# Thread: The biharmonic problem with Dirichlet and Neumann conditions concerning unique sol

1. ## The biharmonic problem with Dirichlet and Neumann conditions concerning unique sol

The function $u \in H_0^2(U)$ (where $U\subset \mathbb{R}^n$ is bounded with smooth boundary $\partial U$) is a weak solution of the biharmonic equation
$\Delta(\Delta u)=f \mbox{ in }U$
and
$u=\frac{\partial u}{\partial\nu}=0\mbox{ on }\partial U$
provided $\int_U \Delta u\Delta v dx=\int_U f v dx$ for all $v\in H_0^2(U)$.

I want to show that if $f\in L^2 (U)$ then the weak solution $u$ is unique. I imagine this problem satisfies the conditions of the Lax Milgram theorem when the integral is considered as a bilinear operator. However, I am wondering why one can't show that $B[u,v]=\int_U \Delta u\Delta v dx$ defines an inner product on the $H_0^2(U)$ and apply the Riesz Representation Theorem directly to the problem? Am I missing something simple here?