Normal sequences without strongly normal subsequences

Abstract

A sequence $(y_n^*)$ in the unit sphere of $E^*$ is strongly normal if the vectors $x\in E$ satisfying $\sum_n|y_n^*(x)|<\infty$ form a dense subspace. For every set $Γ$ of cardinality at least the continuum, we construct a normalized weakly null sequence in $\ell_1(Γ)^*$ with no strongly normal subsequence. This answers a question of Śliwa negatively in ZFC. Together with the classical selection argument below the bounding number $\mathfrak b$, the construction gives positive and negative bounds for this subsequence property on $\ell_1(κ)$. Its validity on $\ell_1(ω_1)$ is independent of ZFC. The intermediate range $\mathfrak b\leqslantκ<\mathfrak c$ remains undecided by these results. We also give a counterexample in a Banach sequence space that is not isomorphic to any $\ell_1(Γ)$.