The Smallest Enclosing Ball Problem and the Smallest Intersecting Ball   Problem: Existence and Uniqueness of Solutions
  In this paper we study the following problems: given a finite number of nonempty closed subsets of a normed space, find a ball with the smallest radius that encloses all of the sets, and find a ball with the smallest radius that intersects all of the sets. These problems can be viewed as generalized versions of the smallest enclosing circle problem introduced in the 19th century by Sylvester which asks for the circle of smallest radius enclosing a given set of finite points in the plane. We will focus on the sufficient conditions for the existence and uniqueness of an optimal solution for each problem, while the study of optimality conditions and numerical implementation will be addressed in our next projects. 
Boris S. Mordukhovich, Nguyen Mau Nam, Cristina Villalobos
math.OC