Does there exist a good reference for the inequality:
$$\Vert u \Vert_{H^2(\Omega)} = \Vert D^2 u\Vert_{L_2(\Omega)} \leq C\left( ||\Delta u||_{L_2(\Omega)} + ||u||_{L_2(\Omega)}\right),$$
where $u \in H^2(\Omega)$?
I noticed this inequality for example here, but could not find any reference that derives it. I would like to know more about the constant $C$.