Generating Function of Single Centered Black Hole Index from the Igusa Cusp Form

Abstract: We introduce manifestly duality invariant generating function of the index of single centered black holes in the heterotic string theory compactified on a six dimensional torus. This function is obtained by subtracting, from the inverse of the Igusa cusp form, the generating function of the index of two centered black holes constructed from the Dedekind eta function. We also study the analytic properties of this function in the Siegel upper half plane.