From the special linear Lie
algebra An= sℓ(n + 1, ℂ) we construct certain indefinite Kac-Moody Lie algebras
IAn(a,b) and then use the representation theory of An to determine explicit closed
form root multiplicity formulas for the roots α of IAn(a,b) whose degree satisfies
|deg(α)|≤ 2a + 1. These expressions involve the well-known Littlewood-Richardson
coefficients and Kostka numbers. Using the Euler-Poincaré Principle and Kostant’s
formula, we derive two expressions, one of which is recursive and the other closed
form, for the multiplicity of an arbitrary root α of IAn(a,b) as a polynomial in
Kostka numbers.