Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 13 / 3 / c

For finite $p>n$, chooseSince $n\ge 3$ and $p>n$, we have $2<q<2^{\ast }=2n/(n-2)$. The zero-boundary versions of the Sobolev embedding theorem and Rellich-Kondrachov compactness theorem say that $W_0^{1,2}(\Omega )$ embeds continuously into $L^{2^{\ast }}$ and compactly into $L^q$ for $q<2^{\ast }$ on a bounded open set. These versions hold on arbitrary bounded open sets by zero extension; a boundary extension operator is not needed.

If $(v_j)$ is bounded in $L^2$, then $(T{v}_j)$ is bounded in $W_0^{1,2}$ by the preceding part. A subsequence consequently converges in $L^q$ to some $z$. Hölder's inequality givesMultiplication by $w$ is bounded from $L^q$ to $L^2$, and the same estimate proves that $K$ is bounded and well defined. Every bounded input sequence has a subsequence whose images converge in $L^2$, so $\boxed{K\text{ is compact}}$. If $p=\infty$ is included, take $q=2$ and use the same argument. This is the compact L2 potential perturbation of the Dirichlet Laplacian mechanism.