Separating Convex Sets in the Plane. J. Urrutia; J. Czyzowicz; E. Rivera-Campo; J. Zaks — 1992 Discrete & computational geometry
A Combinatorial Result About Points and Balls in Euclidean Space. I. Bárány; J.H. Schmerl; S.J. Sidney; J. Urrutia — 1989 Discrete & computational geometry