Exact bijection to prove rationals are countable
5803
Produce an explicit bijection between rationals and naturals?
48804
Enumeration of rationals from Stein-Shakarchi's Real Analysis (Chapter 1, Exercise 24)
94365
How to prove that if $D$ is countable, then $f(D)$ is either finite or countable?
38242
Explicit bijection between the successor of $\alpha$ and $\alpha$ (infinite ordinal).
84936
The set of all finite sequences of members of a countable set is also countable
78412
Representing a countable field by $\Bbb N$ [transport of algebraic structure]
102583
Infinite number of rationals between any two reals.
67066
Proof there is a 1-1 correspondence between an uncountable set and itself minus a countable part of it
53168
Surjective, Injective, Bijective Functions from $\mathbb Z$ to itself
139247
Real Analysis - Prove a set is countable
157513
How can I prove that these sets are countable infinite?
27937
Cartesian Product of Two Countable Sets is Countable
132778
All distances are rational prove the set is countable
35692
The set of all finite subsets of the natural numbers is countable
37398
Proof that$ (a+A)\cap A=\varnothing$ if $A$ is countable
80096
Prove that the union of countably many countable sets is countable.
133699
Proving that there is a bijection between $A$ and $B$.