English

Exponential Taylor domination

Classical Analysis and ODEs 2019-09-12 v1

Abstract

Let f(z)=k=0akzkf(z) = \sum_{k=0}^\infty a_k z^k be an analytic function in a disk DRD_R of radius R>0R>0, and assume that ff is pp-valent in DRD_R, i.e. it takes each value cCc\in{\mathbb C} at most pp times in DRD_R. We consider its Borel transform B(f)(z)=k=0akk!zk, B(f)(z) = \sum_{k=0}^\infty \frac{a_k}{k!} z^k , which is an entire function, and show that, for any R>1R>1, the valency of the Borel transform B(f)B(f) in DRD_R is bounded in terms of p,Rp,R. We give examples, showing that our bounds, provide a reasonable envelope for the expected behavior of the valency of B(f)B(f). These examples also suggest some natural questions, whose expected answer will strongly sharper our estimates. We present a short overview of some basic results on multi-valent functions, in connection with "Taylor domination", which, for f(z)=k=0akzkf(z) = \sum_{k=0}^\infty a_k z^k, is a bound of all its Taylor coefficients aka_k through the first few of them. Taylor domination is our main technical tool, so we also discuss shortly some recent results in this direction.

Keywords

Cite

@article{arxiv.1909.04918,
  title  = {Exponential Taylor domination},
  author = {Omer Friedland and Gil Goldman and Yosef Yomdin},
  journal= {arXiv preprint arXiv:1909.04918},
  year   = {2019}
}
R2 v1 2026-06-23T11:12:02.316Z