Permutation Weights and Modular Poincare Polynomials for Affine Lie Algebras

Abstract

Poincare Polynomial of a Kac-Moody Lie algebra can be obtained by classifying the Weyl orbit $W(ρ)$ of its Weyl vector $ρ$. A remarkable fact for Affine Lie algebras is that the number of elements of $W(ρ)$ is finite at each and every depth level though totally it has infinite number of elements. This allows us to look at $W(ρ)$ as a manifold graded by depths of its elements and hence a new kind of Poincare Polynomial is defined. We give these polynomials for all Affine Kac-Moody Lie algebras, non-twisted or twisted. The remarkable fact is however that, on the contrary to the ones which are classically defined,these new kind of Poincare polynomials have modular properties, namely they all are expressed in the form of eta-quotients. When one recalls Weyl-Kac character formula for irreducible characters, it is natural to think that this modularity properties could be directly related with Kac-Peterson theorem which says affine characters have modular properties. Another point to emphasize is the relation between these modular Poincare Polynomials and the Permutation Weights which we previously introduced for Finite and also Affine Lie algebras. By the aid of permutation weights, we have shown that Weyl orbits of an Affine Lie algebra are decomposed in the form of direct sum of Weyl orbits of its horizontal Lie algebra and this new kind of Poincare Polynomials count exactly these permutation weights at each and every level of weight depths.