topology (point-set topology, point-free topology)
see also differential topology, algebraic topology, functional analysis and topological homotopy theory
Basic concepts
fiber space, space attachment
Extra stuff, structure, properties
Kolmogorov space, Hausdorff space, regular space, normal space
sequentially compact, countably compact, locally compact, sigma-compact, paracompact, countably paracompact, strongly compact
Examples
Basic statements
closed subspaces of compact Hausdorff spaces are equivalently compact subspaces
open subspaces of compact Hausdorff spaces are locally compact
compact spaces equivalently have converging subnet of every net
continuous metric space valued function on compact metric space is uniformly continuous
paracompact Hausdorff spaces equivalently admit subordinate partitions of unity
injective proper maps to locally compact spaces are equivalently the closed embeddings
locally compact and second-countable spaces are sigma-compact
Theorems
Analysis Theorems
A space (such as a topological space) is first-countable if, in a certain sense, there is only a countable amount of information locally in its topology. (Change ‘locally’ to ‘globally’ to get a second-countable space.)
A topological space is first-countable if every point $x$ has a countable local basis $B_x$.
The character of a space at a point $x$ is the minimum of the cardinalities of the possible bases $B_x$. We are implicitly using the axiom of choice here, to suppose that this set of cardinalities (which really is a small set because bounded above by the number of neighbourhoods of $x$, and inhabited by this number as well) has a minimum. But without Choice, we can still consider this collection of cardinalities.
Then a first-countable space is simply one whose characters are all countable.
The character, tout court, of a space is the supremum of the characters of its points; then a first-countable space is simply one with a countable character.
Axioms: axiom of choice (AC), countable choice (CC).
second-countable: there is a countable base of the topology.
metrisable: the topology is induced by a metric.
$\sigma$-locally discrete base: the topology of $X$ is generated by a $\sigma$-locally discrete base.
$\sigma$-locally finite base: the topology of $X$ is generated by a countably locally finite base.
Lindelöf: every open cover has a countable sub-cover.
weakly Lindelöf: every open cover has a countable subcollection the union of which is dense.
metacompact: every open cover has a point-finite open refinement.
countable chain condition: A family of pairwise disjoint open subsets is at most countable.
first-countable: every point has a countable neighborhood base
Frechet-Uryson space: the closure of a set $A$ consists precisely of all limit points of sequences in $A$
sequential topological space: a set $A$ is closed if it contains all limit points of sequences in $A$
countably tight: for each subset $A$ and each point $x\in \overline A$ there is a countable subset $D\subseteq A$ such that $x\in \overline D$.
a second-countable space has a $\sigma$-locally finite base: take the the collection of singeltons of all elements of a countable cover of $X$.
second-countable spaces are separable: use the axiom of countable choice to choose a point in each set of a countable cover.
weakly Lindelöf spaces with countably locally finite base are second countable.
separable spaces satisfy the countable chain condition: given a dense set $D$ and a family $\{U_\alpha : \alpha \in A\}$, the map $D \cap \bigcup_{\alpha \in A} U_\alpha \to A$ assigning $d$ to the unique $\alpha \in A$ with $d \in U_\alpha$ is surjective.
separable spaces are weakly Lindelöf: given a countable dense subset and an open cover choose for each point of the subset an open from the cover.
Lindelöf spaces are trivially also weakly Lindelöf.
a space with a $\sigma$-locally finite base is first countable: obviously, every point is contained in at most countably many sets of a $\sigma$-locally finite base.
a first-countable space is obviously Fréchet-Urysohn.
a Fréchet-Uryson space is obviously sequential.
a sequential space is obviously countably tight.
paracompact spaces satisfying the countable chain condition are Lindelöf.
Last revised on February 4, 2024 at 18:36:15. See the history of this page for a list of all contributions to it.