A Thorough View of Exact Inference in Graphs from the Degree-4 Sum-of-Squares Hierarchy
This work addresses exact inference in graphs for applications like image segmentation and community detection, but it is incremental as it builds on existing SoS methods to analyze improvements in recoverability.
The paper tackles the problem of exactly recovering a binary node labeling from corrupted edge observations in graphs by applying the degree-4 sum-of-squares (SoS) hierarchy, showing that the dual solution relates to edge weights in Johnson and Kneser graphs and deriving a new Cheeger-type lower bound for algebraic connectivity.
Performing inference in graphs is a common task within several machine learning problems, e.g., image segmentation, community detection, among others. For a given undirected connected graph, we tackle the statistical problem of exactly recovering an unknown ground-truth binary labeling of the nodes from a single corrupted observation of each edge. Such problem can be formulated as a quadratic combinatorial optimization problem over the boolean hypercube, where it has been shown before that one can (with high probability and in polynomial time) exactly recover the ground-truth labeling of graphs that have an isoperimetric number that grows with respect to the number of nodes (e.g., complete graphs, regular expanders). In this work, we apply a powerful hierarchy of relaxations, known as the sum-of-squares (SoS) hierarchy, to the combinatorial problem. Motivated by empirical evidence on the improvement in exact recoverability, we center our attention on the degree-4 SoS relaxation and set out to understand the origin of such improvement from a graph theoretical perspective. We show that the solution of the dual of the relaxed problem is related to finding edge weights of the Johnson and Kneser graphs, where the weights fulfill the SoS constraints and intuitively allow the input graph to increase its algebraic connectivity. Finally, as byproduct of our analysis, we derive a novel Cheeger-type lower bound for the algebraic connectivity of graphs with signed edge weights.