Skip to content

Space: $\mathbb{R} \times \omega$ - #1845

Open
JSMassmann wants to merge 4 commits into
mainfrom
JSMassmann/NtimesR
Open

JSMassmann wants to merge 4 commits into
mainfrom
JSMassmann/NtimesR

Conversation

@JSMassmann

Copy link
Copy Markdown
Collaborator

It's a pretty simple space, so @Moniker1998 advised I just submit a PR without an Issue.
It answers a search that currently has no results, namely Polish + homogeneous + ~connected + ~totally disconnected.
It's also P41, P120 and P184, but I didn't want to overwhelm with so many traits in one PR.

@JSMassmann
JSMassmann requested a review from prabau October 2, 2026 23:43
Comment thread spaces/S000227/README.md Outdated
@prabau

prabau commented Oct 3, 2026 •

Copy link
Copy Markdown
Collaborator

observation: $X$ is homeomorphic (for example) to the closed subspace $\mathbb R\times\mathbb Z$ of $\mathbb R^2$. (useful also to show X is embeddable in some Euclidean space)

Possibly worth mentioning later, if we find this useful. For example, it could an alternative way to show that X is Polish.

But both that way and the current justification that X is Polish (P116) rely on some meta-property that is currently not in pi-base and that need to be added to P116. This should be done in the same PR since we are making use of it. (for products and for closed sets in particular -- see https://github.com/pi-base/data/wiki - "Conventions and Style" for usual wording.)

I have not checked the other properties, but please take a look if other meta-properties are missing.

value: false
---

$\{(n, n + 1) \times \omega: n \in \mathbb{Z}\}$ is an open cover with no finite subcover.

@Moniker1998 Moniker1998 Oct 7, 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.

Suggested change
$\{(n, n + 1) \times \omega: n \in \mathbb{Z}\}$ is an open cover with no finite subcover.
$X$ contains {S25} as a closed subspace and {S25|P16}.

value: true
---

$[n, n + 1] \times \{m\}$ is compact for all $n, m \in \mathbb{N}$, and the union of all of these is the whole space.

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
$[n, n + 1] \times \{m\}$ is compact for all $n, m \in \mathbb{N}$, and the union of all of these is the whole space.
Product of {P17} spaces

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.

Also, add meta-property that P17 is closed under countable products

value: false
---

Since {S25|P36}, $\mathbb{R} \times \{n\}$ is a connected component for each $n \in \mathbb{N}$.

@Moniker1998 Moniker1998 Oct 7, 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.

Suggested change
Since {S25|P36}, $\mathbb{R} \times \{n\}$ is a connected component for each $n \in \mathbb{N}$.
Has {S25} as a subspace and {S25|P47}

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.

If we want to rely on a meta-property for P47, we should instead claim {S25|P47} (which expands to X not being totally disconnected).

But maybe just easier, just say $\mathbb R\times\{0\}$ is a connected subspace with more than one point.
(which is closer that what @JSMassmann had initially).

value: false
---

For any $A \subseteq \omega$, $\mathbb{R} \times A$ is clopen since we give $\omega$ the discrete topology.

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
For any $A \subseteq \omega$, $\mathbb{R} \times A$ is clopen since we give $\omega$ the discrete topology.
Product of {S25} and {S2}, and {S2|P36}

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.

Again, that relies on meta-properties that are missing from pi-base.
On the other had, what @JSMassmann had initially is more than we need.

How about just saying:
$\mathbb R\times\{0\}$ is a nonempty proper clopen subset of $X$. ?
(more justification not needed as it's obvious)

@Moniker1998 Moniker1998 Oct 7, 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.

@prabau I've already mentioned that in the other comment. And adding the metaproperty is useful, and easier

value: true
---

{S2|P116}, {S25|P116}, and a countable product of Polish spaces is Polish.

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.

Need to add metaproperty

@Moniker1998

Copy link
Copy Markdown
Collaborator

@JSMassmann I've made it so that the justifications use metaproperties instead of writing things explicitly. @prabau can check for stylystic choices

Co-authored-by: Patrick Rabau <70125716+prabau@users.noreply.github.com>
@JSMassmann

JSMassmann commented Oct 8, 2026 •

Copy link
Copy Markdown
Collaborator Author

Sorry that I haven't written a reply yet, I'm a bit busy. I'm not sure what I should do wrt the metaproperties, since as @prabau pointed out many of them aren't in pi-base yet.

@Moniker1998

Copy link
Copy Markdown
Collaborator

@JSMassmann just two of them

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Labels

None yet

Projects

None yet

Development

Successfully merging this pull request may close these issues.

3 participants