gelfand state of the complementary group {cal U}(2n-2). The labels of {cal U}(2n-2) can be used as the outer multiplicity labels needed to distinguish multiple occurrences of irreducible representations (irreps) in the SU(n)times SU(n)downarrow SU(n) decomposition that is obtained from the Littlewood rule. Furthermore, this realization can be used to determine SU(n)supset SU(n-1)times U(1) Reduced Wigner Coefficients (RWCs) and Clebsch-Gordan Coefficients (CGCs) of SU(n), using algebraic or numeric methods, in either the canonical or a noncanonical basis. The method is recursive in that it uses simpler RWCs or CGCs with one symmetric irrep in conjunction with standard recoupling procedures. New explicit formulae for the multiplicity for SU(3) and SU(4) are used to illustrate the theory.