Sameer Murthy, King's College
Title: Single-centered black hole degeneracies from brane-instantons
Abstract: I shall begin by reviewing the computation of exact black hole entropy using localization in supergravity. I shall then discuss how to include the effects of string worldsheet instantons in the effective action of N=4 supergravity. This leads to a formula consisting of a sum over Bessel functions with successively sub-leading arguments, as in the Rademacher expansion of Jacobi forms -- but begins to disagree with such a structure near an order where the single-centered nature of the black hole becomes relevant. This leads to a systematic method to recover the polar terms of the microscopic black hole degeneracies from the degeneracy of instantons (the Gromov-Witten invariants). The formula can be checked explicitly against expressions for the microscopic BPS counting functions that are known in terms of certain mock modular forms.