Ela Note on Positive Semidefinite Maximum Nullity and Positive Semidefinite Zero Forcing Number of Partial 2-trees

The maximum positive semidefinite nullity of a multigraph G is the largest possible nullity over all real positive semidefinite matrices whose (i, j)th entry (for i 6= j) is zero if i and j are not adjacent in G, is nonzero if {i, j} is a single edge, and is any real number if {i, j} is a multiple edge. The definition of the positive semidefinite zero forcing number for simple graphs is extended to multigraphs; as for simple graphs, this parameter bounds the maximum positive semidefinite nullity from above. The tree cover number T(G) is the minimum number of vertex disjoint induced simple trees that cover all of the vertices of G. The result that M+(G) = T(G) for an outerplanar multigraph G [F. Barioli et al. Minimum semidefinite rank of outerplanar graphs and the tree cover number. Electron. J. Linear Algebra, 22:10–21, 2011.] is extended to show that Z+(G) = M+(G) = T(G) for a multigraph G of tree-width at most 2.