This is a cross post in continuation to this questionthis question on MathstackexchangeMathematics Stack Exchange. I wanted to know if this inequality holds true in two or three dimensions,
$\|\nabla\phi\|_{L^{\infty}(\Omega)}\leq C\|\phi\|_{H^2(\Omega)}.$
Where $\Omega$ is an open-bounded domain and $\phi$ is a test function, so we can assume $H^2(\Omega)$ regularity. We also have some leeway in putting extra conditions on the domain(I think convexity) or the boundaries.
Thank you!