Preprint typeset in JHEP style - HYPER VERSION Quantum Black Holes, Wall Crossing, and Mock Modular Forms Atish Dabholkar1;2, Sameer Murthy3, and Don Zagier4;5 1Theory Division, CERN, PH-TH Case C01600, CH-1211, 23 Geneva, Switzerland 2Sorbonne Universit´es,UPMC Univ Paris 06 UMR 7589, LPTHE, F-75005, Paris, France CNRS, UMR 7589, LPTHE, F-75005, Paris, France 3NIKHEF theory group, Science Park 105, 1098 XG Amsterdam, The Netherlands 4Max-Planck-Institut f¨ur Mathematik,Vivatsgasse 7, 53111 Bonn, Germany 5Coll`egede France, 3 Rue d'Ulm, 75005 Paris, France Emails:
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[email protected] Abstract: We show that the meromorphic Jacobi form that counts the quarter-BPS states in N = 4 string theories can be canonically decomposed as a sum of a mock Jacobi form and an Appell-Lerch sum. The quantum degeneracies of single-centered black holes are Fourier coefficients of this mock Jacobi form, while the Appell-Lerch sum captures the degeneracies of multi-centered black holes which decay upon wall-crossing. The completion of the mock Jacobi form restores the modular symmetries expected from AdS3=CF T2 holography but has a holomorphic anomaly reflecting the non-compactness of the microscopic CFT. For every positive integral value m of the magnetic charge invariant of the black hole, our analysis leads arXiv:1208.4074v2 [hep-th] 3 Apr 2014 to a special mock Jacobi form of weight two and index m, which we characterize uniquely up to a Jacobi cusp form. This family of special forms and another closely related family of weight-one forms contain almost all the known mock modular forms including the mock theta functions of Ramanujan, the generating function of Hurwitz-Kronecker class numbers, the mock modular forms appearing in the Mathieu and Umbral moonshine, as well as an infinite number of new examples.