English

Addendum: A separation in modulus property of the zeros of a partial theta function

Classical Analysis and ODEs 2021-02-24 v1

Abstract

We consider 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, where zCz\in \mathbb{C} is a variable and qCq\in \mathbb{C}, 0<q<10<|q|<1, is a parameter. Set D(a):={qCD(a):=\{ q\in \mathbb{C}, 0<qa0<|q|\leq a, arg(q)[π/2,3π/2]}\arg (q)\in [\pi /2,3\pi /2]\}. We show that for kNk\in \mathbb{N} and qD(0.55)q\in D(0.55), there exists exactly one zero of θ(q,.)\theta (q,.) (which is a simple one) in the open annulus qk+1/2<z<qk1/2|q|^{-k+1/2}<z<|q|^{-k-1/2} (if k2k\geq 2) or in the punctured disk 0<z<q3/20<z<|q|^{-3/2} (if k=1k=1). For k=1k=1, 44, 55, 66, \ldots, this holds true for qD(0.6)q\in D(0.6) as well.

Keywords

Cite

@article{arxiv.1812.02644,
  title  = {Addendum: A separation in modulus property of the zeros of a partial theta function},
  author = {Vladimir Petrov Kostov},
  journal= {arXiv preprint arXiv:1812.02644},
  year   = {2021}
}