 Solution

The Radon-Riesz property of a norm is the implicationIn a Hilbert space, expand the square using its inner product:Weak convergence gives $⟨u_k,u⟩\to \Vert u{\Vert }^2$, and convergence of norms gives $\Vert u_k{\Vert }^2\to \Vert u{\Vert }^2$. Hence the right side tends to zero. This proves the Hilbert-space Radon-Riesz theorem.