the set of countable subsets of $2^ω$
90566
There exists surjections from A onto B and vice versa, and there is no bijection between the sets?
118595
Proving that a set is countable by finding a bijection
86685
Example of a set $S$ that is countable, but the set of limit points is uncountable
12472
Let $X$ and $Y$ be countable sets. Then $X\cup Y$ is countable
33157
Proof that an infinite subset of $\mathbb{N}$ is countable
142583
Subsets of the Cantor set
41661
Show that the set of all finite subsets of $\mathbb{N}$ is countable.
35692
The set of all finite subsets of the natural numbers is countable
9021
An infinite subset of a countable set is countable
92005
Existence of compact/complete metric on countable set
52682
Is the power set of the natural numbers countable?
63276
Is an arbitrary union of a chain of countable sets countable?
80096
Prove that the union of countably many countable sets is countable.
52429
Explicit countable elementary extension of $\mathbb{N}$
51094
Prove that the set of integer coefficients polynomials is countable
99599
Uncountable and countable
79974
The cardinality of Lebesgue sets