Irreducible Combinatorially Symmetric Sign Patterns Requiring a Unique Inertia

Abstract: A sign pattern is a matrix whose entries belong to the set ${+,-,0}$. A sign pattern requires a unique inertia if every real matrix in its qualitative class has the same inertia. Symmetric tree sign patterns requiring a unique inertia has been studied extensively in \cite{2001, 2001a, 2018}. Necessary and sufficient conditions in terms of the symmetric minimal and maximal rank, as well as conditions depending on the position and sign of the loops in the underlying graph of such patterns has been used to characterize inertia of symmetric tree sign patterns. In this paper, we consider combinatorially symmetric sign patterns with a $0$-diagonal and identify some such patterns with interesting combinatorial properties, which does not require a unique inertia. Initially, we begin with combinatorially symmetric tree sign patterns with a $0$-diagonal, with a special focus on tridiagonal sign patterns. We then consider patterns whose underlying undirected graph contain cycles but no loops, and we derive necessary conditions based on the sign of the edges and the distance between the cycles in the underlying graph for such patterns to require a unique inertia.