Let $X$ be a Hilbert space and $(x_n)\in X^{\mathbb N}$ be a sequence. Then the following statements are equivalent (with $x \in X$):
- We have $x_n \to x$, i.e. $\| x_n -x \| \to 0$ and
- we have $x_n \xrightarrow{\sigma} x$ (weak convergence) as well as $\|x_n\| \to \|x\|$.
Proof: Just expand the expression $\|x_n - x\|^2 = (x_n - x, x_n - x)$.
Q1: This is not true in arbitrary Banach spaces (correct?). Is there a nice example?
Q2: Does the above characterisation still hold for certain spaces which are not (pre-)Hilbertian? Does it still hold for $L^p$ with $p \ne 2$ e.g.?
edit: Apparently, this is referred to as the Radon–Riesz property, Kadets–Klee property or property (H).
for Q2 - in general it holds for uniformly convex spaces