Holomorphic bootstrap for RCFT: signs and bounds for quasi-characters

Abstract: Quasi-characters are solutions of simple modular differential equations that form an explicit basis for the space of all admissible characters that can potentially describe rational CFT. They were classified for rank-$2$ in arXiv:1810.09472 [hep-th], where it was conjectured that their $q$-series has coefficients of alternating sign that stabilise to a fixed sign at order $\frac{c}{12}$ and sometimes undergo more exotic behaviour -- where $c$ is the central charge. Here we prove some of these conjectures and estimate the growth of coefficients near the stabilisation point. We also explain what role the signs play in the construction of admissible characters from quasi-characters.