A relation on 2 countable sets
122506
Relation on countable sets
7678
Show "countable complement topology" is a topology
85852
How many total order relations on a set $A$?
4602
Outline of a proof that $\mathbb{R}^2 - A$ where A is countable is path-connected
24685
Expressing a Countable Union of Sets as the Countable Union of Disjoint Sets (e.g. Eliminating Duplicate Intersections)
84936
The set of all finite sequences of members of a countable set is also countable
120522
Showing any countable, dense, linear ordering is isomorphic to a subset of $\mathbb{Q}$
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.
81509
The cardinality of a countable union of countable sets, without the axiom of choice
44974
Infinite set and countable subsets
120562
If A is an infinite set and B is at most countable set, prove that A and $A \cup B$ have the same cardinality
18223
Prove that the union of two disjoint countable sets is countable
13620
$\omega_2$ is a not countable union of countable sets without AC
139247
Real Analysis - Prove a set is countable
80096
Prove that the union of countably many countable sets is countable.
6161
The cartesian product $\mathbb{N} \times \mathbb{N}$ is countable
144731
Disjoint closed sets in a second countable zero-dimensional space can be separated by a clopen set