When boundary of union is equal to union of boundaries
30231
The boundary of union is the union of boundaries when the sets have disjoint closures
67191
When is the union of a family of subspaces of a vector space also a subspace?
31980
Is the boundary of a boundary a subset of the boundary?
22245
Show the union of two subrings is generally not a subring
107564
If two sets have a natural (asymptotic) density, does their union?
122663
Proving $T$ and $U$ are equal if I know $(X,T)$ and $(X,U)$ are hausdorff and compact
57938
When do two functions become equal?
148638
Write $(0,1)$ as a countable union of disjoint open intervals
111830
When is the closure of an open ball equal to the closed ball?
70538
Intersection and union of simply connected domain
10602
Monotone Union of subgroups being subgroup
52990
Rigorously proving equal tension on both ends
68700
Proving Two Limits Are Equal to Each Other
143586
Is $[0,1]$ a union of family of disjoint closed intervals?
142840
Open axioms of equality
130156
Image of the union and intersection of sets.
94266
The union of powersets is contained in the powerset of union