The combinatorics of Borel covers

The combinatorics of splittability

Marion Scheepers, in his studies of the combinatorics of open covers, introduced the property Split(U,V) asserting that a cover of type U can be split into two covers of type V. In the first part of this paper we give an almost complete classification of all properties of this form where U and V are significant families of covers which appear in the literature (namely, large covers, omega-covers, tau-covers, and gamma-covers), using combinatorial characterizations of these properties in terms related to ultrafilters on N. In the second part of the paper we consider the questions whether, given…

Several comments about the combinatorics of tau-covers

In a previous work with Mildenberger and Shelah, we showed that the combinatorics of the selection hypotheses involving tau-covers is sensitive to the selection operator used. We introduce a natural generalization of Scheepers' selection operators, and show that: (1) A slight change in the selection operator, which in classical cases makes no difference, leads to different properties when tau-covers are involved. (2) One of the newly introduced properties sheds some light on a problem of Scheepers concerning tau-covers. Improving an earlier result, we also show that no generalized Luzin set sa…

The combinatorics of tau-covers

We solve four out of the six open problems concerning critical cardinalities of topological diagonalization properties involving tau-covers, show that the remaining two cardinals are equal, and give a consistency result concerning this remaining cardinal. Consequently, 21 open problems concerning potential implications between these properties are settled. We also give structural results based on the combinatorial techniques.

A Borel linear subspace of $\mathbb R^ω$ that cannot be covered by countably many closed Haar-meager sets

We prove that the countable product of lines contains a Borel linear subspace $L\ne\mathbb R^ω$ that cannot be covered by countably many closed Haar-meager sets. This example is applied to studying the interplay between various classes of ``large'' sets and Kuczma--Ger classes in the topological vector spaces $\mathbb R^n$ for $n\le ω$.

高维热带覆盖的组合学

Combinatorics of higher-dimensional tropical covers

本研究构建组合框架研究高维热带覆盖类多面映射，证明平衡条件与带指标分支覆盖的等价性，实现连通性提升并解决重数扩展问题，给出强连通性假设缺失时的反例。

o-bounded groups and other topological groups with strong combinatorial properties

We construct several topological groups with very strong combinatorial properties. In particular, we give simple examples of subgroups of the real line R (thus strictly o-bounded) which have the Hurewicz property but are not sigma-compact, and show that the product of two o-bounded subgroups of R^N may fail to be o-bounded, even when they satisfy the stronger property S1(Borel_Omega,Borel_Omega). This solves a problem of Tkacenko and Hernandez, and extends independent solutions of Krawczyk and Michalewski and of Banakh, Nickolas, and Sanchis. We also construct separable metrizable groups G of …