What’s a topology
[ text{Let X be a set, a topology on X is a family of sets } tau subseteq wp text{(X) (where} wp text{ is the power set) such that:} ] [ begin{align} & (1) emptyset , X in tau \ & (2) tau text{ is closed under unions. For every } {A_{i}}_{i in I} subseteq tau text{ we have }