Intuitive interpretation of open and closed sets in topological spaces
137460
How should I think of an open vs. closed set?
152193
The intersection of a closure with an open set
77563
Homeomorphism between spaces equipped with cofinite topologies
28683
Sum of two closed sets in $\mathbb R$ is closed?
105445
Are the rationals a closed or open set in $\mathbb{R}$?
125962
The set $A'$ is closed in a $T_1$ space
2198
Maps from subsets of $\mathbb{R}^2$ that are either open/closed/continuous
127900
Interior of closure of open set in topological space
127575
Prove that Open Sets in $\mathbb{R}$ are The Disjoint Union of Open Intervals Without the Axioms of Choice
70048
In which topologies do open sets maintain open under countable or arbitrary intersection?
93623
Showing that every connected open set in a locally path connected space is path connected
59424
Showing that $B=\{(x,y) \in \Bbb{R^2} \mid \ xy>0 \}$ is an open set
65316
Explaining Infinite Sets and The Fault in Our Stars
52865
Topological Vector Space: $\dim Z\text{ finite}\implies Z\text{ closed}$
101504
An open ball is an open set
135454
An open interval is an open set?
27661
The closed graph theorem in topology