Definition:Finite Complement Topology/Countable

Definition
Let $S$ be an infinite set.

Let $\tau$ be the finite complement topology on $S$.

Let $S$ be countably infinite.

Then $\tau$ is a finite complement topology on a countable space, and $\left({S, \tau}\right)$ is a countable finite complement space.

Also known as
The term cofinite is sometimes seen in place of finite complement.

Some sources are more explicit about the nature of this topology, and call it the topology of finite complements.

Also see

 * Finite Complement Topology is Topology