Topology and countable base
105358
In a second countable space any base has a countable subset that is still a base
6210
Countable compact spaces as ordinals
23442
Cardinality of the union of infinite and countable sets
81509
The cardinality of a countable union of countable sets, without the axiom of choice
40071
What is the cardinality of the set of all topologies on $\mathbb{R}$?
76240
How is $\epsilon_0$ countable?
103717
Topological Property of the Set of Integers
51871
Characterization of Topology
21892
Any open subset of $\Bbb R$ is a countable union of disjoint open intervals
74877
Every metric space with a countable base is separable
99478
countably infinite union of countably infinite sets is countable
115173
The Cantor set is homeomorphic to infinite product of $\{0,1\}$ with itself - cylinder basis - and it topology
138202
Every open set in $\mathbb{R}$ is a countable union of closed sets
99599
Uncountable and countable
71489
Countable product of Polish spaces
84936
The set of all finite sequences of members of a countable set is also countable
102543
Real definition of "countable set"