WEBMATH 600 Topology Cheatsheet. This cheatsheet is not, of course, complete. topological space is a pair (X; O), where X is a set and O is a collection of open subsets of X so that: X 2 O; ? 2 O. For any collection fO g 2 of open sets, [ O is an open set. 2. For any. N. \ nite collection O1; : : : ; ON of open sets, Oi is an open set. i=1.