3
$\begingroup$

I am interested in examples of dual Banach spaces $X$ with the Schur property (weakly convergent sequences in $X$ are norm convergent) like $\ell_1$. The Lorentz spaces $d(w,1)$ [Lindenstrauss and Tzafriri. Classical Banach spaces I. Sequence spaces. Section 4.3] are candidates because they admit a predual and are hereditarily-$\ell_1$ (Proposition 4.e.3 in the cited reference).

Do the spaces $d(w,1)$ have the Schur property?

$\endgroup$

2 Answers 2

2
$\begingroup$

No. The unit vector basis $(e_n)$ is unconditional and symmetric but not equivalent to the unit vector basis for $\ell_1$, hence $(e_n)$ converges weakly to zero.

$\endgroup$
0
$\begingroup$

By Rosenthal's $\ell_1$ Theorem, every normalized basic sequence in $X$ either admits a subsequence equivalent to the canonical basis of $\ell_1$, or else admits a normalized 2-block basic sequence which is weakly null. If $X$ has the Schur property, it follows that every normalized basic sequence admits a subsequence equivalent to the canonical basis of $\ell_1$. What sort of spaces might have this property?

Two interesting candidates come to mind. First of all, you could try $X=(\oplus\ell_2^n)_{\ell_1}$. Second, you could let $X=S^*$, where $S$ is the Schreier space, i.e. the completion of $c_{00}$ under the norm $\|(a_n)\|_S=\|(a_n)\|_\infty\vee\sup_{F\in\mathcal{S}_1}\|(a_n)_{n\in F}\|_{\ell_1}$. (Here, $\mathcal{S}_1$ denotes the first Schreier family.) I don't know whether these spaces have the Schur property, but I strongly suspect $(\ell_2^n)_{\ell_1}$ has it. I'm less optimistic about $S^*$, but it's still worth looking at.

EDIT: The reason I suspect $S*$ is because $S$ is known to be $c_0$-saturated. Since it is also (uniformly) subprojective, that means $S^*$ is $\ell_1$-saturated. Does this imply the Schur property? Even if not, it makes it plausible.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.