Graph covers and quadratic minimization

We formulate a new approach to understanding the behavior of the min-sum algorithm by exploiting the properties of graph covers. First, we present a new, natural characterization of scaled diagonally dominant matrices in terms of graph covers; this result motivates our approach because scaled diagonal dominance is a known sufficient condition for the convergence of min-sum in the case of quadratic minimization. We use our understanding of graph covers to characterize the periodic behavior of the min-sum algorithm on a single cycle. Lastly, we explain how to extend the single cycle results to understand the 2-periodic behavior of min-sum for general pairwise MRFs. Some of our techniques apply more broadly, and we believe that by capturing the notion of indistinguishability, graph covers represent a valuable tool for understanding the abilities and limitations of general message-passing algorithms.