Szlenk derivation ($K_{\epsilon }^{\prime }$)

For a weak-star compact set $K\subseteq X^{\ast }$, its Szlenk derivation at scale $\epsilon >0$ isIt removes points having a relatively weak-star open neighbourhood of norm diameter at most $\epsilon$.

If $X^{\ast }$ is separable and $K$ is a nonempty weak-star closed subset of $B_{X^{\ast }}$, then $K_{\epsilon }^{\prime }$ is a proper subset of $K$ for every $\epsilon >0$. Metrizability produces sequences witnessing membership in the derivative, while a Baire-category argument applied to a universal weakly null sequence yields the strict inclusion.