Convexity of balls and fixed-point theorems for mappings with nonexpansive square K. Goebel — 1970 Compositio Mathematica
Irregular convex sets with fixed-point property for nonexpansive mappings K. Goebel; T. Kuczumow — 1979 Colloquium Mathematicae
A fixed point theorem for transformations whose iterates have uniform Lipschitz constant K. Goebel; W. Kirk — 1973 Studia Mathematica