Abstract
A class of posets, called thin posets, is introduced, and it is shown that every thin poset can be covered by a finite family of trees. This fact is used to show that (within ZFC) every separable monotonically Menger space is first countable. This contrasts with the previously known fact that under CH there are countable monotonically Lindelöf spaces which are not first countable.
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Bonanzinga, M., Matveev, M. Combinatorics of Thin Posets: Application to Monotone Covering Properties. Order 28, 173–179 (2011). https://doi.org/10.1007/s11083-010-9161-5
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DOI: https://doi.org/10.1007/s11083-010-9161-5
Keywords
- Tree
- Thin poset
- Lindelöf space
- Monotonically Lindelöf space
- Menger space
- Monotonically Menger space
- Separable space
- First countable space