Solution
= Solution
A <topology> on $X$ contains $\varnothing$ and $X$, is closed under arbitrary unions, and is closed under finite intersections. Its members are the open sets. A subset is closed precisely when its complement in $X$ is open.
= Solution
A <topology> on $X$ contains $\varnothing$ and $X$, is closed under arbitrary unions, and is closed under finite intersections. Its members are the open sets. A subset is closed precisely when its complement in $X$ is open.