Abstract
A Tychonoff space Xp = X ∪ {p} is called a one-point extension of X if X is dense in Xp and the reminder Xp \X consists of the singleton {p}. We study the following problem: Characterize the spaces X such that every (some) one-point extension Xp of X has a given local topological property P at the point p. The list of properties P considered in the paper includes, among others: 1) {p} is a Gδ-set in Xp; 2) Xp admits a local countable base at p; 3) Xp has the Fréchet-Urysohn property at p; 4) Xp has countable tightness at p. One of our main results states that a Tychonoff space X is Lindelöf (not pseudocompact) iff the point p is of type Gδ in Xp, for every (for some, respectively) one-point extension Xp of X. We pose several open problems for various concrete properties P.
| Original language | American English |
|---|---|
| Pages (from-to) | 195-208 |
| Number of pages | 14 |
| Journal | Topology Proceedings |
| Volume | 59 |
| Early online date | 8 May 2021 |
| State | Published - 1 Jan 2022 |
Keywords
- character
- Fréchet-Urysohn property
- G-set
- Lindelöf space
- One-point extension
- Stone-Čech compactification
- zero-set
All Science Journal Classification (ASJC) codes
- Geometry and Topology