Moons, bridges, birds … and nonexpansive mappings in Hilbert space
Kasimierz Goebel, Rainald Schöneberg · Bulletin of the Australian Mathematical Society · 1977
In recent years some fixed point theorems have been proved for nonexpansive mappings in Hilbert space, which have non-convex domains. The purpose of this paper is to present a simple but very useful new result of that kind and to indicate some of its consequences.