Prove that a set is countable
100333
Show A is countable infinity
70143
Mutually exclusive countable subsets of a countable set
66398
Union of a finite set and a countably infinite set is countably infinite
157513
How can I prove that these sets are countable infinite?
138768
Showing that the family of Borel sets is countably generated
41192
Neatest proof that set of finite subsets is countable?
52682
Is the power set of the natural numbers countable?
48523
Let $A$ be any uncountable set, and let $B$ be a countable subset of $A$. Prove that the cardinality of $A = A - B $
46967
What is wrong with this proof that the power set of natural numbers is countable?
99656
Prove that the set of all algebraic numbers is countable
16722
Proving Gaussian Integers are countable
136321
Show that if $\sup\big\{\sum\lvert\, f(a)\rvert\big\}  0\}$ is countable.
118595
Proving that a set is countable by finding a bijection
157081
The cartesian product of a finite amount of countable sets is countable.
97697
Countable union of sets of cardinality $c$ has cardinality $c$
35692
The set of all finite subsets of the natural numbers is countable
44623
Closed set as a countable intersection of open sets