Does anyone know a simple way to derive the following inequality for smooth, compactly supported functions in $\mathbb R^3$?
$$ \max \,\{ |u(x)| \mid x \in \mathbb R^3 \} \;\leq\; K\|\, Du \|_{L^2(\mathbb R^3)}^{1/2} \| D^2 u \|_{L^2(\mathbb R^3)}^{1/2} $$
(This is sometimes called "Moser inequality")