The paper consists of two parts, both related to the complex geometry of the universal Teichmüller space. We reprove that all contractible invariant metrics on this space coincide and apply this important fact to solving the general extremal problems for univalent functions with quasiconformal extensions.The complex metric geometry of the universal Teichmüller space causes some interesting phenomena in the classical variational problems for univalent functions. In this paper, we investigate an interaction between these topics.First, we reprove that all contractible invariant metrics on the universal Teichmüller space agree with its intrinsic Teichmüller metric.Theorem 1.1. The Carathéodory metric of the universal Teichmüller space T coincides with its Kobayashi metric, hence all invariant metrics on T are equal to the Teichmüller metric.The infinitesimal forms of these metrics coincide with the canonical Finsler structure on T that generates the Teichmüller metric.This important fact, established in [14] by the renormalization of sequences of holomorphic quadratic differentials generating the extremal Beltrami coefficients, underlies various profound applications. Its new proof is much simpler and relies on some results that have intrinsic interest. Note that such a result is still unknown for other Teichmüller spaces.Theorem 1.1 is deeply connected with general extremal problems and the distortion theorem for univalent functions with quasiconformal extensions and gives rise to their strengthening. We consider these problems in the last section.
BackgroundHere, we briefly present certain results underlying the proof of Theorem 1.1. This exposition is adapted to our special cases. Let ∆ = {|z| < 1}, ∆ * = C \ ∆.