Union of definable sets is a definable set
12404
Union of definable sets
27899
Can we prove the existence of $A\cup B$ without the union axiom?
81509
The cardinality of a countable union of countable sets, without the axiom of choice
149317
An open subset of $\mathbb{R}$ is an at-most-countable union of disjoint open intervals
95244
Prove that a finite union of closed sets is also closed
27937
Cartesian Product of Two Countable Sets is Countable
88904
Help with a proof. Countable sets.
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
68994
Countable closed sets
94825
Open set as a finite union of closed sets
87830
Relating the union of the power set of a set to the set itself
17022
Definable real numbers
13620
$\omega_2$ is a not countable union of countable sets without AC
67279
Is the union of finitely many open sets in an omega-cover contained within some member of the cover?
23442
Cardinality of the union of infinite and countable sets
77791
Proving an inclusion related to algebraic sets and interpreting it
17842
Is every set in a separable metric space the union of a perfect set and a set that is at most countable?