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Let $\Omega \subset \mathbb{R}^n$ be a bounded domain with Lipschitz boundary (for Sobolev embedding and elliptic regularity).
The $H^2$ norm is: $$ \|u\|_{H^2(\Omega)}^2 = \|u\|_{L^2(\Omega)}^2 + \|\nabla u\|_{L^2(\Omega)}^2 + \|D^2 u\|_{L^2(\Omega)}^2, $$ where $$ \|D^2 u\|_{L^2(\Omega)}^2 = \sum_{i,j=1}^n \|\partial_{ij} u\|_{L^2(\Omega)}^2. $$

I am trying to prove: $$ \|u\|_{H^2(\Omega)}^2 \leq C \left( \|u\|_{L^2(\Omega)}^2 + \|\nabla u\|_{L^2(\Omega)}^2 + \|\Delta u\|_{L^2(\Omega)}^2 \right), \quad \forall\, u \in H^2(\Omega). $$

I don't know if this is true. If this is the case, is there some book with this proof?

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    $\begingroup$ Something does not look quite right in the second equation. You may want to check it. $\endgroup$ Commented Sep 30 at 7:08
  • $\begingroup$ Fixed, thank you. $\endgroup$ Commented Sep 30 at 10:06

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Let $E := \{u \in H^1(\Omega) \mid \Delta u \in L^2(\Omega)\}$ be equipped with the norm: $$\|u\|_E := \sqrt{\|u\|_{L^2}^2 + \|\nabla u\|_{L^2}^2 + \|\Delta u\|_{L^2}^2}$$ Almost by definition, $E$ is a Banach space with said norm.

Per the answers to the question Do $u\in H^1(\Omega)$ and $\Delta u\in L^2(\Omega)$ imply $u\in H^2(\Omega)$?, $E$ is generally distinct from $H^2$ even if $\partial \Omega$ is smooth due to integrability-up-to-the-boundary issues. On the other hand, the inequality you desire would imply that $H^2$ is closed in $E$.
It thus suffices to check that $H^2$ is dense in $E$ to show that your inequality cannot exist when $E$ is distinct from $H^2$, since dense and closed would imply that $H^2 = E$.

Now the procedure is a bit involved due to having to deal with the boundary, but an adaptation of the proof of Theorem $3$ of paragraph $5.3.3.$ in Evans' Partial differential equations should yield that $C^\infty\left(\overline{\Omega}\right) \cap E$ is dense in $E$ whenever $\partial \Omega$ is $C^1$, yet $C^\infty\left(\overline{\Omega}\right) \cap E \subseteq H^2 \subset E$, hence $H^2$ is dense in $E$ in that case, and we are done.

A more self-contained and confident answer would write down all the modifications, so you can see this more as a comment-answer if you want, but essentially the reason this should work is that you can get and use estimates on one combination of partial derivatives at a time (in this instance, I'm referring to the Laplacian) without having to make use of all the derivatives at the same time or only being able to use triangle inequalities with estimates on the "elementary" partial derivatives (each second derivative here).

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  • $\begingroup$ If the domain is a ball, do you think my inequality would be true? $\endgroup$ Commented Sep 30 at 20:39
  • $\begingroup$ @ThiagoGM Yes, the boundary of a ball is $C^1$ so it should work out. $\endgroup$ Commented Sep 30 at 21:14
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You need extra boundary conditions: either on $u|_{\partial \Omega}$ or the smoothness of the boundary itself (the domain being convex for example).

If for a moment you assume $u=0$ on $\partial \Omega$, this is essentially proving the $H^2$ regularity estimate for the weak solution $\Delta u = f$ when $f\in L^2$. The context is that $f$ is usually required to be a distribution not an $L^p$ function. So if one has extra regularity on $f$, $u$ got a regularity boost from $H^1$ to $H^2$.

For a long proof you can check Gilbarg-Trudinger Chapter 9 Theorem 9.9, basically proving a weak solution is a strong solution if the r.h.s. is $L^p$.

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  • $\begingroup$ The second example in the second answer to the post I linked shows that there can be a problem even when the domain is the unit disk, which is as smooth and convex as one can get, so I do think it's moreso a problem of traces and/or integrability up to the boundary. $\endgroup$ Commented Sep 30 at 15:23
  • $\begingroup$ @BrunoB The second example domain is not a unit disc, though, it is a domain with a crack (nonconvex). $\endgroup$ Commented Sep 30 at 16:49
  • $\begingroup$ Ah I see, I had misinterpreted the discussion in the comments below their answer. $\endgroup$ Commented Sep 30 at 17:50
  • $\begingroup$ So, what I need is true if the domain is a ball? $\endgroup$ Commented Sep 30 at 20:36
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    $\begingroup$ @ThiagoGM Yes, convexity suffices. You can check Grisvard's book as well. $\endgroup$ Commented Oct 1 at 14:04

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