Past exam of the mathematics course of the University of Cambridge / 2025 / ii / Paper 4 / 23H / b / i / Solution

Weak convergence $u_n\rightharpoonup u$ in $H_0^1(\Omega )$ impliesin $L^2$. The $L^2$ norm is weakly lower semicontinuous, so

The Rellich-Kondrashov compactness theorem for H01 upgrades the weak convergence tostrongly in $L^2(\Omega )$. Indeed, compactness gives this along every subsequence after passage to a further subsequence, and the weak limit uniquely identifies every such strong limit as $u$. Consequently,Combining the two terms proves the weak lower semicontinuity of a bounded-domain Schrodinger energy: