On existence of Becker extension

Abstract

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