Proving $\mathbb{N} \times \mathbb{N}$ is countable without theorem of arithmetic.
110569
Does anyone know a closed-form expression for a bijection between $\mathbb{N}^k$ and $\mathbb{N}$?
55978
Prove that for every countable ordinal $\alpha$, $\alpha\times[0,1)$ is order isomoprhic to $[0,1)$
27937
Cartesian Product of Two Countable Sets is Countable
157081
The cartesian product of a finite amount of countable sets is countable.
83116
Uncountability of basis of $\mathbb R^{\mathbb N}$
127978
Another point of view that $\mathbb{Z}/m\mathbb{Z} \times\mathbb{Z}/n\mathbb{Z}$ is cyclic.
60678
Does 'A countable infinite Cartesian product of countable sets is not necessarily countable' violate induction principle?
157513
How can I prove that these sets are countable infinite?
40937
Is the set of partial order relations over $ \mathbb{N}$ countable?
50939
If $A$ and $B$ are nonempty sets, prove that $A \times B = B \times A$ if and only if $A = B$
72750
Uncountable number of mutually disjoint countable sets in $\mathbb R$
136321
Show that if $\sup\big\{\sum\lvert\, f(a)\rvert\big\}  0\}$ is countable.
134844
Summablity and countability
136006
Prove that any countable cartesian product of countable sets is countable
64130
Prove that $\mathbb{Q}^{\times}$ not isomorphic to $\mathbb{Z}^{n}$
84936
The set of all finite sequences of members of a countable set is also countable
155168
Is the collection of finite subsets of $\mathbb{Z}$ countable?