# Math Help - find the closure of A in the co-finite topology

1. ## find the closure of A in the co-finite topology

Suppose that $X$ is an infinite set with the co-finite topology, and $A \subseteq X$. Find $Cl(A)$ in the cases where
(a) $A$ finite and (b) $A$ infinite.

I really dont know where to start. Hints would be very useful.

Thanks

2. ## Re: find the closure of A in the co-finite topology

Hint: the closed set are the finite subsets of X and X.

3. ## Re: find the closure of A in the co-finite topology

Originally Posted by FGT12
Suppose that $X$ is an infinite set with the co-finite topology, and $A \subseteq X$. Find $Cl(A)$ in the cases where
(a) $A$ finite and (b) $A$ infinite.
Question: What is closure of a closed set?
In part a), is $X\setminus A$ an open set? Why or why not?