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The Ky Fan fixed point theorem on star-shaped domains. (English) Zbl 1094.47048

Ky Fan’s theorem states that a set-valued map \(A\) defined on a nonempty compact convex subset \(X\) of a topological vector space with values in the nonempty convex subsets of \(X\) has a fixed point provided the preimage of each point is open in \(X\). The present author shows that it suffices to assume that \(X\) is star-shaped instead of convex.

MSC:

47H10 Fixed-point theorems
54H25 Fixed-point and coincidence theorems (topological aspects)
47H04 Set-valued operators