  We prove a generating function formula for the Betti numbers of Nakajima
quiver varieties. We prove that it is a q-deformation of the Weyl-Kac character
formula. In particular this implies that the constant term of the polynomial
counting the number of absolutely indecomposable representations of a quiver
equals the multiplicity of a a certain weight in the corresponding Kac-Moody
algebra, which was conjectured by Kac in 1982.
