Collection of open intervals $(a,b) \ a,b\in \mathbb{Q}$ is a basis for euclidean topology on $\mathbb{R}$
57037
How does $(a,b)$ where $a,b\in \mathbb{Q}$ form a basis for $\mathbb{R}$?
73927
Is $\mathbb{R}$ a subset of $\mathbb{R}^2$?
16654
Unions and Intersections of Open Sets are Open
2575
Showing that $\mathbb{R}$ is connected
55729
Family of Closed/Open Nested Intervals
72468
Open and closed balls in $C[a,b]$
41611
Open and closed set in n by n matrices space.
27855
Show that $(a,b)\times (c,d)$ is an open set in $\mathbb{R}^2$ with the Euclidian metric.
36921
An Open Interval and a Half-open Interval are not Homeomorphic
128987
A collection of pairwise disjoint open intervals must be countable
130361
Basis for a topology with a countable number of sets
21938
describing all possible topologies on a set
86537
Prove that an open interval and a closed interval are not homeomorphic
84104
Definition of neighborhood and open set in topology
39692
The length of an interval covered by an infinite family of open intervals
99236
Interior of closure of an open set
55486
Topological proof to show a complement set of a set is polygonally connected