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Some Krasnosel'skii numbers for finitely starlike sets in the plane
Authors:Marilyn Breen
Institution:(1) Department of Mathematics, University of Oklahoma, 73019 Norman, Oklahoma, USA
Abstract:Let S be a subset of the plane. In case (int cl S) sim S = Ropf, then S is finitely starlike if and only if every 4 points of S see via S a common point. In case (int cl S) sim S has at most countably many components, each a singleton set, then S is finitely starlike if and only if every 5 points of S see via S a common point. Each of the numbers 4 and 5 is best possible. Examples show that these results fail without suitable restrictions on (int cl S) sim S. Moreover, a final example shows that if a general Krasnosel'skii number lambda. exists to characterize finitely starlike sets in the plane, then lambda > 9.
Keywords:Primary 52  A30  52  A35  Secondary 52  A10
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