Is every subset  of $\mathbb{Z^+}$ countable?
9021
An infinite subset of a countable set is countable
146436
Cardinality of the set of bijective functions on $\mathbb{N}$?
51094
Prove that the set of integer coefficients polynomials is countable
63276
Is an arbitrary union of a chain of countable sets countable?
88904
Help with a proof. Countable sets.
41192
Neatest proof that set of finite subsets is countable?
83185
How to show countability of $\omega^\omega$ or $\epsilon_0$ in ZF?
99656
Prove that the set of all algebraic numbers is countable
8434
Order of $\mathbb{Z}[i]/(1+i)$
74720
Is every set of measure zero countable?
76240
How is $\epsilon_0$ countable?
156006
Cardinality of set containing all infinite subsets of $\mathbb{Q}$
75970
$\mathbb Z[1/2]$ is not finitely generated?
125411
Infinite set always has a countably infinite subset
102112
Idempotents in $\mathbb Z_n$
135441
When is $\mathbb{Z}[\alpha]$ dense in $\mathbb{C}$?
147136
Subset of a countable set is itself countable