The problem is:
For any closed 2-form in the Minkowski space $\mathbb{R}^{3,1}$ satisfiying $dF=0$ and $\delta F \ne 0$ (with $F$$\delta$ denoting the codifferential), does there exist a Lorentz manifold $(M,g)$ and a corrsponding diffeomorpism $f:M\to\mathbb{R}^{3,1}$ such that the pullback of $F$ on $M$, $f^{*}F$, satisfies $d(f^{*}F)=0$ and $\delta(f^{*}F)=0$?