Connected Sub-graph Detection

We characterize the family of connected subgraphs in terms of linear matrix inequalities (LMI) with additional integrality constraints. We then show that convex relaxations of the integral LMI lead to parameterization of all weighted connected subgraphs. These developments allow for optimizing arbitrary graph functionals under connectivity constraints. For concreteness we consider the connected sub-graph detection problem that arises in a number of applications including network intrusion, disease outbreaks, and video surveillance. In these applications feature vectors are associated with nodes and edges of a graph. The problem is to decide whether or not the null hypothesis is true based on the measured features. For simplicity we consider the elevated mean problem wherein feature values at various nodes are distributed IID under the null hypothesis. The non-null (positive) hypothesis is distinguished from the null hypothesis by the fact that feature values on some unknown connected sub-graph has elevated mean.