New polar-finite forms of generalized Euler identities for $A_1^{(1)}$-string functions and mock theta conjecture-like identities

Abstract

Determining the explicit forms and modularity for string functions and branching coefficients for Kac--Moody algebras after Kac, Peterson, and Wakimoto is an important problem. For positive admissible-level string functions for the affine Kac--Moody algebra $A_1^{(1)}$, very little is known. Here we apply the notion of quasi-periodicity to a generalized Euler identity of Schilling and Warnaar for the affine Kac--Moody algebra $A_1^{(1)}$. For integral-level string functions the classical periodicity reduces the infinite sum of string functions in the generalized Euler identity to a finite sum of string functions with theta function coefficients. For admissible-level, we similarly reduce to an analogous finite sum of string functions, but we also gain an additional finite sum of the form \begin{equation*} \sum_{i}Φ_{i}(q)Ψ_{i}(q), \end{equation*} where the $\hat{I}{¦}_i(q)$'s are modular and depend only on the spin and the $\hat{I}{¨}_i(q)$'s are (mixed) mock modular Hecke-type double-sums and depend only on the quantum number. For levels $1/2$, $1/3$, and $2/3$, we shall also see that the $\hat{I}{¨}_i(q)$'s give us families of mock theta conjecture-like identities for symmetric Hecke-type double-sums. Our work here focuses on evaluating the $\hat{I}{¨}_i(q)$'s, and our expressions utilize Ramanujan's second-order mock theta function $\hat{I}{¼}_2(q)$ and third-order mock theta functions $f_3(q)$, $\ddot{I}{‰}_3(q)$, $\ddot{I}{ˆ}_3(q)$, and $\ddot{I}{‡}_3(q)$.

77 pages