arXiv subjects
Publications and source records attributed to Samuel L. Krushkal.
This paper introduces a new approach to estimating polynomial coefficients, an important problem in complex analysis. This approach is intrinsically connected with features of univalent functions and of Teichmuller spaces.
The well-known Zalcman conjecture, which implies the Bieberbach conjecture, states that the coefficients of univalent functions $f(z) = z + \sum\limits_2^{\infty} a_n z^n$ on the unit disk satisfy $|a_n^2 - a_{2n-1}| \le (n-1)^2$ for all $n > 2$, with equality only for the Koebe function and its rotations. The conjecture was proved by the author for $n \le 6$ (using geometric arguments related to the Ahlfors-Schwarz lemma) and remains open for $n \ge 7$. The main theorem of this paper states that these conjectures are equivalent and provides their simultaneous proof for all $n \ge 3$ combining the indicated geometric arguments with a new author's approach to extremal problems for holomorphic functions based on lifting the rotationally homogeneous coefficient functionals to the Bers fiber space over universal Teichmuller space.
Estimating the coefficient functionals on various classes of holomorphic functions traditionally forms an important field of geometric complex analysis and its mathematical and physical applications. These coefficients reflect fundamental intrinsic features of holomorphy and of conformality. This paper surveys the results obtained by a new approach involving deep features of Teichmuller spaces. This approach was recently suggested by the author. The paper also contains some new results generalizing the classical coefficient conjectures and presents open problems.
Recently the author presented a new approach to solving the coefficient problems for various classes of holomorphic functions $f(z) = \sum\limits_0^\infty c_n z^n$, not necessarily univalent. This approach is based on lifting the given polynomial coefficient functionals $J(f) = J(c_{m_1}, \dots, c_{m_s}), 2 < c_{m_1} < \dots < c_{m_s} < \infty$, onto the Bers fiber space over universal Teichmuller space and applying the analytic and geometric features of Teichmüller spaces, especially the Bers isomorphism theorem for Teichmuller spaces of punctured Riemann surfaces. In this paper, we extend this approach to more general classes of functions. In particular, this provides a strengthening of de Branges' theorem solving the Bieberbach conjecture.
The Grunsky operator arises from univalence and plays a crucial role in geometric function theory. This operator also implies quasiconformal extendibility and has an intrinsic connection with Teichmuller space theory and its interactions with complex analysis and pluripotential theory. This paper surveys recent results in this field and simultaneously provides solutions of two old problems.
Biunivalent holomorphic functions form an interesting class in geometric function theory and are connected with special functions and solutions of complex differential equations. The paper reveals a deep connection between biunivalence and geometry of Teichmuller balls and provides some sufficient conditions for biunivalence of holomorphic functions on the disk. Among the consequences, one obtains new sharp distortion theorems for univalent functions with quasiconformal extension.
This paper is the final step in solving the problem of starlikeness of Teichmuller spaces in Bers' embedding. This step concerns the case of finite dimensional Teichmuller spaces ${\mathbf T}(g, n)$ of positive dimension (corresponding to punctured Riemann surfaces of finite conformal type $(g, n)$ with $2g - 2 + n > 0$).
The problem of starlikeness of Teichmuller spaces in Bers' embedding was raised in 1974 and is solved (negatively) for Teichmuller spaces of sufficiently large dimensions. The original proof given by the author relies on the existence of conformally rigid domains established by Thurston. Later the author found another proof of non-starlikeness of universal Teichmuller space based on geometric features of rectilinear polygons. This paper provides a complete solution of the problem for Teichmuller spaces $T(g,0)$ of closed Riemann surfaces of genus $g \ge 2$.
The paper continues the author's research in the problem of quantitative investigation of basic curvelinear quasiinvariants of quasiconformal curves. It concerns polygons with infinite number of vertices and provides various distortion estimates in terms of intrinsic geometric characteristics of polygons. In particular, this implies the coarse upper and lower estimates for the Grunsky and Teichmuller norms of a conformal map of the disk onto any piecewise $C^{1+}$-smooth bounded quasicircle.
We establish that the Grunsky norm of any normalized univalent function on the disk is completely determined by the squares of holomorphic abelian differentials (in contrast to the Teichmuller norm, which relates to all integrable holomorphic quadratic differentials). This result has important interesting applications. In particular, it provides an explicit representation of Fredholm eigevalues of all quasiconformal curves.
An important problem in applications of quasiconformal analysis and in its numerical aspect is to establish algorithms for explicit or approximate determination of the basic quasiinvariant curvelinear and analytic functionals intrinsically connected with conformal and quasiconformal maps, such as their Teichmuller and Grunsky norms, Fredholm eigenvalues and the quasireflection coefficients of associated quasicircles. We prove a general theorem of new type answering this question for univalent functions in arbitrary quasiconformal domains and provide its applications. The results are strengthened in the case of maps of the disk and give rise to extremal Beltrami coefficients of a new type.
Recently the author proved that the 1977 Hummel-Scheinberg-Zalcman conjecture on coefficients of nonvanishing $H^p$ functions is true for all $p = 2m, m \in \mathbb{N}$, i.e., for the Hilbertian Hardy spaces $H^{2m}$. As a consequence, this also implies a proof of the Krzyz conjecture for bounded nonvanishing functions which originated this direction. In the present paper, we solve the problem for all spaces $H^p$ with $p \ge 2$.
A. Grinshpan posed a deep conjecture on the norm of the Grunsky operator generated by univalent functions in the disk. It gives a quantitative answer in terms of the Grunsky coefficients, to which extent a univalent function determines the bound of dilatations of its quasiconformal extensions. We provide the proof of this conjecture and its various analytic, geometric and potential applications. Another result concerns the model of universal Teichmuller space by Grunsky coefficients.
Recently the author presented a new approach to solving the coefficient problems for holomorphic functions based on the deep features of Teichmuller spaces. It involves the Bers isomorphism theorem for Teichmuller spaces of punctured Riemann surfaces. The aim of the present paper is to provide new applications of this approach and extend the indicated results to more general classes of functions
The problem of holomorphic contractibilty of the Teichmuller spaces $T(0, n)$ of punctured spheres ($n > 4$) arose in the 1970s in connection with solving algebraic equations in Banach algebras. Recently it was solved by the author in \cite{Kr2}. In the present paper we improve the statement of Lemma 3 in \cite{Kr2} and provide an alternate proof of holomorphic contractibility of low dimensional Teichmuller spaces.
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.
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.
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 Teichmüller space theory and geometric complex analysis. In particular, it provides an alternate and direct proof of the Bieberbach conjecture.