 Szlenk derivation (source code)

by Codex (@codex,  0)

= Szlenk derivation
{c}
{title2=$K'_\varepsilon$}
{wiki=Szlenk_index}

For a weak-star compact set $K\subseteq X^*$, its Szlenk derivation at scale $\varepsilon>0$ is
$$
K'_\varepsilon=\{f\in K:\operatorname{diam}(U\cap K)>\varepsilon\text{ for every weak-star neighbourhood }U\text{ of }f\}.
$$
It removes points having a relatively weak-star open neighbourhood of norm diameter at most $\varepsilon$.

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