Skip to content

Some traits for S174 - #1683

Open
felixpernegger wants to merge 3 commits into
mainfrom
t6traits
Open

Some traits for S174#1683
felixpernegger wants to merge 3 commits into
mainfrom
t6traits

Conversation

@felixpernegger

@felixpernegger felixpernegger commented Mar 18, 2026

Copy link
Copy Markdown
Collaborator

This PR has low priority!

@felixpernegger

Copy link
Copy Markdown
Collaborator Author

Just to make clear this contains MORE that just the subspaces as in the newer PRs

Comment thread spaces/S000174/properties/P000048.md Outdated
@felixpernegger
felixpernegger marked this pull request as draft March 23, 2026 02:48
@felixpernegger
felixpernegger marked this pull request as ready for review March 26, 2026 16:35
@Moniker1998

Copy link
Copy Markdown
Collaborator

@felixpernegger can you delete ~P58 and P163 and add P65

@Moniker1998
Moniker1998 self-requested a review August 5, 2026 09:13

Copy link
Copy Markdown
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This is not trivial enough to just assert this as facts. One needs an elaboration. Clearly sets of the form $\{\omega\}^n\times (\omega+1)^\omega$ form a clopen base for this point, so it's enough to show that they're not compact. And this follows because it has topology larger than that of the product topology if $\omega+1$ were given discrete topology, which is not compact.

@Moniker1998 Moniker1998 Aug 5, 2026

Copy link
Copy Markdown
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Replace with not P62

value: true
---

The point $p := (0, 0, \dots) \in X$ is isolated via $\{p\} = 0 ^ \omega \cap \omega_1 ^ \omega$.

Copy link
Copy Markdown
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Suggested change
The point $p := (0, 0, \dots) \in X$ is isolated via $\{p\} = 0 ^ \omega \cap \omega_1 ^ \omega$.
The point $p := (0, 0, \dots) \in X$ is isolated since $\{p\} = 1^\omega$ where $1 = \{0\}$.

value: false
---

The subspace $2 ^ \omega \setminus \{(0,0,\dots)\} \subseteq X$ has no isolated point.

Copy link
Copy Markdown
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Suggested change
The subspace $2 ^ \omega \setminus \{(0,0,\dots)\} \subseteq X$ has no isolated point.
The subspace $2 ^ \omega \setminus \{(0,0,\dots)\} \subseteq X$ is homeomorphic to Cantor set without a point, and so has no isolated points.

@Moniker1998 Moniker1998 Aug 5, 2026

Copy link
Copy Markdown
Collaborator

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Replace with not P62

@Moniker1998

Moniker1998 commented Aug 5, 2026

Copy link
Copy Markdown
Collaborator

@felixpernegger so I've figured out that Pol, in proposition 2, actually proves a lot more than just lack of paracompactness.

Namely, this argument works to show Pol's space is not weakly $\delta\theta$-refinable space. In such space, a locally countable collection is $\sigma$-discrete. See theorem 13.3 of Borel measures by Gardner and Pfeffer in Handbook of set-theoretic topology

In particular, we can add that Pol's space is not meta-Lindelof, and not submetacompact.

@Moniker1998

Moniker1998 commented Aug 5, 2026

Copy link
Copy Markdown
Collaborator

It's not weakly Lindelof, since the cover by sets $\xi^\omega$ for $\xi\in\omega_1$, a countable subcollection would be some set of the form $\xi^\omega$, and it's closed, so not dense.

Easy argument, gets rid of two properties you wanted to add.

@Moniker1998

Copy link
Copy Markdown
Collaborator

@felixpernegger

@Moniker1998 Moniker1998 added the awaiting-author This PR requires the author to take further action in order to continue. label Aug 10, 2026
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Labels

awaiting-author This PR requires the author to take further action in order to continue. trait

Projects

None yet

Development

Successfully merging this pull request may close these issues.

3 participants