“…Necessary and sufficient conditions for optimality can be obtained from complementary slackness, see [2,15,5]. Problem (12) has been widely studied [1,2,17,12,4,5,6] and many approaches for its (approximate) large-scale optimization have been proposed, typically based on blockcoordinate descent [8,9,10,2,7,11,20,18] or constraint propagation [12,13,2,27,14]. If f is optimal for (12), then the CSP A * (f ) has a non-empty pairwise-consistency (PWC) closure (for binary WCSPs, PWC reduces to arc consistency) [15].…”