On a decomposition of Banach spaces

Tom 108 / 2007

Jakub Duda Colloquium Mathematicum 108 (2007), 147-157 MSC: Primary 28A05; Secondary 46B04. DOI: 10.4064/cm108-1-13


By using D. Preiss' approach to a construction from a paper by J. Matoušek and E. Matoušková, and some results of E. Matoušková, we prove that we can decompose a separable Banach space with modulus of convexity of power type $p$ as a union of a ball small set (in a rather strong symmetric sense) and a set which is Aronszajn null. This improves an earlier unpublished result of E. Matoušková. As a corollary, in each separable Banach space with modulus of convexity of power type $p$, there exists a closed nonempty set $A$ and a Borel non-Haar null set $Q$ such that no point from $Q$ has a nearest point in $A$. Another corollary is that $\ell_1$ and $L_1$ can be decomposed as unions of a ball small set and an Aronszajn null set.


  • Jakub DudaDepartment of Mathematics
    Weizmann Institute of Science
    Rehovot 76100, Israel
    Faculty of Mathematics and Physics
    Charles University
    Sokolovská 83
    186 75 Praha 8, Czech Republic

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