How to prove that a union of cardinals is a cardinal
39956
Union of uncountable cardinals
147213
Prove that finite unions of compact sets are compact
127575
Prove that Open Sets in $\mathbb{R}$ are The Disjoint Union of Open Intervals Without the Axioms of Choice
7785
Cardinality of sum-set
117047
exponentiation of cardinal number
50880
Expressing $\Bbb N$ as an infinite union of disjoint infinite subsets.
95244
Prove that a finite union of closed sets is also closed
136069
Is there a simple formula for the cardinality of $\{A\subseteq\kappa\mid |A|\leq\lambda\}$ when $\lambda\leq\kappa$?
112084
Proof of a formula for the power set of a union of two sets
5840
divides, not divides and cardinalities
59813
Prove that for all sets A and B, A ∩ B = ∅ implies ( A ∪ B ) - B = A
9268
Proving that the Union of Two Compact Sets is Compact
87830
Relating the union of the power set of a set to the set itself
142993
Proof of A ⊆ C and B ⊆ C ⇔ A ∪ B ⊆ C
9158
Proving that a propositional theory of any cardinality has an independent set of axioms
12404
Union of definable sets
63489
The class of cardinal numbers is well ordered