English

On existence of Becker extension

Complex Variables 2020-03-26 v2

Abstract

A well-known theorem by J. Becker states that if a normalized univalent function ff in the unit disk D\mathbb{D} can be embedded as the initial element into a Loewner chain (ft)t0(f_t)_{t\geqslant 0} such that the Herglotz function pp in the Loewner -- Kufarev PDE ft(z)/f=zft(z)p(z,t),zD,a.e. t0,\partial f_t(z)/\partial f=zf'_t(z)p(z,t),\qquad z\in\mathbb{D},\quad\mathrm{a.e.}~t\ge0, satisfies (p(z,t)1)/(p(z,t)+1)k<1\big|(p(z,t)-1)/(p(z,t)+1)\big|\le k<1, then ff admits a kk-q.c. (="kk-quasiconformal") extension F:CCF:\mathbb{C}\to\mathbb{C}. The converse is not true. However, a simple argument shows that if ff has a qq-q.c. extension with q(0,1/6)q\in(0,1/6), then Becker's condition holds with k:=6qk:=6q. In this paper we address the following problem: find the largest k(0,1]k_*\in(0,1] with the property that for any q(0,k)q\in(0,k_*) there exists k0(q)(0,1)k_0(q)\in(0,1) such that every normalized univalent function f:DCf:\mathbb D\to\mathbb C with a qq-q.c. extension to C\mathbb C satisfies Becker's condition with k:=k0(q)k:=k_0(q). We prove that k1/3k_*\ge1/3.

Keywords

Cite

@article{arxiv.2003.10037,
  title  = {On existence of Becker extension},
  author = {Pavel Gumenyuk},
  journal= {arXiv preprint arXiv:2003.10037},
  year   = {2020}
}

Comments

A few minor errors and misprints are corrected. Some details are added

R2 v1 2026-06-23T14:23:26.208Z