Suppose that a sequence $\{x_n\}$ of points in a normed linear space satisfies that $\lim{l(x_n)} = l(x)$ for a set $A$ of $l$ dense in $X'$(the dual of $X$, the collection of all continuous linear functionals, with the norm $|f|_{\infty} = \sup_{X}|f(x)|$).

Is there an example such that the sequence $x_n$ doesn't weakly converge to $x$, which is, there is $g \in X'$ such that $\lim{g(x_n)} \neq g(x)$?


1 Answer 1


Take $X:=\ell^2$ the space of real square summable sequences and $x_n(k):=n\delta_{nk}$, where $\delta_{nk}=0$ if $n\neq k$ and $1$ otherwise. Consider $A$ the space of sequences with finitely many non-zero terms. Then $\langle l;x_n\rangle\to 0$ for all $l\in A$, but $\{x_n\}$ is not bounded.

However, we could conclude weak convergence if we had add boundedness of $\{x_n\}$.


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