The Green function of Teichm\"uller spaces with applications

We describe briefly a new approach to some problems related to Teichm\"uller spaces, invariant metrics, and extremal quasiconformal maps. This approach is based on the properties of plurisubharmonic functions, especially of the plurisubharmonic Green function. The main theorem gives an explicit representation of the Green function for Teichm\"uller spaces by the Kobayashi-Teichm\"uller metric of these spaces. This leads to various applications. In particular, this gives a new characterization of extremal quasiconformal maps.Comment: 5 page

Milin's coefficients, complex geometry of Teichm\"{u}ller spaces and variational calculus for univalent functions

We investigate the invariant metrics and complex geodesics in the universal Teichm\"{u}ller space and Teichm\"{u}ller space of the punctured disk using Milin's coefficient inequalities. This technique allows us to establish that all non-expanding invariant metrics in either of these spaces coincide with its intrinsic Teichm\"{u}ller metric. Other applications concern the variational theory for univalent functions with quasiconformal extension. It turns out that geometric features caused by the equality of metrics and connection with complex geodesics provide deep distortion results for various classes of such functions and create new phenomena which do not appear in the classical geometric function theory

Hyperbolic distances, nonvanishing holomorphic functions and Krzyz's conjecture

The goal of this paper is to prove the conjecture of Krzyz posed in 1968 that for nonvanishing holomorphic functions $f(z) = c_0 + c_1 z + ...$ in the unit disk with $|f(z)| \le 1$, we have the sharp bound $|c_n| \le 2/e$ for all $n \ge 1$, with equality only for the function $f(z) = \exp [(z^n - 1)/(z^n + 1)]$ and its rotations. The problem was considered by many researchers, but only partial results have been established. The desired estimate has been proved only for $n \le 5$. Our approach is completely different and relies on complex geometry and pluripotential features of convex domains in complex Banach spaces

Hyperbolic metrics, homogeneous holomorphic functionals and Zalcman's conjecture

We show, using the Kobayashi and Caratheodory metrics on special holomorphic disks in the universal Teichmuller space, that a wide class of holomorphic functionals on the space of univalent functions in the disk is maximized by the Koebe function or by its root transforms; their extremality is forced by hyperbolic features. As consequences, this implies the proofs of the famous Zalcman and Bieberbach conjectures.Comment: This is a revised and improved version of arXiv:0907.3623. Comments are welcom

The Zalcman conjecture and related problems

At the end of 1960's, Lawrence Zalcman posed a conjecture that the coefficients of univalent functions $f(z) = z + \sum\limits_2^\infty a_n z^n$ on the unit disk satisfy the sharp inequality $|a_n^2 - a_{2n-1}| \le (n-1)^2$, with equality only for the Koebe function. This remarkable conjecture implies the Bieberbach conjecture, investigated by many mathematicians, and still remains a very difficult open problem for all n > 3; it was proved only in certain special cases. We provide a proof of Zalcman's conjecture based on results concerning the plurisubharmonic functionals and metrics on the universal Teichm\"uller space. As a corollary, this implies a new proof of the Bieberbach conjecture. Our method gives also other new sharp estimates for large coefficients.Comment: This paper has been replaced with a revised version arXiv:1109.4646v

A new look at Krzyz's conjecture

Recently the author has presented a new approach to solving extremal problems of geometric function theory. It involves the Bers isomorphism theorem for Teichmuller spaces of punctured Riemann surfaces. We show here that this approach, combined with quasiconformal theory, can be also applied to nonvanishing holomorphic functions from $H^\infty$. In particular this gives a proof of an old open Krzyz conjecture for such functions and of its generalizations. The unit ball $H_1^\infty$ of $H^\infty$ is naturally embedded into the universal Teichmuller space, and the functions $f \in H_1^\infty$ are regarded as the Schwarzian derivatives of univalent functions in the unit disk.Comment: arXiv admin note: text overlap with arXiv:1908.0518

Extremal quasiconformality vs rational approximation

We show that on most of the hyperbolic simply connected domains the weighted bounded rational approximation in a natural sup norm is possible only for a very sparse set of holomorphic functions (in contrast to integral approximation). The obstructions are caused by the features of extremal quasiconformality

A general coefficient theorem for univalent functions

Using the Bers isomorphism theorem for Teichmuller spaces of punctured Riemann surfaces and some of their other complex geometric features, we prove a general theorem on maximization of homogeneous polynomial (in fact, more general holomorphic) coefficient functionals $J(f) = J(a_{m_1}, a_{m_2},\dots, a_{m_n})$ on some classes of univalent functions in the unit disk naturally connected with the canonical class $S$. The given functional $J$ is lifted to the Teichmuller space $\mathbf T_1$ of the punctured disk $\mathbb{D}_{*} = \{0 < |z| < 1\}$ which is biholomorphically equivalent to the Bers fiber space over the universal Teichmuller space. This generates a positive subharmonic function on the disk $\{|t| < 4\}$ with $\sup_{|t|<4} u(t) = \max_{\mathbf T_1} |J|$ attaining this maximal value only on the boundary circle, which correspond to rotations of the Koebe function. This theorem implies new sharp distortion estimates for univalent functions giving explicitly the extremal functions, and creates a new bridge between Teichm\"{u}ller space theory and geometric complex analysis. In particular, it provides an alternate and direct proof of the Bieberbach conjecture

A question of Kuhnau

It is well known that the square is not a Strebel's point (i.e., its extremal Beltrami coefficient is not Teichmuller). Many years ago, Reiner Kuhnau posed the question: does there exist in the case of a "long" rectangle the corresponding holomorphic quadratic differential? We prove that the answer is negative for any bounded convex quadrilateral and establish a stronger result for rectangles.Comment: arXiv admin note: text overlap with arXiv:1806.0276

Quasiconformal features and Fredholm eigenvalues of convex polygons

An important open problem in geometric complex analysis is to find algorithms for explicit determination of basic functionals intrinsically connected with conformal and quasiconformal maps, such as their Teichmuller and Grunsky norms, Fredholm eigenvalues and the quasireflection coefficient. This has not been solved even for convex polygons. This case has intrinsic interest in view of the connection of such polygons with the geometry of the universal Teichmuller space. We provide a new approach, based on affine transformations of univalent functions