Eventual cone invariance revisited

Abstract

We consider finite-dimensional real vector spaces $X$ ordered by a closed cone $X_+$ with non-empty interior and study eventual nonnegativity of matrix semigroups $(e^{tA})_{t \ge 0}$ with respect to this cone. Our first contribution is the observation that, for general cones, one needs to distinguish between different notions of eventual nonnegativity: (i) uniform eventual nonnegativity means that $e^{tA}$ maps $X_+$ into $X_+$ for all sufficiently large times $t$; (ii) individual eventual nonnegativity means that for each $x \in X_+$ the vector $e^{tA}x$ is in $X_+$ for all $t$ larger than an $x$-dependent time $t_0$; and (iii) weak eventual nonnegativity means that for each $x \in X_+$ and each functional $x'$ in the dual cone $X'_+$ the value $\langle x', e^{tA} x \rangle$ is in $[0,\infty)$ for all $t$ larger than an $x$- and $x'$-dependent time $t_0$. Until now, only the first of these notions has been studied in the literature. We demonstrate by examples that, somewhat surprisingly for finite-dimensional spaces, all three notions are different. Our second contribution is to show that typical Perron-Frobenius like properties remain valid under the weakest of the above notions. Third, we study a strengthened form of the above mentioned concepts, namely eventual positivity. We prove that uniform, individual and weak versions of this property are - in contrast to the nonnegative case - equivalent, and that they can be characterized by spectral properties.