The saga of a fish: from a survival guide to closing lemmas

TL;DR

This paper extends global controllability results for incompressible fluid flows to unbounded domains and connects it to structural stability, providing constructive methods and extending Pugh's closing lemma.

Contribution

It offers a constructive approach to modify velocity fields for conservative dynamics and extends Pugh's closing lemma to unbounded incompressible flows.

Findings

Constructive method for velocity field modification

Extension of Pugh's closing lemma to unbounded domains

Relationship between controllability and structural stability

Abstract

In the paper by D.~Burago, S.~Ivanov and A.~Novikov, "A survival guide for feeble fish", it has been shown that a fish with limited velocity can reach any point in the (possibly unbounded) ocean provided that the fluid velocity field is incompressible, bounded and has vanishing mean drift. This result extends some known global controllability theorems though being substantially nonconstructive. We give a fish a different recipe of how to survive in a turbulent ocean, and show its relationship to structural stability of dynamical systems by providing a constructive way to change slightly the velocity field to produce conservative (in the sense of not having wandering sets of positive measure) dynamics. In particular, this leads to the extension of C.~Pugh's closing lemma to incompressible vector fields over unbounded domains. The results are based on an extension of the Poincar\'{e} recurrence theorem to some $\sigma$-finite measures and on specially constructed Newtonian potentials.