Proving a basic property about derived sets and unions
107383
Derived sets - prove $(A \cup B)' = A' \cup B'$
12404
Union of definable sets
16654
Unions and Intersections of Open Sets are Open
115606
Proofs involving $ (A\setminus B) \cup (A \cap B)$
43024
Well-ordered sets, properties of an element that is not largest in the set
59813
Prove that for all sets A and B, A ∩ B = ∅ implies ( A ∪ B ) - B = A
33563
Is the axiom of choice used in the proof that every open set is the union of basis elements (Mukres, Lemma 13.1)
49438
Prove that $X^\ast$ separable implies $X$ separable
52451
Ordered Triples and Set Theory
77286
Proving set theory question
115649
Proving two sets $A, B$ are order isomorphic
25553
elementary set theory (cartesian product and symmetric difference proof)
68230
The cardinality of a countable union of sets with less than continuum cardinality
89506
Union of disjoint sets : Equivalence relations
116527
Validating a proof for necessary and sufficient conditions on sets A & B such that $P(A) \cup P(B) = P(A\cup B)$
47680
Prove that the union of the interior of a set and the boundary of the set is the closure of the set
87327
Unions and intersections: $(A \cup B = A ∪ C) \land (A \cap B = A ∩ C) \implies B = C.$