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Continuum theory, point-set topology, spaces with algebraic structure, foundations, dimension theory, local and global properties.

2 votes

Connected, maximal compact, but not $T_2$

Yes. Let Y be any compactly generated connected Hausdorff space such that Y fails to be locally compact. Let X be the one point compactification. X is maximal compact (since compact subsets of X are c …
Paul Fabel's user avatar
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1 vote

Is every T0 2nd countable space the quotient of a separable metric space?

The following canonical construction yields a `Yes' answer if $X$ is countable. Each $x \in X$ is assigned a metric space (or spaces) as follows. Let $U_{1},U_{2},...$ be a countable basis for $X$. …
Paul Fabel's user avatar
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5 votes

US does not imply AB

Take [0,1] with the usual topology and attach a new point y so that the neighborhoods of y are precisely those which are and open dense in [0,1]. To obtain a 2nd disjoint copy, repeat the construction …
Paul Fabel's user avatar
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10 votes

Is any function taking compact sets to compact sets, and connected sets to connected sets, n...

Replace [connected sets map to connected sets] with [point preimages are closed], and we have the following. Theorem. Suppose $X$ and $Y$ are compactly generated weakly Hausdorff spaces and suppose $ …
Paul Fabel's user avatar
  • 1,968
11 votes

A space in which sequences have unique limits but compact sets need not be closed

To create a counterexample X, start with the closed interval [0,1] (with the usual topology) and attach a new point z whose neighborhoods are open dense subsets of [0,1]. Observe [0,1] is a compact n …
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2 votes

Is “factoring through a dendrite loop” preserved under deletion?

The answer to the original question is apparently no. If a starting loop factors through a dendrite, and we delete a subloop which also factors through a dendrite, the surviving loop might not factor …
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4 votes
1 answer
186 views

Is “factoring through a dendrite loop” preserved under deletion?

Definition 1. A dendrite $X$ is a 1 dimensional retract of the closed unit disk. Equivalently, $X$ is a compact, locally path connected, and uniquely arcwise connected metric space. Definition 2. The …
Paul Fabel's user avatar
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10 votes
Accepted

"All retracts are closed" as separation axiom

No. Let $X$ be a compactly generated Hausdorff space which fails to be locally compact at precisely one point. Now take the Alexandroff compactification of $X$, adding exactly one new point, whose nei …
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8 votes

A space in which sequences have unique limits but compact sets need not be closed

Start with the one point compactification of the minimal uncountable well ordered space and then split the maximum point into two points.
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12 votes
Accepted

Existence of a continuous section

No, even if $Y=[0,1]$. The piecewise linear continuous nondecreasing surjection $f:[0,1] \rightarrow [0,1]$ which maps $[1/3,2/3]$ to $1/2$ and is otherwise 1-1 and linear has no continuous section.
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12 votes
3 answers
6k views

Is every T0 2nd countable space the quotient of a separable metric space?

Suppose the space $X$ has a countable basis and $X$ is $T_{0}$. Must there exist a separable metrizable space $Y$ and a quotient map q:$Y \rightarrow X$? (Some surrounding facts: Every metrizable sp …
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2 votes

Is a certain subset of the disc a convex set?

Start with a 3 point set {a,b,c} on the unit circle. The union of the 3 chords forms not a convex set. Blow up the points into 3 very small Cantor sets. The union of all the new chords still fails mis …
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5 votes

Inserting an open and simply-connected set between a compact set and an open set

Perhaps the theorem at the common intersection of the various ambiguous interpretations of the inquiry at hand is the following: If the planar open set U contains a continuum K and if the planar com …
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2 votes

Largest Hausdorff quotient

To augment very slightly Mariano's nice answer, Hausdorff quotients (as opposed to surjections) suffice. To obtain the finest Hausdorff quotient of an arbitrary space $X$, take the quotient of $X$ b …
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3 votes
1 answer
167 views

The finest countably generated free topological group so that $x^{m_{n}}_n\rightarrow 1$?

What is the finest free topological group $H$ with generators ${x_{1},x_{2},...}$ so that $x^{m_{n}}_n\rightarrow 1$ for all sequences $m_{1},m_{2},...$? Is $H \simeq K$, with $K$ the natural subspac …
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