Homemath.CAarXiv:2605.29903

Where not to find the spectrum of the partial theta function

math.CA2026-05v1license

Abstract

The spectrum of Ramanujan's partial theta function θ(q,x):=j=0qj(j+1)/2xj\theta (q,x):=\sum _{j=0}^{\infty}q^{j(j+1)/2}x^j, qD1q\in \mathbb{D}_1 (the unit disk centered at the origin), xCx\in \mathbb{C}, is the set of values of the parameter qq for which θ(q,.)\theta (q,.) has a multiple zero. We show that there is no spectral value in the set SDc0\mathbb{S}\cup \mathbb{D}_{c_0}, c0=0.2078750206c_0=0.2078750206\ldots, where S\mathbb{S} is the sector {0<z<0.6,arg(z)[π/4,7π/4]}\{ 0<|z|<0.6,{\rm arg}(z)\in [\pi /4 ,7\pi /4 ]\}. There is a single spectral value in the set SD0.31\mathbb{S}\cup \mathbb{D}_{0.31} which equals 0.3092490.309249\ldots. For qSDc0q\in \mathbb{S}\cup \mathbb{D}_{c_0}, the moduli of the zeros of θ\theta are separated by the negative half-integer powers of q|q|.

Cite

@article{arxiv.2605.29903,
  title  = {Where not to find the spectrum of the partial theta function},
  author = {Yousra Gati and Vladimir Petrov Kostov},
  journal= {arXiv preprint arXiv:2605.29903},
  year   = {2026}
}