Borsuk Number for Planar Convex Bodies

Abstract

By using some simple tools from graph theory, we obtain a characterization of the compact sets in Rn with Borsuk number equal to two. This result allows to give some examples of planar (convex) compact sets with Borsuk number equal to three. Moreover, we also prove that the unique centrally symmetric planar convex compact sets with Borsuk number equal to three are the Euclidean balls.