On the convexity of piecewise-defined functions
  Functions that are piecewise defined are a common sight in mathematics while convexity is a property especially desired in optimization. Suppose now a piecewise-defined function is convex on each of its defining components - when can we conclude that the entire function is convex? In this paper we provide several convenient, verifiable conditions guaranteeing convexity (or the lack thereof). Several examples are presented to illustrate our results. 
Heinz H. Bauschke, Yves Lucet, and Hung M. Phan
math.CA math.OC