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arXiv · 2112.10023

Rademacher expansion of a Siegel modular form for ${\cal N}= 4$ counting

Gabriel Lopes Cardoso·Suresh Nampuri·Martí Rosselló
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Abstract

The degeneracies of $1/4$ BPS states with unit torsion in heterotic string theory compactified on a six-torus are given in terms of the Fourier coefficients of the reciprocal of the Igusa cusp Siegel modular form $\Phi_{10}$ of weight $10$. We use the symplectic symmetries of the latter to construct a fine-grained Rademacher type expansion which expresses these BPS degeneracies as a regularized sum over residues of the poles of $1/\Phi_{10}$. The construction uses two distinct ${\rm SL}(2, \mathbb{Z})$ subgroups of ${\rm Sp}(2, \mathbb{Z})$ which encode multiplier systems, Kloosterman sums and Eichler integrals appearing therein. Additionally, it shows how the polar data are explicitly built from the Fourier coefficients of $1/\eta^{24}$ by means of a continued fraction structure.

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Gabriel Lopes Cardoso, Suresh Nampuri, Martí Rosselló. 2021-12-18. Rademacher expansion of a Siegel modular form for ${\cal N}= 4$ counting. https://doi.org/10.1007/s00023-023-01400-3

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