Preliminaries 1.1 Function spaces 1.2 Basic facts from measure theory 1.3 Markov processes and random dynamical systems Notes and comments 2 Two-dimensional Navier-Stokes equations 2.1 Cauchy problem for the deterministic system 2.2 Stochastic Navier-Stokes equations 2.3 Navier-Stokes equations perturbed by a random kick force 2.4 Navier-Stokes equations perturbed by spatially regular white noise 2.5 Existence of a stationary distribution 2.6 Appendix: some technical proofs Notes and comments 3 Uniqueness of stationary measure and mixing 3.1 Three results on uniqueness and mixing 3.2 Dissipative RDS with bounded kicks 3.3 Navier-Stokes system perturbed by white noise 3.4 Navier-Stokes system with unbounded kicks 3.5 Further results and generalisations 3.6 Appendix: some technical proofs 3.7 Relevance of the results for physics Notes and comments vii www.cambridge.org
We formulate two results on controllability properties of the 3D NavierStokes (NS) system. They concern the approximate controllability and exact controllability in finite-dimensional projections of the problem in question. As a consequence, we obtain the existence of a strong solution of the Cauchy problem for the 3D NS system with an arbitrary initial function and a large class of right-hand sides. We also discuss some qualitative properties of admissible weak solutions for randomly forced NS equations.
AMS subject classifications: 35Q30, 60H15, 76D05, 93B05, 93C20Keywords: Approximate controllability, exact controllability in projections, 3D Navier-Stokes system, Agrachev-Sarychev method, stationary solutions, irreducibility.
Main resultsLet D ⊂ R 3 be a bounded domain with C 2 -smooth boundary ∂D. Consider 3D Navier-Stokes (NS) equationṡwhere u = (u 1 , u 2 , u 3 ) and p are unknown velocity and pressure fields, ν > 0 is the viscosity, and f (t, x) is an external force. We introduce the spaceswhere n stands for the outward unit normal to ∂D and ·, · denotes the scalar product in R 3 . It is well known (e.g., see [Tem79]) that H is a closed vector 1
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