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 $AdS_3/CFT_2$ 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 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.
Users
Please
log in to take part in the discussion (add own reviews or comments).