An exercise on union of open sets
21892
Any open subset of $\Bbb R$ is a countable union of disjoint open intervals
72468
Open and closed balls in $C[a,b]$
149673
Cardinality of an infinite union of finite sets
148638
Write $(0,1)$ as a countable union of disjoint open intervals
19289
complement of a set and its measure
90590
Prove that the following set is dense
67451
Building a bijection from a finite union of countable sets to $\mathbb{N}$
157753
Show that any set in a metric space can be written as the intersection of open sets
67279
Is the union of finitely many open sets in an omega-cover contained within some member of the cover?
80860
How to call a space in which every G$_\kappa$-set is open
127575
Prove that Open Sets in $\mathbb{R}$ are The Disjoint Union of Open Intervals Without the Axioms of Choice
146664
The union of two linearly independent subsets
92639
Prove that the family of open sets in $\mathbb{R}$ has cardinality equal to $2^{\aleph_0}$
59424
Showing that $B=\{(x,y) \in \Bbb{R^2} \mid \ xy>0 \}$ is an open set
33563
Is the axiom of choice used in the proof that every open set is the union of basis elements (Mukres, Lemma 13.1)
87830
Relating the union of the power set of a set to the set itself
132868
Bijection between an infinite set and its union of a countably infinite set