A remark on ω-limit sets for non-expansive dynamics

Alon Duvall, Eduardo D. Sontag

Abstract: In this paper, we study systems of time-invariant ordinary differential equations whose flows are non-expansive with respect to a norm, meaning that the distance between solutions may not increase. Since non-expansiveness (and contractivity) are norm-dependent notions, the topology of $\omega$-limit sets of solutions may depend on the norm. For example, and at least for systems defined by real-analytic vector fields, the only possible $\omega$-limit sets of systems that are non-expansive with respect to polyhedral norms (such as $\ell^{p}$ norms with $p=1$ or $p=\infty$) are equilibria. In contrast, for non-expansive systems with respect to Euclidean $\left(\ell^{2}\right)$ norm, other limit sets may arise (such as multi-dimensional tori): for example linear harmonic oscillators are non-expansive (and even isometric) flows, yet have periodic orbits as $\omega$-limit sets. This paper shows that the Euclidean linear case is what can be expected in general: for flows that are non-expansive with respect to any strictly convex norm (such as $\ell^{p}$ for any $p \neq 1, \infty$), and if there is at least one bounded solution, then the $\omega$-limit set of every trajectory is also an $\omega$-limit set of a linear time-invariant system.