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

Abstract

We investigate univalent functions $f(z)=z+a_2{z}^2+a_3{z}^3+\dots$ 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 Kühnau 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 $k_{\ast }\in (0,1]$ such that every univalent function $f$ in $\mathbb{D}$ having a $k$-q.c. extension to $\mathbb{C}$ with $k⩽k_{\ast }$ admits also a Becker q.c.-extension, possibly with a larger upper bound for the dilatation. We prove that $k_{\ast }>1/6$. 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 $k^{\prime }$-q.c. extensions to $\mathbb{C}$ with $k^{\prime }<k$.