Is My System of ODEs k-Cooperative?

Abstract

A linear dynamical system is called positive if its flow maps the non-negative orthant to itself. More precisely, it maps the set of vectors with zero sign variations to itself. A linear dynamical system is called $k$-positive if its flow maps the set of vectors with up to $k-1$ sign variations to itself. A nonlinear dynamical system is called $k$-cooperative if its variational system, which is a time-varying linear dynamical system, is $k$-positive. These systems have special asymptotic properties. For example, it was recently shown that strongly $2$-cooperative systems satisfy a strong Poincar\'{e}-Bendixson property. Positivity and~$k$-positivity are easy to verify in terms of the sign-pattern of the matrix in the dynamics. However, these sign conditions are not invariant under a coordinate transformation. A natural question is to determine if a given~$n$-dimensional system is $k$-positive up to a coordinate transformation. We study this problem for two special kinds of transformations: permutations and scaling by a signature matrix. For any $n\geq 4$ and~$k\in\{2,\dots, n-2\}$, we provide a graph-theoretical necessary and sufficient condition for $k$-positivity up to such coordinate transformations. We describe an application of our results to a specific class of Lotka-Volterra systems.