In this note, we obtain uniqueness results for Beltrami flow in both bounded and unbounded domain with nonempty boundary by establishing an elementary but useful formula involving operators div and curl. We also use this formula to deal with Maxwell and Stokes eigenvalue problems.
KEYWORDSBeltrami flow, Liouville-type theorem, star-shaped domain, the first Maxwell eigenvalue, the first Stokes eigenvalue
INTRODUCTIONIn this note, we study the Beltrami flow, that is, a vector field u, which satisfies the systemBy establishing an elementary but useful identity, we obtain uniqueness results for Beltrami flow in both bounded and unbounded domain with nonempty boundary. As another interesting application, we use this identity to deal with Maxwell and Stokes eigenvalue problems.Since curlu × u = (u · ∇)u − ∇|u| 2 ∕2, each Beltrami flow will then give a special solution to the stationary Euler system. We refer the reader to Arnold and Khesin 1 for the basic properties of Beltrami flows. For some recent results, see previous studies 2-6 and references therein. We mention here that the Beltrami flows are also called force-free magnetic fields in magnetohydrodynamics, since the term curlu × u models the Lorentz force when u represents the magnetic field, see previous works 7-9 and references therein.Enciso and Peralta-Salas 3 constructed Beltrami fields, which satisfy curlu = u in R 3 for nonzero constant and fall off as |u(x)| < C|x| −1 at infinity. In particular, they are in L (R 3 , R 3 ) for all p > 3. A similar result can be also found in Lei et al. 10 Recently, Nadirashvili 11 proved a Liouville-type theorem for the globally defined Beltrami flow. He proved that when Ω = R 3 , a C 1 Beltrami flow satisfying either u ∈ L (R 3 , R 3 ), ∈ [2,3] or |u(x)| = o(|x| −1 ) as |x| → +∞ is in fact trivial, ie, u ≡ 0 in R 3 . Chae and Constantin 12 gave a new and elementary proof to a similar result, which partially covers the result of Nadirashvili. Chae and Wolf 13 succeeded in covering the result of Nadirashvili and got some improvements. Concerned with the exterior problem, Enciso, Poyato, and Soler 5 considered a related system 3632