If $X$ and $Y$ are countable, then $X \times Y$ is countable.
103769
The Cartesian product of a finite number of countable sets is countable
23442
Cardinality of the union of infinite and countable sets
40937
Is the set of partial order relations over $ \mathbb{N}$ countable?
9021
An infinite subset of a countable set is countable
99656
Prove that the set of all algebraic numbers is countable
44974
Infinite set and countable subsets
68139
Show that the set of polynomials with rational coefficients is countable.
51206
If $X$ and $Y$ are sets, their cartesian product $X\times Y$ is a set.
51094
Prove that the set of integer coefficients polynomials is countable
102543
Real definition of "countable set"
97697
Countable union of sets of cardinality $c$ has cardinality $c$
105358
In a second countable space any base has a countable subset that is still a base
41661
Show that the set of all finite subsets of $\mathbb{N}$ is countable.
122506
Relation on countable sets
110635
Proving $\mathbb{N}^k$ is countable
18429
Counting measure proof
2269
Proof that the real numbers are countable: Help with why this is wrong