The complexity of minimum difference cover

The complexity of searching minimum difference covers  , both in Z+Z+ and in ZnZn, is studied. We prove that these two optimization problems are NP-hard. To obtain this result, we characterize those sets—called extrema—having themselves plus zero as minimum difference cover. Such a combinatorial characterization enables us to show that testing whether sets are not extrema, both in Z+Z+ and in ZnZn, is NP-complete. However, for these two decision problems we exhibit pseudo-polynomial time algorithms.