The Diameter of a Hyperbolic Disc |
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Authors: | A F Beardon D Minda |
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Institution: | (3) Inst. Elie Cartan, Nancy, France;(4) Systems Res. Inst., Warsaw, Poland |
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Abstract: | Recently, Matti Vuorinen asked whether the set-theoretic diameter of a hyperbolic disc of radius r in a hyperbolic plane region Ω is 2r. The answer is affirmative if Ω is simply or doubly connected. However, there are a hyperbolic discs in the triply-punctured
sphere whose set-theoretic diameter is less than twice the radius. Also, for finitely connected hyperbolic plane regions all
hyperbolic discs sufficiently close to the boundary have set-theoretic diameter equal to twice the radius. Precisely, if Ω
is a hyperbolic plane region of finite connectivity, then there is a compact subset K of Ω such that any hyperbolic disc which is disjoint from K has diameter equal to twice the radius. |
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