Multiplicity function for tensor powers of modules of the A n algebra

Abstract

We consider the decomposition of the pth tensor power of the module \(L^{\omega _1 }\) over the algebra An into irreducible modules, \((L^{\omega _1 } )^{ \otimes p} = \sum\nolimits_v {m(v,p)L^v }\). This problem occurs, for example, in finding the spectrum of an invariant Hamiltonian of a spin chain with p nodes. To solve the problem, we propose using the Weyl symmetry properties. For constructing the coefficients m(ν, p) as functions of p, we develop an algorithm applicable to powers of an arbitrary module. We explicitly write an expression for the multiplicities m(ν, p) in the decomposition of powers of the first fundamental module of sl(n+1). Based on the obtained results, we find new properties of systems of orthogonal polynomials (multivariate Chebyshev polynomials). Our algorithm can also be applied to tensor powers of modules of other simple Lie algebras.