English

Two properties of the partial theta function

Classical Analysis and ODEs 2025-12-18 v1

Abstract

For the partial theta function θ(q,z):=j=0qj(j+1)/2zj\theta (q,z):=\sum_{j=0}^{\infty}q^{j(j+1)/2}z^j, qq, zCz\in \mathbb{C}, q<1|q|<1, we prove that its zero set is connected. This set is smooth at every point (q,z)(q^{\flat},z^{\flat}) such that zz^{\flat} is a simple or double zero of θ(q,.)\theta (q^{\flat},.). For q(0,1)q\in (0,1), q1q\rightarrow 1^- and aeπa\geq e^{\pi}, there are o(1/(1q))o(1/(1-q)) and (ln(a/eπ))/(1q)+o(1/(1q))(\ln (a/e^{\pi}))/(1-q)+o(1/(1-q)) real zeros of θ(q,.)\theta (q,.) in the intervals [eπ,0)[-e^{\pi},0) and [a,eπ][-a,-e^{-\pi}] respectively (and none in [0,)[0,\infty)). For q(1,0)q\in (-1,0), q1+q\rightarrow -1^+ and aeπ/2a\geq e^{\pi /2}, there are o(1/(1+q))o(1/(1+q)) real zeros of θ(q,.)\theta (q,.) in the interval [eπ/2,eπ/2][-e^{\pi /2},e^{\pi /2}] and (ln(a/eπ/2)/2)/(1+q)+o(1/(1+q))(\ln (a/e^{\pi /2})/2)/(1+q)+o(1/(1+q)) in each of the intervals [a,eπ/2][-a,-e^{\pi /2}] and [eπ/2,a][e^{\pi /2},a].

Keywords

Cite

@article{arxiv.1911.08841,
  title  = {Two properties of the partial theta function},
  author = {Vladimir Petrov Kostov},
  journal= {arXiv preprint arXiv:1911.08841},
  year   = {2025}
}
R2 v1 2026-06-23T12:22:07.040Z