Loewner chains with quasiconformal extensions: an approximation approach

A new approach in Loewner Theory proposed by Bracci, Contreras, D\'iaz-Madrigal and Gumenyuk provides a unified treatment of the radial and the chordal versions of the Loewner equations. In this framework, a generalized Loewner chain satisfies the differential equation $$\partial_{t}f_{t}(z) = (z - \tau(t))(1-\overline{\tau(t)}z)\partial_{z}f_{t}(z)p(z,t),$$ where $\tau : [0,\infty) \to \overline{\mathbb{D}}$ is measurable and $p$ is called a Herglotz function. In this paper, we will show that if there exists a $k \in [0,1)$ such that $p$ satisfies $$|p(z,t) - 1| \leq k |p(z,t) + 1|$$ for all $z \in \mathbb{D}$ and almost all $t \in [0,\infty)$, then $f_{t}$ has a $k$-quasiconformal extension to the whole Riemann sphere for all $t \in [0,\infty)$. The radial case ($\tau =0$) and the chordal case ($\tau=1$) have been proven by Becker [J. Reine Angew. Math. \textbf{255} (1972), 23-43] and Gumenyuk and the author (Math. Z. \textbf{285} (2017), no.3, 1063--1089). In our theorem, no superfluous assumption is imposed on $\tau \in \overline{\mathbb{D}}$. As a key foundation of our proof is an approximation method using the continuous dependence of evolution families.