Let
- $d\in\mathbb N$
- $\Omega\subseteq\mathbb R^d$ be open
- $\mathcal D(\Omega):=C_c^\infty(\Omega)$ and $\mathcal D(\Omega,\mathbb R^d):=C_c^\infty(\Omega,\mathbb R^d)$
- $H^{-1}(\Omega):=H_0^1(\Omega)'$ and $H^{-1}(\Omega,\mathbb R^d):=H_0^1(\Omega,\mathbb R^d)'$
Let me cite a well-known theorem:
Let $u\in\mathcal D'(\Omega,\mathbb R^d)$ $\Rightarrow$ $$\left.u\right|_{\mathfrak D(\Omega)}=0\;\Leftrightarrow\;\exists p\in\mathcal D'(\Omega):u=\nabla p\tag 1$$ where $$\mathfrak D(\Omega):=\left\{\phi\in\mathcal D(\Omega,\mathbb R^d):\nabla\cdot\phi=0\right\}\;.$$
Now, if $F\in H^{-1}(\Omega,\mathbb R^d)$, then $$f:=\left.F\right|_{\mathcal D(\Omega,\:\mathbb R^d)}\in\mathcal D'(\Omega,\mathbb R^d)\;.$$ And since $\mathfrak D(\Omega)\subseteq\mathcal D(\Omega,\mathbb R^d)$, we obtain $$\left.f\right|_{\mathfrak D(\Omega)}=\left.F\right|_{\mathfrak D(\Omega)}\;.$$ Thus, $$\left.F\right|_{\mathfrak D(\Omega)}=0\;\Leftrightarrow\;\exists p\in\mathcal D'(\Omega):\left.F\right|_{\mathcal D(\Omega,\:\mathbb R^d)}=\nabla p\tag 2$$ by $(1)$.
I would like to conclude that $$\left.F\right|_{\mathfrak D(\Omega)}=0\;\Leftrightarrow\;\exists\overline p\in H^{-1}(\Omega):F=\nabla\overline p\;.\tag 3$$
Is that possible (at least the direction "$\Rightarrow$")? And if so, how?