A Self-dual Polar Factorization for Vector Fields

We show that any non-degenerate vector field $u$ in $ L^{\infty}(\Omega, \R^N)$,where$\Omega$isaboundeddomainin$\R^N$, can be written as {equation} \hbox{$u(x)={\mathrm{\nabla }}_{1}H(S(x),x)$ for a.e. $x\in \mathrm{\Omega }$}, {equation} where $S$ is a measure preserving point transformation on $\mathrm{\Omega }$ such that $S^2=I$ a.e (an involution), and $H:{\mathbb{R}}^N\times {\mathbb{R}}^N\to \mathbb{R}$ is a globally Lipschitz anti-symmetric convex-concave Hamiltonian. Moreover, $u$ is a monotone map if and only if $S$ can be taken to be the identity, which suggests that our result is a self-dual version of Brenier’s polar decomposition for the vector field $u$ as $u(x)=\mathrm{\nabla }\varphi (S(x))$, where $\varphi$ is convex and $S$ is a measure preserving transformation. We also describe how our polar decomposition can be reformulated as a self-dual mass transport problem.