All Questions
5,184 questions
2
votes
1
answer
174
views
Understanding the picture of monoidal space
Ogus in his slides https://math.berkeley.edu/~ogus/preprints/colloqhandout.pdf presents the following picture of a monoidal space $\operatorname{Spec}(\mathbb{N} \longrightarrow \mathbb{C}[\mathbb{N}])...
- 1,349
2
votes
0
answers
339
views
Blow up at an ordinary double point
Let $X \subset \mathbb{C}^n$ be a complex complete intersection surface with only ordinary double point singularities. Let $o$ be such an ordinary double point.
Let $\tilde{X}$ be the strict transform ...
2
votes
1
answer
387
views
Embeddings of vector spaces
Let $V$ be an $n$-dimensional vector space. Is the space of embeddings
$$
\coprod_1^{k} V \to V
$$
path connected for large enough $n$? Clearly $n=1$ is not enough, but I feel like $n=2$ is enough ...
- 981
3
votes
1
answer
192
views
Homeomorphic extension of a discrete function
Let $f : \{ 0,1 \} ^ {n} \rightarrow \{ 0,1 \} ^ {n}$ be a bijective map. Then is there a known computable way to extend it to a homeomorphism $g:[ 0,1 ] ^ {n} \rightarrow [ 0,1 ] ^ {n}?$
- 7,893
10
votes
1
answer
460
views
An incomplete characterisation of the Euclidean line?
We say that a metric space $(X, d)$ is a Banakh space if for every $\rho \in \mathbb{R}_{> 0}$ and every $x \in X$, there are
$a,b \in X$ such that $\{y \in X \, \vert \, d(x, y) = \rho\} = \{a, b\}...
- 47.4k
4
votes
1
answer
353
views
Almost compact sets
Update:
Q1 is answered in the comments.
I think that the usual arguments show that every relatively almost compact set in a space is closed in the space.
Original question:
A set $K$ in a space $X$ ...
- 1,412
3
votes
1
answer
126
views
What are the names of the following classes of topological spaces?
The closure of any countable is compact.
The closure of any countable is sequentially compact.
The closure of any countable is pseudocompact.
The closure of any countable is a metric compact set.
- 1,412
26
votes
5
answers
10k
views
Locally compact Hausdorff space that is not normal
What is a good example of a locally compact Hausdorff space that is not normal? It seems to be well-known that not all locally compact Hausdorff spaces are normal (and only a weaker version of Urysohn'...
- 4,713
26
votes
2
answers
1k
views
Is there an infinite topological space with only countably many continuous functions to itself?
Cross-posted from MSE.
Is there an infinite countable topological space $X$ with only countably many continuous functions to itself?
It cannot be a metrizable space. Another large class of examples ...
- 1,412
4
votes
0
answers
182
views
Symmetric line spaces are homeomorphic to Euclidean spaces
For points $x,y,z$ of a metric space $(X,d)$ we write $\mathbf Mxyz$ and say that $y$ is a midpoint between $x$ and $z$ if $d(x,z)=d(x,y)+d(y,z)$ and $d(x,y)=d(y,z)$.
Definition: A metric space $(X,d)$...
- 42k
15
votes
1
answer
1k
views
In ZF, when is a disjoint union of metrizable spaces metrizable?
It is easy to see that the disjoint union $\bigsqcup_i X_i$ of a collection of
metric spaces is metrizable, simply by rescaling or chopping off
the individual metrics to have diameter at most one, and ...
- 185
2
votes
1
answer
172
views
Non-Hausdorff CGWH-group
Is there a group $G$ which is at the same time a (compact-Hausdorff)-ly generated weakly Hausdorff space (or short CGWH space) such that inverse and product are continuous maps and the space is not ...
- 185
6
votes
2
answers
1k
views
Does every locally compact Hausdorff space admit a locally finite open covering by relatively compact sets?
Let $X$ be a locally compact Hausdorff space. Does there exist a locally finite open covering consisting of relatively compact sets?
- 1,412
3
votes
2
answers
194
views
Is every uncountable, homogeneous connected $T_2$-space isomorphic to a subspace of $\mathbb{R}^\omega$?
We say a space $(X,\tau)$ is homogeneous if for any $x,y\in X$ there is a homeomorphism $\varphi:X\to X$ such that $\varphi(x) = y$.
What is an example of a connected, homogeneous $T_2$-space $(X,\...
- 1,412
2
votes
1
answer
94
views
Connected box products of Hausdorff spaces
This is a follow-up on an older question.
Let $\Box_{i\in I} X_i$ denote the box product of the spaces $X_i$. Is there a Hausdorff space $(X,\tau)$ with $|X|>1$ such that $\Box_{n\in\omega}X$ ...
- 1,412
7
votes
1
answer
462
views
Does second countable and functionally Hausdorff imply submetrizable?
A topological space $\mathbf{X}$ is functionally Hausdorff, if for any two distinct $x, y \in \mathbf{X}$ there exists a continuous function $f_{xy} : \mathbf{X} \to [0,1]$ with $f(x) = 0$ and $f(y) = ...
- 1,412
3
votes
2
answers
257
views
Cancelable commutative monoids with finite maximal subgroups
Suppose $\mathcal{M} = (M, +, 0)$ is a cancelable commutative monoid. Let $G$ be the maximal subgroup of $M$, i.e.
$$G = \{ a \in M \colon (\exists b \in M)\, a + b = 0 \}.$$
For $a, b \in M$ say $a \...
- 38.6k
2
votes
1
answer
232
views
Existence of diffeomorphism interpolating affine map and identity
$\newcommand{\R}{\mathbb{R}}$Suppose $\Omega$ is a bounded, convex domain in $\R^{m}$. Fix $x_1, x_2\in\Omega$ and an invertible matrix $A\in\mathrm{GL}^{+}(m)$ with positive determinant.
Let $U\...
- 193
2
votes
0
answers
74
views
Is there a literature name for this concept of a "graded metric"?
Given a space $X$, I have been thinking about a function $d\colon X \times X \times \mathbb{N} \to \mathbb{R}_{\geq 0}$ (i.e. with values that are nonnegative reals) with the properties below. One may ...
- 5,429
6
votes
1
answer
171
views
When can we find a net, defined on a totally ordered index set, converging to a non-isolated point in a compact Hausdorff space?
Let $X$ be a compact Hausdorff space and $p\in X$ be a non-isolated point. Is it always possible to find a net $(x_\alpha)_{\alpha\in (I,\leq)}$ in $X\setminus\{p\}$ converging to $p$ such that $(I,\...
- 609
1
vote
0
answers
50
views
Is there some kind of construction of a "canonical unirational variety" like the one for toric varieties?
Toric varieties in some sense a "canonical rational variety" in that one can construct them from purely combinatorial data and this combinatorial data makes it possible to turn many ...
- 912
3
votes
1
answer
159
views
Closed graph correspondence which never contains the whole support
Let $I=[a,b]$ with $a<b\in\mathbb{R}$ and denote by $\mathcal{M}(I)$ the set of Borel probability measures on $I$ equipped with the topology induced by the weak convergence of measures.
Does there ...
- 128k
1
vote
1
answer
165
views
Trivial convergent sequences in $\beta X$
Let $X$ be a Tychonoff space and denote by $\beta X$ its Stone-Čech compactification. We know, for example, that if $X$ is an $F$-space then $\beta X$ is an $F$-space and, therefore, in $\beta X$, the ...
- 1,364
2
votes
0
answers
57
views
The graph topologies for powersets
Given a topological space $X$ and a metric space $(Y,d_Y)$, there are a number of topologies one may put on the space $\mathcal{C}(X,Y)$ of continuous functions from $X$ to $Y$. Perhaps the most ...
- 11.8k
1
vote
0
answers
70
views
A ZFC example of a star-$K$-Menger space which is not star-$K$-Hurewicz
An open cover $\mathcal U$ of a space $X$ is said to be $\gamma$-cover if $\mathcal U$ is infinite and for each $x\in X$, the set $\{U\in\mathcal U : x\notin U\}$ is finite.
A space $X$ is said to be ...
- 505
2
votes
2
answers
354
views
Construct a homeomorphism whose periodic points set is not closed
I'm looking for a simple example in discrete dynamical systems whose periodic points set is not necessary closed.
I've seen some example in websites but they are not that simple and discrete.
Note ...
- 18.7k
2
votes
0
answers
95
views
References (and a question) on the "fine" topology of powersets
Recently I've been trying to understand powerset topologies better, and came upon the following reference:
Frank Wattenberg, Topologies on the set of closed subsets. Pacific J. Math. 68(2): 537-551 (...
- 869
7
votes
0
answers
295
views
A minimal semigroup generating subset of the additive reals
I asked this on MSE, but I was told to ask it here because it is a difficult question. Consider the additive magma of the real numbers, $(\mathbb{R};+)$. Does there exist a subset $S$ of the reals ...
- 63.9k
6
votes
1
answer
260
views
A ZFC example of a Menger space which is not Scheepers
$\Omega$: The collection of all $\omega$-covers of a space $X$. An open cover $\mathcal U$ of $X$ is said to be $\omega$-cover if $X\notin\mathcal U$ and for each finite $F\subseteq X$ there exists a $...
- 3,104
4
votes
3
answers
515
views
Does every Lindelof uniform space have a Lindelof completion?
Recall Lindelof = every open cover has a countable subcover. Is the question of the title answered by known material from some standard text on uniform spaces?
Note it is well known to be true for ...
- 2,821
3
votes
1
answer
242
views
Closed subset of unit ball with peculiar connected components
Let $n\geq 2$ and denote by $B\subset \mathbb{R}^n$ the closed unit ball.
Does there exist a closed subset $A\subset B$ containing $0\in \mathbb{R}^n$ with the following properties i,ii,iii?
i) $\{0\}$...
- 45k
0
votes
0
answers
91
views
Describing a time-varying process with a manifold
I am a beginner in topology and I am trying to define a model for some computations. My questions are speculative:
I am wondering what is the proper way to add time in a manifold so as to describe a ...
1
vote
1
answer
112
views
Changing a metric to that 2 points have different distance
Let $X$ be a compact metric space. Assume that $X$ has more than $2$ points (or even better, that $X$ is connected with more than 1 point). Given a metric $d$ on $X$ we define $$d(x,X)=\max\{d(x,z):z\...
- 10.6k
1
vote
0
answers
48
views
Neighborhoods of idempotents in topological inverse semigroups
In a topological group, for any neighborhood $U$ of the origin, there is another such neighborhood with the property that $V.V\subseteq U.$ I conjecture a similar property for topological inverse ...
- 63.9k
10
votes
1
answer
448
views
Do compactly generated spaces have a more direct definition?
Is there an elementary way to define Haussdorf-compactly generated weakly Hausdorff topological spaces in a way that does not need defining topological space first?
Weakly Hausdorff sequential spaces ...
- 64k
8
votes
0
answers
172
views
The pro-discrete space of quasicomponents of a topological space
Let $X$ be a topological space.
Consider the functor $P^X : \textbf{Set} \to \textbf{Set}$ that sends each set $Y$ to the set of continuous maps $X \to Y$.
It is not hard to check that $P^X : \textbf{...
- 15.9k
2
votes
0
answers
67
views
When did derivative algebras first appear?
In the paper "The Algebra of Topology" (Annals of Mathematics, 45, 1944), McKinsey and Tarski proposed derivative algebras (p183) to define the derive set in topology as follows.
Suppose $K$ ...
- 663
7
votes
1
answer
501
views
Non-homeomorphic connected one-dimensional Hausdorff spaces that have continuous bijections between them in both sides
I need to construct an example of two non-homeomorphic connected one-dimensional Hausdorff spaces that have continuous bijections between them in both sides. Spaces should have induced ("good&...
- 63.9k
5
votes
2
answers
496
views
Do germs of open sets around a point form a frame?
Let $X$ be a topological space and $x \in X$ a point. Let $\Omega$ be the set of open sets (viꝫ. the topology) of $X$, and $\Omega_x$ the set of germs around $x$ of open sets, that is, $\Omega_x = \...
- 1,574
5
votes
1
answer
311
views
Infinite tensor/Fubini product of ultrafilters
Given an infinite family $\{\mathcal{F}_{\lambda}$, $\lambda <\kappa\}$, $\kappa \geq \omega_0$, of (ultra)filters of a set $X$, how it is defined the infinite tensor/Fubini product $$\bigotimes_{\...
- 53
2
votes
1
answer
163
views
Homological restrictions on certain $4$-manifolds
I am not very familiar with the non-compact $4$-manifold theory. So I apologize if the following question is very silly.
Let $X$ be a non-compact, orientable $4$ manifold that is homotopic to an ...
- 25k
4
votes
1
answer
335
views
When is this topology compatible with the partial ordering?
This question was first asked here, on math stack exchange, but wasn't able to attract any attention. Now that I am thinking more, it feels like the most suitable place for this question is here.
...
- 236k
9
votes
1
answer
734
views
Does the category of locally compact Hausdorff spaces with proper maps have products?
nlab presents a proof that the category of locally compact Hausdorff spaces does not admit infinite products in general. In particular it shows that there is no infinite product of $\mathbb{R}$, since ...
- 63.9k
4
votes
0
answers
152
views
Maximally fine topologies on $B(H)$ making the unit ball compact
Let $H$ be a Hilbert space, and $B(H)$ its algebra of bounded operators. One of the reasons the Ultraweak topology is (in a way) more useful than the weak operator topology is that the Ultraweak ...
- 190
13
votes
1
answer
1k
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Is every $\sigma$-algebra of sets *abstractly* the Borel algebra of a topology on perhaps some other set?
Is every sigma-algebra the Borel algebra of a topology?
inspires the present question which asks for less.
Question: Given a $\sigma$-algebra $\mathcal A$ on a set $X$, does there exist a topology $\...
- 12.9k
2
votes
0
answers
107
views
Existence of a nice-ish topology on the powerset of a topological space
This is a follow-up question to my previous question, Existence of a *really* nice topology on the powerset of a topological space, which, in a few words, asked about whether given a topological space ...
- 11.8k
12
votes
1
answer
635
views
Ultrafilter subtraction and "zero"
This is related to a couple recent MO/MSE questions of mine, namely 1,2. Belatedly, I've tweaked this post to remove an overly-ambitious secondary question; see the edit history if interested.
Let $\...
- 20.5k
3
votes
1
answer
89
views
Can the set of compact metrisable topologies naturally be equipped with the structure of a standard Borel space?
Let $X$ be a compact metric space, and let $K_X$ be the set of non-empty closed subsets of $X$, equipped with the $\sigma$-algebra
$$ \mathcal{B}(K_X) \ := \ \sigma(\{C \in K_X : C \cap U = \emptyset\}...
- 780
0
votes
1
answer
188
views
A question about uniformities generated by pseudometrics
Suppose that for all $n$ natural numbers, $d_{n}$ is a pseudometric on set $X
$. Define $d=\sum_{n=1}^{\infty }a_{n}\frac{d_{n}}{1+d_{n}}$, where $\left(
a_{n}\right) $ is a sequence of positive ...
- 1,367
19
votes
2
answers
804
views
Existence of a *really* nice topology on the powerset of a topological space
TL;DR. Given a topological space $X$, is there a natural way to "induce" a topology on $\mathcal{P}(X)$ from the topology of $X$ in such a way that 1) all the basic operations of set theory (...
- 11.8k