On coverings of Banach spaces and their subsets by hyperplanes

TL;DR

This paper studies how subsets of Banach spaces can be covered by hyperplanes, analyzing their cardinal characteristics, and explores whether certain covering properties hold universally in ZFC, with results for specific classes and consistency results.

Contribution

It determines cardinal characteristics of hyperplane coverings in Banach spaces and proves covering properties for various classes, connecting to set-theoretic and topological questions.

Findings

Values of cardinal characteristics for separable Banach spaces are determined.

Proves that certain covering properties hold for all Banach spaces in many classes.

Shows the consistency of these properties with various continuum sizes.

Abstract

Given a Banach space we consider the $\sigma$-ideal of all of its subsets which are covered by countably many hyperplanes and investigate its standard cardinal characteristics as the additivity, the covering number, the uniformity, the cofinality. We determine their values for separable Banach spaces, and approximate them for nonseparable Banach spaces. The remaining questions reduce to deciding if the following can be proved in ZFC for every nonseparable Banach space $X$: (1) $X$ can be covered by $\omega_1$-many of its hyperplanes; (2) All subsets of $X$ of cardinalities less than ${\rm cf}([{\rm dens}(X)]^\omega)$ can be covered by countably many hyperplanes. We prove (1) and (2) for all Banach spaces in many well-investigated classes and that they are consistent with any possible size of the continuum. (1) is related to the problem whether every compact Hausdorff space which has small diagonal is metrizable and (2) to large cardinals.