Helicity is the only invariant of incompressible flows whose derivative is continuous in $C^1$-topology

Elena A. Kudryavtseva

Let $Q$ be a smooth compact orientable 3--manifold with smooth boundary $\partial Q$. Let $\mathcal{B}$ be the set of exact 2--forms $B\in {\Omega }^2(Q)$ such that $j_{\partial Q}^{\ast }B=0$, w…