So to prove $"\implies "$ I go by contradiction:
- Let $A$ be open and $A^c$ (the complement of $A$) also be open.
- since $A^c$ is open it does not contain its limit points.
- let $y$ be a limit point of $A^c$, then $y \in (A^c)^c =A$.
- since $y$ is a limit point of $A$ complement there must always be an element of $A$ complement in any epsilon ball around $y$.
- But the thing is that A is open: which means any point in a is interior (that is for any point in a, there is a ball around that point that is entirely contained in A)
- since $y$ is in $A$, this should have been the case for y too.
- But we see that there is no ball around $y$ which does not contain an element of $A^c$.
- contradiction: 7) is contradicting 5) & 6) [end of proof $"\implies "$]
*** The other direction though is tricky:
I want to go by contradiction again: I say
- By contradiction let $A^c$ be closed and $A$ also be closed.
- Since $A$ is closed it contains all its limit points.
- ...*** at this point my brain just stops "seeing" what's up [I can probably look this up but I learn better by discussing stuff so any help is appreciated].