Théorème de fermeture/complémentaire de Kuratowski
theorem that the monoid generated by taking closures and complements of a given set in a topological space has size at most 14, attained by e.g. the set (0, 1) ∪ (1, 2) ∪ {3} ∪ ([4, 5] ∩ ℚ) of reals