Why are the countable ordinals a set?
28764
How do we know an $ \aleph_1 $ exists at all?
2269
Proof that the real numbers are countable: Help with why this is wrong
99478
countably infinite union of countably infinite sets is countable
131163
There is a well ordering of the class of all finite sequences of ordinals
19924
Uncountable ordinals without power set axiom
68230
The cardinality of a countable union of sets with less than continuum cardinality
127233
Cardinality of order types in $\mathbb{N}$
55978
Prove that for every countable ordinal $\alpha$, $\alpha\times[0,1)$ is order isomoprhic to $[0,1)$
18223
Prove that the union of two disjoint countable sets is countable
9021
An infinite subset of a countable set is countable
142045
What's the definition of limit of sets(esp. ordinals) in set theory?
23442
Cardinality of the union of infinite and countable sets
3135
Is a limit ordinal necessarily a cardinal?
108922
Cardinality and infinite sets: naturals, integers, rationals, bijections
105146
$M$ contains all ordinals
148507
Why an open set in $R^n$ might not be written as countable union of disjoint open intervals?
35692
The set of all finite subsets of the natural numbers is countable