Weak Banach–Saks theorem in a Hilbert space

If $x_n\rightharpoonup x$ in a Hilbert space, some subsequence $(x_{n_k})$ has Cesàro means converging in norm to $x$. For $x=0$, choose the subsequence so successive vectors have summably small inner products; expansion of the squared norm of its averages then proves convergence.

Thus Hilbert spaces have the weak form of the Banach-Saks property.