On the zero set of the partial theta function

Abstract

We consider the partial theta function $\theta (q,x):=\sum _{j=0}^{\infty}q^{j(j+1)/2}x^j$, where $q\in (-1,0)\cup (0,1)$ and either $x\in \mathbb{R}$ or $x\in \mathbb{C}$. We prove that for $x\in \mathbb{R}$, in each of the two cases $q\in (-1,0)$ and $q\in (0,1)$, its zero set consists of countably-many smooth curves in the $(q,x)$-plane each of which (with the exception of one curve for $q\in (-1,0)$) has a single point with a tangent line parallel to the $x$-axis. These points define double zeros of the function $\theta (q,.)$; their $x$-coordinates belong to the interval $[-38.83\ldots ,-e^{1.4}=4.05\ldots )$ for $q\in (0,1)$ and to the interval $(-13.29,23.65)$ for $q\in (-1,0)$. For $q\in (0,1)$, infinitely-many of the complex conjugate pairs of zeros to which the double zeros give rise cross the imaginary axis and then remain in the half-disk $\{ |x|<18$, Re\,$x>0\}$. For $q\in (-1,0)$, complex conjugate pairs do not cross the imaginary axis.