Condenser Capacity and Hyperbolic Perimeter
CONDENSER CAPACITY AND HYPERBOLIC PERIMETER MOHAMED M. S. NASSER, OONA RAINIO, AND MATTI VUORINEN Abstract. We apply domain functionals to study the conformal capacity of condensers (G; E) where G is a simply connected domain in the complex plane and E is a compact subset of G. Due to conformal invariance, our main tools are the hyperbolic geometry and functionals such as the hyperbolic perimeter of E. Novel computational algorithms based on implementations of the fast multipole method are combined with analytic techniques. Computational experiments are used throughout to, for instance, demonstrate sharpness of established inequalities. In the case of model problems with known analytic solutions, very high precision of computation is observed. 1. Introduction A condenser is a pair (G; E), where G ⊂ Rn is a domain and E is a compact non-empty subset of G. The conformal capacity of this condenser is defined as [14, 17, 20] Z (1.1) cap(G; E) = inf jrujndm; u2A G 1 where A is the class of C0 (G) functions u : G ! [0; 1) with u(x) ≥ 1 for all x 2 E and dm is the n-dimensional Lebesgue measure. The conformal capacity of a condenser is one of the key notions in potential theory of elliptic partial differential equations [20, 29] and it has numerous applications to geometric function theory, both in the plane and in higher dimensions, [10, 14, 15, 17]. The isoperimetric problem is to determine a plane figure of the largest possible area whose boundary has a specified length, or, perimeter. Constrained extremal problems of this type, where the constraints involve geometric or physical quantities, have been studied in several thousands of papers, see e.g.
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