Extremal decomposition of the complex plane with free poles

2019 ◽  
Vol 16 (3) ◽  
pp. 307-328
Author(s):  
Aleksandr Bakhtin ◽  
Iryna Denega

We consider an open extremal problem in geometric function theory of complex variables on the maximum of the functional $$r^\gamma\left(B_0,0\right)\prod\limits_{k=1}^n r\left(B_k,a_k\right),$$ where \(B_{0}\), ..., \(B_{n}\), \(n\ge 2\), are pairwise disjoint domains in \(\overline{\mathbb{C}}\), \(a_0 = 0\), \(|a_{k}| = 1\), \(k=\overline{1,n}\), and \(\gamma\in (0, n]\) (\(r(B,a)\) is the inner radius of the domain \(B\subset\overline{\mathbb{C}}\) relative to a point \(a\in B\)). For all values of the parameter \(\gamma\in (0, n]\), it is necessary to show that its maximum is attained for a configuration of domains \(B_{k}\) and points \(a_{k}\), \(k=\overline{0,n}\), possessing the \(n\)-fold symmetry. The problem was solved by V.N. Dubinin [1, 2] for \(\gamma=1\) and by G.V. Kuz’mina [4] for \(0 \lt \gamma \lt 1\). L.V. Kovalev [4] obtained its solution for \(n \ge 5\) under the additional assumption that the angles between neighbouring line segments \([0, a_{k}]\) do not exceed \(2\pi /\sqrt{\gamma}\). In particular, this problem will be solved in the present paper for \(n=2\) and \(\gamma\in(1,\,2]\).

2019 ◽  
Vol 27 (1) ◽  
pp. 61-77
Author(s):  
Iryna Denega ◽  
Yaroslav Zabolotnii

Abstract In geometric function theory of a complex variable problems on extremal decomposition with free poles on the unit circle are well known. One of such problem is the problem on maximum of the functional $${r^\gamma }({B_0},0)\prod\limits_{k = 1}^n r ({B_k},{a_k}),$$ where B0, B1, B2,..., Bn, n ≥ 2, are pairwise disjoint domains in ¯𝔺, a0 = 0, |ak| = 1, $k = \overline {1,n}$ and γ ∈ 2 (0; n], r(B, a) is the inner radius of the domain, B ⊂ ¯𝔺, with respect to a point a ∈ B. In the paper we consider a more general problem in which restrictions on the geometry of the location of points ak, $k = \overline {1,n}$ are weakened.


Author(s):  
Iryna Denega

In geometric function theory of complex variable extremal problems on non-overlapping domains are well-known classic direction. A lot of such problems are reduced to determination of the maximum of product of inner radii on the system of non-overlapping domains satisfying a certain conditions. In this paper, we consider the well-known problem of maximum of the functional \(r^\gamma\left(B_0,0\right)\prod\limits_{k=1}^n r\left(B_k,a_k\right)\), where \(B_{0}\),..., \(B_{n}\) are pairwise disjoint domains in \(\overline{\mathbb{C}}\), \( a_0=0 \), \(|a_{k}|=1\), \(k=\overline{1,n}\) are different points of the circle, \(\gamma\in (0, n]\), and \(r(B,a)\) is the inner radius of the domain \(B\subset\overline{\mathbb{C}}\) relative to the point \( a \). This problem was posed as an open problem in the Dubinin paper in 1994. Till now, this problem has not been solved, though some partial solutions are available. In the paper an estimate for the inner radius of the domain that contains the point zero is found. The main result of the paper generalizes the analogous results of [1, 2] to the case of an arbitrary arrangement of systems of points on \(\overline{\mathbb{C}}\).


Author(s):  
Andrey L. Targonskii

The sharp estimates of the product of the inner radius for pairwise disjoint domains are obtained. In particular, we solve an extremal problem in the case of an arbitrary finite number of the free poles on the unit circle for the following functional (see formula in paper)


Author(s):  
Liudmyla Vyhivska

The problem of extreme decomposition of a complex plane with fixed poles on a circle. Investigation on geometric function theory has been conducted by several researchers, however, few studies have reported on the problem considering extremal configurations the product of inner radii of non-overlapping domains with respect to fixed poles. The paper describes the problem of finding the maximum of the product of inner radii of mutually non-overlapping symmetric domains with respect to points on a unit circle multiply by a certain positive degree \(\gamma\) of the inner radius of the domain with respect to the zero. The problem was studied using the method of separating transformation. Proving the theorem shows that the maximum is obtained if \(\gamma\in(1,n^2]\) and for all \(n\geqslant 2\). Its results and the method for the obtaining of these results can be used in the theory of potential, approximations, holomorphic dynamics, estimation of the distortion problems in conformal mapping, and complex analysis.


Author(s):  
ELENA CHERKAEV ◽  
MINWOO KIM ◽  
MIKYOUNG LIM

The Neumann–Poincaré (NP) operator, a singular integral operator on the boundary of a domain, naturally appears when one solves a conductivity transmission problem via the boundary integral formulation. Recently, a series expression of the NP operator was developed in two dimensions based on geometric function theory [34]. In this paper, we investigate geometric properties of composite materials using this series expansion. In particular, we obtain explicit formulas for the polarisation tensor and the effective conductivity for an inclusion or a periodic array of inclusions of arbitrary shape with extremal conductivity, in terms of the associated exterior conformal mapping. Also, we observe by numerical computations that the spectrum of the NP operator has a monotonic behaviour with respect to the shape deformation of the inclusion. Additionally, we derive inequality relations of the coefficients of the Riemann mapping of an arbitrary Lipschitz domain using the properties of the polarisation tensor corresponding to the domain.


2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Hugo Arbeláez ◽  
Víctor Bravo ◽  
Rodrigo Hernández ◽  
Willy Sierra ◽  
Osvaldo Venegas

AbstractBieberbach’s conjecture was very important in the development of geometric function theory, not only because of the result itself, but also due to the large amount of methods that have been developed in search of its proof. It is in this context that the integral transformations of the type $f_{\alpha }(z)=\int _{0}^{z}(f(\zeta )/\zeta )^{\alpha }\,d\zeta $ f α ( z ) = ∫ 0 z ( f ( ζ ) / ζ ) α d ζ or $F_{\alpha }(z)=\int _{0}^{z}(f'(\zeta ))^{\alpha }\,d\zeta $ F α ( z ) = ∫ 0 z ( f ′ ( ζ ) ) α d ζ appear. In this note we extend the classical problem of finding the values of $\alpha \in \mathbb{C}$ α ∈ C for which either $f_{\alpha }$ f α or $F_{\alpha }$ F α are univalent, whenever f belongs to some subclasses of univalent mappings in $\mathbb{D}$ D , to the case of logharmonic mappings by considering the extension of the shear construction introduced by Clunie and Sheil-Small in (Clunie and Sheil-Small in Ann. Acad. Sci. Fenn., Ser. A I 9:3–25, 1984) to this new scenario.


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