Let $\mathcal{H}$ be a Hilbert space. Show:
$(\forall \psi \in \mathcal{H}: \lim \langle \psi, \phi_{n}\rangle = \langle \psi, \phi_{n}\rangle \implies \lim \phi_{n}=\phi )\implies \mathcal{H}$ finite-dimensional. $(*)$
The idea:
Assume that $\mathcal{H}$ is infinite dimensional, then in particular there exists a countable orthonormal system $(e_{n})_{n\in \mathbb N}$ such that by the Bessel inequality, we have:
$\sum\limits_{n \in \mathbb N}\lvert \langle e_{n}, \phi_{m}\rangle\rvert^{2}\leq \lvert \lvert \phi_{m}\rvert \rvert^{2}<\infty $
Thus for any $m \in \mathbb N$, we obtain $ \lim\limits_{n \to \infty}\langle e_{n}, \phi_{m}\rangle=0$.
I do not see how this shows that the left-hand side of $(*)$ is false
Best Answer
$e_n \to 0$ weakly because $\sum (\langle e_n, x \rangle)^{2} <\infty$ which implies $\langle e_n, x \rangle \to 0$ for all $x$. But $\|e_n\|=1 $ so $e_n$ does not tend to $0$ in the norm..