English

On centered local $\pi$-bases

General Topology 2026-05-01 v1

Abstract

In 1967 Hajnal and Juh{\'a}sz showed that the cardinality of a first-countable Hausdorff space with the countable chain condition has cardinality at most c\mathfrak{c}, the cardinality of the real line. We give an improvement of this celebrated theorem by replacing ``first-countable" with the weaker condition ``each point has a countable centered local π\pi-base". Given a point pp in a topological space XX, a \emph{local} π\pi-\emph{base} \scrB\scr{B} at pp acts like a neighborhood base at pp except that pp may not be in any member of \scrB\scr{B}. A local π\pi-base \scrB\scr{B} has the \emph{finite intersection property} if any finite intersection of members of \scrB\scr{B} is nonempty. We call this type of local π\pi-base \emph{centered}. A centered local π\pi-base behaves even more like a neighborhood base in a sense. A space has the \emph{countable chain condition} if every family of pairwise disjoint open sets is countable. We also improve a theorem of Pospi{\v s}il from 1937 using centered local π\pi-bases. As is customary, examples are given to demonstrate these improvements are strict. Compact Hausdorff spaces are also explored in this connection, along with variations on the notion of a centered local π\pi-base.

Cite

@article{arxiv.2604.28092,
  title  = {On centered local $\pi$-bases},
  author = {Nathan Carlson},
  journal= {arXiv preprint arXiv:2604.28092},
  year   = {2026}
}

Comments

10 pages

R2 v1 2026-07-01T12:43:58.837Z