Question about regular open sets
90782
Examples of topologies in which all open sets are regular?
6491
Not every open set in $\mathbb{R}^n$, for $n\geq2$, is a disjoint union of countable open balls.
44430
Is there an open dense set $S \subset [0,1]$ such that $m(S)<1$?
60053
Does the boundary of an open set have measure zero (in $\mathbb{R}^n$)?
43535
show that the interval of the form $[0,a)$ or $(a, 1]$ is open set in metric subspace $[0,1]$ but not open in $\mathbb R^1$
106944
Is the set defined by y=sin(1/x) open or closed?
153934
How to prove that a set R\Z is open
110957
No non-trivial clopen sets in $\mathbb{R}$? How to give a direct proof?
122463
How to show the that a set $A$ nowhere dense is equivalent to the complement of $A$ containing a dense open set?
75829
Dimension of open subsets of $R^n$
107160
Open Set and Measure Theory
69542
Why do the $n \times n$ non-singular matrices form an "open" set?
121626
Every open cover of the real numbers has a countable subcover (Lindelöf's lemma)
32723
Open sets and holomorphic functions in Complex analysis
70264
Prove that the inverse image of an open set is open
17607
Open maps which are not continuous
101504
An open ball is an open set