May 1, 2014
In topology, Kuratowski's closure-complement problem asks for the largest number of distinct sets which can be obtained by repeatedly applying the closure and complement operator to some subset of a topological space.
The answer, first published by Kazimierz Kuratowski in 1922, is 14.
This applet lets you play with subsets of the reals. The goal of the page is to help you understand why 14 is the maximum obtainable, and to let you create an example of a set attaining this maximum.