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Discrete Decreasing Minimization, Part II: Views from Discrete Convex Analysis

We continue to consider the discrete decreasing minimization problem on an integral base-polyhedron treated in Part I. The problem is to find a lexicographically minimal integral vector in an integral base-polyhedron, where the components of a vector are arranged in a decreasing order. This study can be regarded as a discrete counter-part of the work by Fujishige (1980) on the lexicographically optimal base and the principal partition of a base-polyhedron in continuous variables. The objective of Part II is two-fold. The first is to offer structural views from discrete convex analysis (DCA) on the results of Part I obtained by the constructive and algorithmic approach. The second objective is to pave the way of DCA approach to discrete decreasing minimization on other discrete structures such as the intersection of M-convex sets, flows, and submodular flows. We derive the structural results in Part I from fundamental facts on M-convex sets and M-convex functions in DCA. A direct characterization is given to the canonical partition, which was constructed by an iterative procedure in Part I. This reveals the precise relationship between the canonical partition for the discrete case and the principal partition for the continuous case. Moreover, this result entails a proximity theorem, stating that every decreasingly minimal element is contained in the small box containing the (unique) fractional decreasingly minimal element (the minimum-norm point), leading further to a continuous relaxation algorithm for finding a decreasingly minimal element of an M-convex set. Thus the relationship between the continuous and discrete cases is completely clarified. Furthermore, we present DCA min-max formulas for network flows, the intersection of two M-convex sets, and submodular flows.