Question 1
Which statement is true for every closure operator on a topological space?
Question 2
What is the closure of the empty set in any topological space?
Question 3
Which identity expresses the idempotent property of closure?
Question 4
If $A \subseteq B$, what must be true of their closures?
Question 5
In the usual topology on $\mathbb{R}$, what is $\operatorname{cl}(\mathbb{Q})$?
