Discrete Convexity in Joint Winner Property

In this paper, we reveal a relation between joint winner property (JWP) in the field of valued constraint satisfaction problems (VCSPs) and M-convexity in the field of discrete convex analysis (DCA). We introduce the M-convex completion problem, and show that a function f satisfying the JWP is Z-free if and only if a certain function f associated with f is M-convex completable. This means that if a function is Z-free, then the function can be minimized in polynomial time via M-convex intersection algorithms. Furthermore we propose a new algorithm for Z-free function minimization, which is faster than previous algorithms for some parameter values.