Root multiplicities of hyperbolic Kac-Moody algebras and Fourier coefficients of modular forms

In this paper we consider the hyperbolic Kac-Moody algebra associated with the generalized Cartan matrix . Its connection to Siegel modular forms of genus 2 was first studied by A. Feingold and I. Frenkel. The denominator function of is not an automorphic form. However, Gritsenko and Nikulin extended to a generalized Kac-Moody algebra whose denominator function is a Siegel modular form. Using the Borcherds denominator identity, the denominator function can be written as an infinite product. The exponents that appear in the product are given by Fourier coefficients of a weak Jacobi form. P. Niemann also constructed a generalized Kac-Moody algebra which contains and whose denominator function is related to a product of Dedekind eta-functions. In particular, root multiplicities of the generalized Kac-Moody algebra are determined by Fourier coefficients of a modular form. As the main results of this paper, we compute asymptotic formulas for these Fourier coefficients using the method of Hardy-Ramanujan-Rademacher, and obtain an asymptotic bound for root multiplicities of the algebra . Our method can be applied to other hyperbolic Kac-Moody algebras and to other modular forms as demonstrated in the later part of the paper.