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.
Reviewer: Christian Fenske (Gießen)
MSC:
47H10 | Fixed-point theorems |
54H25 | Fixed-point and coincidence theorems (topological aspects) |
47H04 | Set-valued operators |