Minimum Rank of Graphs with Loops

A loop graph S is a finite undirected graph that allows loops but does not allow multiple edges. The set S(S) of real symmetric matrices associated with a loop graph of order n is the set of symmetric matrices A = [a(ij)] is an element of R-nxn such that a(ij) not equal 0 if and only if ij is an element of E(S). The minimum (maximum) rank of a loop graph is the minimum (maximum) of t he ranks of the matrices in S(S). Loop graphs having minimum rank at most two are characterized (by forbidden induced subgraphs and graph complements) and loop graphs having minimum rank equal to the order of the graph are characterized. A Schur complement reduction technique is used to determine the minimum ranks of cycles with various loop configurations; the minimum ranks of complete graphs and paths with various configurations of loops are also determined. Unlike simple graphs, loop graphs can have maximum rank less than the order of the graph. Some results are presented on maximum rank and which ranks between minimum and maximum can be realized. Interesting open questions remain