On the minimum semidefinite rank of signed graphs

Abstract

The (real) minimum semidefinite rank of a signed graph is the minimum rank among all real symmetric positive semidefinite matrices associated to the graph and having the given sign pattern. We give a new lower bound for the minimum semidefinite rank of a signed multigraph and show it equals a new upper bound for signed complete multigraphs. This allows a complete characterization of signed multigraphs with minimum semidefinite rank two. We also determine the minimum semidefinite rank of all signed wheel graphs.