The $q$-Laplace Transforms compared: the basic confluent hypergeometric function ${}_2\phi_0$

Abstract

In solving $q$-difference equations, and in the definition of $q$-special functions, we encounter formal power series in which the $n$th coefficient is of size $q^{-\binom{n}{2}}$ with $q\in(0,1)$ fixed. To make sense of these formal series, a $q$-Borel-Laplace resummation is required. There are three candidates for the $q$-Laplace transform, resulting in three different resummations. Surprisingly, the differences between these resummations have hardly been discussed in the literature. Our main result provides explicit formulas for these $q$-exponentially small differences. We also give simple Mellin--Barnes integral representations for all the basic hypergeometric ${}_r\phi_s$ functions and derive a third (discrete) orthogonality condition for the Stieltjes--Wigert polynomials. As the main application, we introduce three resummations for the ${}_2\phi_0$ functions which can be seen as $q$ versions of the Kummer $U$ functions. We derive many of their properties, including interesting integral and sum representations, connection formulas, and error bounds.