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Space Suggestion: [newspacename]

Open justforphone42069-cloud opened this issue 1 month ago • 5 comments

Space Suggestion

$X = \lbrace 0 \times [0,1] \rbrace \bigcup \lbrace [0,1] \times 1 \rbrace \bigcup \lbrace \frac{1}{n} \times (0, \frac{1}{n}] \rbrace_\mathbb{N} \bigcup \lbrace [\frac{1}{n+1}, \frac{1}{n}] \times \frac{1}{n+1} \rbrace_\mathbb{N} \subset \mathbb{R}^2$

This space provides an example satisfying the search connected, locally connected, separable metric space that's $\sigma$-compact but not locally compact. It's also path-connected and locally path-connected. By replacing the segments with pseudo-arcs it loses path-connected and locally path-connected everywhere.

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justforphone42069-cloud avatar Oct 29 '25 18:10 justforphone42069-cloud

Actually, the harmonic comb with $(0,0)$ included but the vertical endpoints removed also works, that's easier.

justforphone42069-cloud avatar Oct 29 '25 18:10 justforphone42069-cloud

FYI, when you create an issue, you are supposed to modify the title to reflect the content. I modified the title of #1501 for you, but you can do it here also.

prabau avatar Oct 29 '25 19:10 prabau

That space does not seem locally path connected?

Wait, yes I see it now. It is locally path connected.

But the "harmonic comb" with $(0,0)$ included would not be locally connected or locally path connected, right?

prabau avatar Oct 29 '25 19:10 prabau

pi-base currently does not have an example of path connected, metrizable, sigma-compact and non-locally compact space: https://topology.pi-base.org/spaces?q=37%2B53%2B17%2B%7Ewlc

How about this even simpler example: "comb with decreasing open tines": $X$ = the subspace of $\mathbb R^2$ consisting of the closed interval $[0,1]\times\{0\}$ together with all open vertical segments $\{\frac1n\}\times[0,\frac1n)$ for $n=1,2,\dots$.

prabau avatar Oct 29 '25 20:10 prabau

pi-base currently does not have an example of path connected, metrizable, sigma-compact and non-locally compact space: https://topology.pi-base.org/spaces?q=37%2B53%2B17%2B%7Ewlc

In fact S75 is, as long as #990 gets merged.

#1234 is an another example satisfying “Path connected + Metrizable + σ-compact + !Locally compact”.

yhx-12243 avatar Oct 29 '25 23:10 yhx-12243