Graphs with sparsity order at most two: The complex case.

The sparsity order of a (simple undirected) graph is the highest possible rank (over ${\mathbb R}$ or ${\mathbb C}$) of the extremal elements in the matrix cone that consists of positive semidefinite matrices with prescribed zeros on the positions that correspond to non-edges of the graph (excluding the diagonal entries). The graphs of sparsity order 1 (for both ${\mathbb R}$ and ${\mathbb C}$) correspond to chordal graphs, those graphs that do not contain a cycle of length greater than three, as an induced subgraph, or equivalently, is a clique-sum of cliques. There exist analogues, though more complicated, characterizations of the case where the sparsity order is at most 2, which are different for ${\mathbb R}$ and ${\mathbb C}$. The existing proof for the complex case, is based on the result for the real case. In this paper we provide a more elementary proof of the characterization of the graphs whose complex sparsity order is at most two. Part of our proof relies on a characterization of the $\{P_4,\overline{K}_3\}$-free graphs, with $P_4$ the path of length 3 and $\overline{K}_3$ the stable set of cardinality 3, and of the class of clique-sums of such graphs.