All Questions
Tagged with gn.general-topology terminology
46 questions
7
votes
2
answers
529
views
What is the name for a point that is periodic to within $\varepsilon$?
Let $X$ be a set and $f: X \to X$ a function. A point $x \in X$ is, of course, said to be periodic for $f$ if $x \in \{f(x), f^2(x), \ldots\}$.
Now suppose that $X$ is a topological space and $f$ is ...
- 5,407
3
votes
0
answers
94
views
Pseudocompactness, countable compactness and locally finite open covers
Let $(P_1)$ be the property: Every locally finite open cover of $X$ has finite subcover.
Let $(P_2)$ be the property: Every locally finite open cover of $X$ is finite.
Let $(P_3)$ be the property: ...
- 1,201
1
vote
0
answers
90
views
Well-embedded type property for bounded functions
According to @Tyrone the term well-embedded set was first used in Measures on Metacompact Spaces by W. Moran.
In the article Extensions of Zero-sets and of Real-valued Functions by R. Blair and A. ...
- 1,201
0
votes
0
answers
42
views
Name for a sequence of open sets, each dense in the complement of the previous ones in the subspace topology
Let $X$ be a topological space. Let $\mathfrak{U} = \langle U_\alpha:\alpha\in\gamma\rangle$ be a sequence of non-empty open subsets of $X$ of length $\gamma$ ($\gamma$ an ordinal). Say (for now) that ...
- 5,429
15
votes
1
answer
796
views
What is this equivalence relation on topological spaces: there are bijective continuous maps in both directions
Consider the following equivalence relation on topological spaces:
$X\sim Y$ $:\Longleftrightarrow$ there are bijective continuous maps $\phi:X\to Y$ and $\psi:Y\to X$.
Note that there are no ...
- 24.2k
1
vote
0
answers
84
views
Is there a standard name for the following class of functions on non-Hausdorff manifolds?
Let $M$ be a (not necessarily Hausdorff) smooth manifold. Given an open chart $U\subset M$ and a compactly-supported smooth function $f:U\to\mathbb{R}$ on $U$, define $\widetilde{f}:M\to\mathbb{R}$ by ...
- 5,429
6
votes
1
answer
289
views
Topological property of convergent sequences being eventually constant
Is there a name in the literature for the topological property that all convergent sequences are eventually constant?
This property seems to occur with some frequency and it would be nice to have a ...
- 351
11
votes
1
answer
355
views
Name for topological spaces where "every point has a local base wellordered by reverse inclusion"?
There are many properties regarding local bases of a topological space, like first countable if every point has a countable local base.
Is there a similar name for a space where "every point has a ...
- 103
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
2
votes
1
answer
198
views
A stronger version of paracompactness
Given a topological space $(X,\tau)$, recall that a cover $\mathcal{U}$ of $X$ is locally finite if for every point $x\in \mathcal{U}$ has a neighborhood $U$ that intersects finitely many elements of $...
3
votes
1
answer
355
views
Extremely disconnected or extremally disconnected?
In the context of Banach space theory, what is the correct terminology: extremally disconnected or extremely disconnected. Looking through the internet I have met using both extremely and extremally ...
- 63.9k
1
vote
1
answer
204
views
Name of a space with both a topology and a metric that are not compatible?
Let $(X,\tau,d)$ be a space where $\tau$ is a topology and $d$ is a metric, where the topology $\tau$ is not necessarily compatible with $d$.
Is there a canonical name for such a structure (maybe ...
- 770
2
votes
0
answers
103
views
$n$-connected spaces (terminology)
A graph is called $n$-connected if it remains connected after removal $\le n$ vertices.
Question. What is the name of an analogous property of topological spaces: a space that remains connected after ...
- 41.9k
1
vote
0
answers
84
views
Terminology for the property: "Each uncountable disjoint open family is locally countable"
Suppose that a topological space $X$ satisfies the following property
(P): "Each uncountable disjoint open family is locally countable",
where a family $\mathcal U$ of subsets of $X$ is ...
- 63.9k
4
votes
0
answers
177
views
Continuity of equivalence relations
A function $\varphi : X \rightarrow Y$ between two topological spaces is continuous if and only if $\varphi(\,\overline{A}\,) \subset \overline{\varphi(A)}$ for all $A \subset X$.
This property can ...
- 18.7k
0
votes
0
answers
41
views
Selectively countable Boolean algebras of sets (terminology)
I am interested in the name for the following property of a Boolean algebra $\mathcal A$ of subsets of a set $X$:
$(\star)$ for any sequence $(A_n)_{n\in\omega}$ of pairwise disjoint nonempty sets in $...
- 41.9k
2
votes
2
answers
588
views
What to call a continuous function with preimage preserving nowhere-density?
Currently I am reading some basic literature on descriptive set theory and boolean algebras. And this comes out a lot, for example in results like:
Let $X$ and $Y$ be topological spaces, and $f:X \to ...
3
votes
0
answers
78
views
Classification of limit points
Let $X$ be a subset of a topolgical space with no open points. Then
$$\overline{X}=X_1\sqcup X_2\sqcup X_3\sqcup X_4\sqcup X_5$$
where $X_1$ are isolated points of $X$,
$X_2$ are interior points, $X_3=...
- 63.9k
0
votes
0
answers
139
views
Why the name 'regular' space?
It is well known that a regular space is a topological space $X$ with these two properties:
1)All one point sets are closed.
2)For every $x\in X$ and every closed set $B$ (such that $x\notin B$), ...
- 883
0
votes
1
answer
277
views
Are knot invariants topological invariants? [closed]
I am a bit confused about terminology considering topology and knot theory.
A topological invariant is considered to be a topological property that does not change under a homeomorphism of the space.
...
- 3,065
3
votes
0
answers
132
views
Terminology for a generalization of the initial topology
This may be a simple piece of terminology, but I have not located it. For the initial topology, we are given a set of functions, indexed by $\alpha$, $f_{\alpha}:X\rightarrow Y_{\alpha}$, where each ...
- 1,422
3
votes
0
answers
74
views
Equivalence relation induced by Kolmogorov quotients
Recall: given a (possibly non-$T_0$) topological space $X$, its Kolmogorov quotient $KX$ is the $T_0$ topological space formed by $X/\sim$ where $x\sim y$ if they are topologically indistinguishable. ...
- 39k
43
votes
3
answers
9k
views
Why the name 'separable' space?
It is well known that a separable space is a topological space that has a countable dense subset. I am wondering how is this related to the name 'separable'? Any intuition where the name come from?
CommunityBot
- 1
12
votes
2
answers
806
views
Term for a metric space for which the triangle inequality is strict?
Is there a standard term for a metric space in which $\rho(p,r) < \rho(p,q) + \rho(q,r)$ for any distinct $p$, $q$, $r$? Sort of the opposite of metric convexity.
For instance, a subset of ...
- 188k
3
votes
0
answers
78
views
Name for mappings that are "not quite projections"
Is there a known name for the following definition?
Consider topological spaces $X$, $Y$ and $f: X \rightarrow Y$ a continuous mapping. Then, $f$ is an "almost projection" if there is a topological ...
- 1,368
6
votes
1
answer
186
views
Reference request: A collection of topologies on $\mathbb{N}$ formed via series
First, some quick notation: for any series $\sum_{n=1}^\infty a_n$ whose terms are positive real numbers, and for any subset $M = \{m_1, m_2,...\} \subseteq \mathbb{N}$, we write $\sum_M a_n$ to mean ...
- 1,615
9
votes
0
answers
569
views
A standard name for a function satisfying the intermediate value theorem?
Do you know any (standard) name for a function $f:\mathbb R\to\mathbb R$ having the following weak intermediate value property:
$(*)$ for any connected subset $C\subset \mathbb R$ and points $a,b\...
- 4,713
6
votes
1
answer
101
views
Name for $\omega_1$-DCC / Noetherian condition?
I recently asked (and then answered) this question:
https://math.stackexchange.com/questions/2756777/decreasing-sequence-of-closed-sets-in-a-separable-metric-space.
In a separable metric space ...
- 13.4k
9
votes
1
answer
322
views
What is the (genuine) name for the Gutik hedgehog?
Working with non-regular topological semigroups, my collegue Oleg Gutik discovered a special space $H$ which we named Gutik's hedgehog. It is homeomorphic to the space
$$H:=\{(0,0)\}\cup\{(\tfrac1n,0):...
- 41.1k
2
votes
2
answers
379
views
Is there a standard term for this graph/set theoretic concept?
For a fixed set $X$ and a finite collection $E_1,E_2,\ldots,E_k\subseteq X$, define the binary relation adjacency as follows: $E_i,E_j$ are adjacent
if their intersection is nonempty.
We term the ...
- 4,713
6
votes
0
answers
322
views
Terminology for notion dual to "support"
If $X$ is a set (feel free to think of it as finite, but it doesn't have to be) and $f$ a real function on $X$, call the support $\operatorname{supp} f$ the subset of $X$ consisting of all elements $i\...
CommunityBot
- 1
1
vote
1
answer
164
views
The proper name for a kind of ordered space [closed]
I'm trying to find the correct term for a specific kind of totally ordered space:
Let $S$ be a totally ordered space with strict total order $<$.
Property: For any two $s_{1}$ and $s_{2}$ in $S$ ...
- 6,210
6
votes
1
answer
1k
views
Who coined "mob" and "clan" and why these words?
A mob is a word used for a topological semigroup which is a Hausdorff space. A clan is a compact connected mob with a two-sided identity element.
Who used these words with these meanings first and ...
- 188k
4
votes
5
answers
1k
views
A generalized diagonal?
A simple question. Let $ f:X\to Y $ be a function and let $ E_f:=\{(x, y): f (x)=f (y)\}\subset X\times X $. What is the name of the set $ E(f) $? It would be nice to have some reference also. It ...
- 7,500
1
vote
1
answer
100
views
Name for (function, set) pairs?
Right now I'm working on a topological graph theory problem. To prove a theorem I introduced some objects. Has anyone heard of something similar before? I would like to call them by the right name.
...
- 39k
5
votes
1
answer
1k
views
Decomposition vs filtration vs stratification
Are there accepted/standard definitions of "decomposition", "filtration", and "stratification" of a topological space (or of a manifold, or of an algebraic variety) $X$?
I tend to understand "...
CommunityBot
- 1
4
votes
3
answers
849
views
"countable" topology
Given universal set $U$. Is there any name of the collection of subsets of $U$ (call them quasi-open) satisfying the following axioms:
i) $\emptyset$ and $U$ are quasi-open;
ii) finite intersections ...
- 9,650
6
votes
0
answers
108
views
How to call a point in a space having the property that there is essentially one $\omega$-sequence converging to it?
Consider the point $x=\langle \omega_1,\omega\rangle$ in the Tychonov plank $(\omega_1 + 1)\times(\omega + 1)$. Then there is essentially only one sequence (of length $\omega$) converging to it, ...
- 1,855
2
votes
0
answers
146
views
How do you call a map which sends convergent sequences to pre-compact ones ?
In my work I encountered a map $f$ between two metric spaces $X$ and $Y$ that was not continuous (at least I couldn't prove it was), but I was able to prove that convergent sequences $(x_n)$ in $X$ ...
- 4,123
9
votes
0
answers
685
views
Name for a topological space where every closed set contains a closed point
A coauthor and I have stumbled upon a useful topological property -- namely, we are interested in the property that every nonempty closed set contains a closed point. However, neither of us are ...
- 1,802
4
votes
0
answers
137
views
Does this property of scattered spaces have a name?
(Note: I asked this question at MSE a week ago and received no answer, so I am now reposting it here.)
Let $K$ be a (Hausdorff) scattered topological space and for each ordinal $\alpha$ denote by $K^{...
- 2,378
6
votes
1
answer
555
views
Is there a name for the class of metric spaces such that the closure of the open ball of radius $r$ around each point $x$ is the set of elements $y$ such that $d(x,y)\leq r$ ?
Let $(X,d)$ be a metric space, let $B(x,r)$ be the open ball of radius $r$ about $x$ and $N(x,r)$ be the set of elements $y\in X$ such that $d(x,y)\leq r$. It is well-known that it is not always true ...
- 13.6k
3
votes
4
answers
514
views
Better terminology than "equivalence class of functions"
Let $X = C(\mathbb R)$ be the Fréchet space of real-valued continuous functions. For each $f \in X$ and each compact set $D \subseteq \mathbb R$, let $$[f]_D = \{ g \in X : \mbox{$g(t) = f(t)$ for ...
- 17k
4
votes
2
answers
3k
views
What do you call a topology that is closed under arbitrary intersections?
An arbitrary union, or a finite intersection, of open sets in a topological space is again open. What name is given to the hypothetical property that an arbitrary intersection of open sets is open?
...
CommunityBot
- 1
3
votes
3
answers
699
views
Is there a name for this property of a topology?
This property seems like it should have a nice name, but I can't find one anywhere. Does anyone know a name for this?
For each non-empty open set $U$, there exist proper open subsets $\{U_i\}_{i\in ...
- 115
3
votes
3
answers
728
views
What do you call the product of a circle and an annulus?
What would you call the product of an annulus and $S^1$ (a 'thickened' torus like 3-manifold)?
More generally, is there an archive or list online of names assigned to various (non-standard) manifolds ...
- 345