On a Convex Operator for Finite Sets

Abstract

Let $S$ be a finite set with $n$ elements in a real linear space. Let $\cJ_S$ be a set of $n$ intervals in $\nR$. We introduce a convex operator $\co(S,\cJ_S)$ which generalizes the familiar concepts of the convex hull $\conv S$ and the affine hull $\aff S$ of $S$. We establish basic properties of this operator. It is proved that each homothet of $\conv S$ that is contained in $\aff S$ can be obtained using this operator. A variety of convex subsets of $\aff S$ can also be obtained. For example, this operator assigns a regular dodecagon to the 4-element set consisting of the vertices and the orthocenter of an equilateral triangle. For $\cJ_S$ which consists of bounded intervals, we give the upper bound for the number of vertices of the polytope $\co(S,\cJ_S)$.