Two properties of the partial theta function

For the partial theta function $\theta(q,z):=\sum_{j=0}^{\infty}q^{j(j+1)/2}z^j$, $q$, $z\in\mathbb{C}$, $|q|<1$, we prove that its zero locus is connected. This set is smooth at every point $(q^{\flat},z^{\flat})$ such that $z^{\flat}$ is a simple or double zero of $\theta(q^{\flat},.)$. For $q\in (0,1)$, $q\to 1^-$ and $a\geq e^{\pi}$, there are $o(1/(1-q))$ and $(\ln(a/e^{\pi}))/(1-q)+o(1/(1-q))$ real zeros of $\theta(q,.)$ in the intervals $[-e^{\pi},0)$ and $[-a,-e^{-\pi}]$ respectively (and none in $[0,\infty)$). For $q\in (-1,0)$, $q\to -1^+$ and $a\geq e^{\pi/2}$, there are $o(1/(1+q))$ real zeros of $\theta(q,.)$ in the interval $[-e^{\pi/2},e^{\pi/2}]$ and $(\ln(a/e^{\pi/2})/2)/(1+q)+o(1/(1+q))$ in each of the intervals $[-a,-e^{\pi/2}]$ and $[e^{\pi /2},a]$.

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Publication Details

Journal
Annual of Sofia University St Kliment Ohridski Faculty of Mathematics and Informatics
Published
2025-12-05
DOI
https://doi.org/10.60063/gsu.fmi.112.77-91
Primary Topic
Advanced Mathematical Identities
Type
article
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article

Two properties of the partial theta function

Vladimir Petrov Kostov
Annual of Sofia University St Kliment Ohridski Faculty of Mathematics and Informatics
Advanced Mathematical Identities
article

Two properties of the partial theta function

Vladimir Petrov Kostov
article en

Abstract

For the partial theta function $\theta(q,z):=\sum_{j=0}^{\infty}q^{j(j+1)/2}z^j$, $q$, $z\in\mathbb{C}$, $|q|<1$, we prove that its zero locus is connected. This set is smooth at every point $(q^{\flat},z^{\flat})$ such that $z^{\flat}$ is a simple or double zero of $\theta(q^{\flat},.)$. For $q\in (0,1)$, $q\to 1^-$ and $a\geq e^{\pi}$, there are $o(1/(1-q))$ and $(\ln(a/e^{\pi}))/(1-q)+o(1/(1-q))$ real zeros of $\theta(q,.)$ in the intervals $[-e^{\pi},0)$ and $[-a,-e^{-\pi}]$ respectively (and none in $[0,\infty)$). For $q\in (-1,0)$, $q\to -1^+$ and $a\geq e^{\pi/2}$, there are $o(1/(1+q))$ real zeros of $\theta(q,.)$ in the interval $[-e^{\pi/2},e^{\pi/2}]$ and $(\ln(a/e^{\pi/2})/2)/(1+q)+o(1/(1+q))$ in each of the intervals $[-a,-e^{\pi/2}]$ and $[e^{\pi /2},a]$.

Annual of Sofia University St Kliment Ohridski Faculty of Mathematics and InformaticsVol. 112
Openalex Percentile: Top 55%
Advanced Mathematical Identities
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