# Univalent functions with quasiconformal extensions: Becker's class and estimates of the third coefficient

## TL;DR

This paper studies univalent functions with quasiconformal extensions, focusing on coefficient estimates, Becker's class, and the maximal dilatation for extensions, providing new bounds and optimality results.

## Contribution

It introduces new bounds for the third coefficient of univalent functions with quasiconformal extensions and characterizes the maximal dilatation for Becker's class.

## Key findings

- Established that the maximal $k_*$ for Becker extensions is at least 1/6.
- Provided estimates for the third coefficient $a_3$ of univalent functions with q.c. extensions.
- Showed that Becker's extension can be optimal in certain cases.

## Abstract

We investigate univalent functions $f(z)=z+a_2z^2+a_3z^3+\ldots$ in the unit disk $\mathbb D$ extendible to $k$-q.c.(=quasiconformal) automorphisms of $\mathbb C$. In particular, we answer a question on estimation of $|a_3|$ raised by K\"uhnau and Niske [Math. Nachr. 78 (1977) 185-192]. This is one of the results we obtain studying univalent functions that admit q.c.-extensions via a construction, based on Loewner's parametric representation method, due to Becker [J. Reine Angew. Math. 255 (1972) 23-43]. Another problem we consider is to find the maximal $k_*\in(0,1]$ such that every univalent function $f$ in $\mathbb D$ having a $k$-q.c. extension to $\mathbb C$ with $k\leqslant k_*$ admits also a Becker q.c.-extension, possibly with a larger upper bound for the dilatation. We prove that $k_*>1/6$. Moreover, we show that in some cases, Becker's extension turns out to be the optimal one. Namely, given any $k\in(0,1)$, to each finite Blaschke product there corresponds a univalent function $f$ in $\mathbb D$ that admits a Becker $k$-q.c. extension but no $k'$-q.c. extensions to $\mathbb C$ with $k'<k$.