nLab second-countable space

Second-countable spaces



topology (point-set topology, point-free topology)

see also differential topology, algebraic topology, functional analysis and topological homotopy theory


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topological homotopy theory

Second-countable spaces


A space (such as a topological space) is second-countable if, in a certain sense, there is only a countable amount of information globally in its topology. (Change ‘globally’ to ‘locally’ to get a first-countable space.)



(second-countable topological space)

A topological space is second-countable if it has a base for its topology consisting of a countable set of subsets.


A locale is second-countable if there is a countable set BB of open subspaces (elements of the frame of opens) such that every open GG is a join of some subset of BB. That is, we have

G={U:B|UG}. G = \bigvee \{ U\colon B \;|\; U \subseteq G \} .


The weight of a space is the minimum of the cardinalities of the possible bases BB. 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 open subspaces, and inhabited by this number as well) has a minimum. But without Choice, we can still consider this collection of cardinalities.

Then a second-countable space is simply one with a countable weight.



(Euclidean space is second-countable)

Let nn \in \mathbb{N}. Consider the Euclidean space n\mathbb{R}^n with its Euclidean metric topology. Then n\mathbb{R}^n is second countable.

A countable set of base open subsets is given by the open balls B x (ϵ)B^\circ_x(\epsilon) of rational radius ϵ 0 0\epsilon \in \mathbb{Q}_{\geq 0} \subset \mathbb{R}_{\geq 0} and centered at points with rational coordinates: x n nx \in \mathbb{Q}^n \subset \mathbb{R}^n.


A compact metric space is second-countable.


A separable metric space, e.g., a Polish space, is second-countable.


It is not true that separable first-countable spaces are second-countable; a counterexample is the real line equipped with the half-open or lower limit topology that has as basis the collection of half-open intervals [a,b)[a, b).


A Hausdorff locally Euclidean space is second-countable precisely it is paracompact and has a countable set of connected components. In this case it is called a topological manifold.

See at topological space this prop..


A countable coproduct (disjoint union space) of second-countable spaces is second-countable.

Countable products (product topological spaces) of second-countable spaces are second-countable.

Subspaces of second-countable spaces are second-countable.


If XX is second-countable and there is an open surjection f:XYf \colon X \to Y, then YY is second-countable.


For second-countable T_3 spaces X,YX, Y, if XX is locally compact, then the mapping space Y XY^X with the compact-open topology is second-countable.

Cf. Urysohn metrization theorem and Polish space. I (Todd Trimble) am uncertain to what extent the T 3T_3 assumption can be removed.


Axioms: axiom of choice (AC), countable choice (CC).



Last revised on June 1, 2024 at 01:02:54. See the history of this page for a list of all contributions to it.