Past exam of the mathematics course of the University of Cambridge / 2017 / iii / Paper 105 / 2 / c

Test the weak formulation with $u$ itself. Uniform ellipticity and the Cauchy-Schwarz inequality giveThe Poincaré inequality for the bounded shell with zero trace gives $\Vert u{\Vert }_2\le C_P\Vert \nabla u{\Vert }_2$. If the gradient is nonzero, divide to obtain $\Vert \nabla u{\Vert }_2\le C_P{\lambda }^{-1}\Vert f{\Vert }_2$; if it is zero the estimate is immediate and the zero trace forces $u=0$. ThusThe constant depends on the shell and $g$, not on $u$ or $f$. Subtracting two solutions with the same forcing gives a zero-forcing solution and hence proves uniqueness. This is a coercive energy argument, not an appeal to real analytic Cauchy solvability.