- #1
doodlepin
- 8
- 0
Question is: X is a set and tau is a collection of subsets O of X such that X - O is either finite or all of X. Show this is a topology and completely described when a point x in X is a limit of a subset A in X.
I already proved that this satisfies the conditions for defining a topology (called the finite complement topology) But I am having a lot of trouble with defining when a point is a limit. I know that any open subset (defined by tau) containing such a limit point must have a non-empty intersection with A for it to be a limit point.
I have tried considering multiple cases: I know if X is finite then it is the discrete topology and therefore no limit points exist. So the non trivial case if when X is infinite.
Now, if A is finite then for any open subset containing x, O, the complement with A would obviously be finite so therefore non empty?
I'm sort of stuck and could use a nudge in some helpful direction.
I already proved that this satisfies the conditions for defining a topology (called the finite complement topology) But I am having a lot of trouble with defining when a point is a limit. I know that any open subset (defined by tau) containing such a limit point must have a non-empty intersection with A for it to be a limit point.
I have tried considering multiple cases: I know if X is finite then it is the discrete topology and therefore no limit points exist. So the non trivial case if when X is infinite.
Now, if A is finite then for any open subset containing x, O, the complement with A would obviously be finite so therefore non empty?
I'm sort of stuck and could use a nudge in some helpful direction.