# $u\in C_0^3(\Omega)$ satisfis $-\Delta u=f(x), x\in\Omega.$Prove that $\sum_{i,j=1}^{n}\int_\Omega u^2_{x_i x_j} dx\leq n\int_{\Omega}f^2 dx.$

Suppose $$u\in C_0^3(\Omega)$$ satisfis $$-\Delta u=f(x), x\in\Omega.$$ Prove that $$\sum_{i,j=1}^{n}\int_\Omega u^2_{x_i x_j} \mathrm{d}x\leq n\int_{\Omega}f^2\mathrm{d}x.$$

I have tried as follows: Let $$v=\Delta u$$ in the Green's formula $$\int_\Omega \nabla u\cdot\nabla v \mathrm{d}x=-\int_\Omega v\Delta u \mathrm{d}x+\int_{\partial\Omega}\frac{\partial u}{\partial n}v\mathrm{d}S$$, then we get $$\int_\Omega \nabla u\cdot\nabla(\Delta u)dx=-\int_\Omega f^2+\int_{\partial\Omega}\frac{\partial u}{\partial n}(-f)\mathrm{d}S.$$ By the Integration by parts formula or Green's formula, I find the left side can be written as $$LHS=\sum_{i,j=1}^n\left[-\int_\Omega(u_{x_ix_j})^2\mathrm{d}x+\int_{\partial\Omega}u_{x_ix_j}\cdot u_{x_i}\cdot n^j\mathrm{d}S\right]\\=-\sum_{i,j=1}^n\int_\Omega(u_{x_ix_j})^2\mathrm{d}x+\int_{\partial\Omega}\mathrm{div}u\cdot\dfrac{\partial \mathrm{div}u}{\partial n}\mathrm{d}S.$$ I do not know how to proceed. Where to find the coeffcient $$n$$? Also, how to estimate the term $$\int_{\partial\Omega}\frac{\partial u}{\partial n}(-f)\mathrm{d}S$$?

Did I get wrong somewhere? Appreciate any help!

In fact a stronger statement holds and it follows directly from integration by parts. Indeed, integrating by parts twice we obtain \begin{align*}\sum_{i,j=1}^n \int_\Omega u^2_{x_ix_j} \, dx &= -\sum_{i,j=1}^n \int_\Omega u_{x_ix_ix_j}u_{x_j} \, dx \\ &=\sum_{i,j=1}^n \int_\Omega u_{x_ix_i}u_{x_jx_j} \, dx \\ &= \int_\Omega (\Delta u )^2 \, dx \\ &=\int_\Omega f^2 \, dx. \end{align*} There are no boundary terms since $$u$$ has compact support in $$\Omega$$.
• Why the first "=" holds? $u$ vanishes on the boundary but not $u_{x_j}$. What is your definition of $C_0^3(\Omega)$? Commented Dec 15, 2021 at 12:48
• The definition of $C^3_0(\Omega)$ is it is the space of functions $u \in C^3(\Omega)$ such that the support of $u$ is contained in some $\Omega' \subset \subset \Omega$. This implies that all the derivatives that exist vanish as you approach the boundary Commented Dec 15, 2021 at 13:29