$X\subset \mathbb N$ is countable
9021
An infinite subset of a countable set is countable
40937
Is the set of partial order relations over $ \mathbb{N}$ countable?
93419
Let $\mathcal A$ be a collection of pairwise disjoint subsets of a $\sigma$-finite measure space,Show that $\mathcal A$ is at most countable.
105358
In a second countable space any base has a countable subset that is still a base
51094
Prove that the set of integer coefficients polynomials is countable
133266
If $(X, \mathcal{T})$ has a countable subbasis, then it has a countable basis
120522
Showing any countable, dense, linear ordering is isomorphic to a subset of $\mathbb{Q}$
95747
Number of countable subsets of $\mathbb{R}$
130361
Basis for a topology with a countable number of sets
32774
Finite Cartesian Product of Countable sets is countable?
138768
Showing that the family of Borel sets is countably generated
52429
Explicit countable elementary extension of $\mathbb{N}$
70143
Mutually exclusive countable subsets of a countable set
125411
Infinite set always has a countably infinite subset
155168
Is the collection of finite subsets of $\mathbb{Z}$ countable?
42794
Prove that functions map countable sets to countable sets
91237
Every infinite set contains a countable subset