A remark on omega limit sets for non-expansive dynamics

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 ω-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 ω-limit sets of systems that are non-expansive with respect to polyhedral norms (such as ℓ^p norms with p =1 or p=∞) are equilibria. In contrast, for non-expansive systems with respect to Euclidean (ℓ^2) 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 ω-limit sets. This paper shows that the Euclidean linear case is what can be expected in general: for flows that are contractive with respect to any strictly convex norm (such as ℓ^p for any p≠1,∞), and if there is at least one bounded solution, then the ω-limit set of every trajectory is also an omega limit set of a linear time-invariant system.