How to show the set of all algebraic real number is countable
68430
Proving the countability of algebraic numbers
27937
Cartesian Product of Two Countable Sets is Countable
98151
Suppose that A is a countable set. Show that the set B is also countable if there is an onto function f from A to B.
136321
Show that if $\sup\big\{\sum\lvert\, f(a)\rvert\big\}  0\}$ is countable.
75019
Infinite Cartesian product of countable sets is uncountable
103769
The Cartesian product of a finite number of countable sets is countable
63276
Is an arbitrary union of a chain of countable sets countable?
6161
The cartesian product $\mathbb{N} \times \mathbb{N}$ is countable
83842
Alternate proofs (other than diagonalization and topological nested sets) for uncountability of the reals?
118595
Proving that a set is countable by finding a bijection
155168
Is the collection of finite subsets of $\mathbb{Z}$ countable?
43268
How to prove that the set of rational numbers are countable?
132778
All distances are rational prove the set is countable
91237
Every infinite set contains a countable subset
32774
Finite Cartesian Product of Countable sets is countable?
138768
Showing that the family of Borel sets is countably generated
127362
How do I prove that there doesn't exist a set whose power set is countable?