In point-set topology, Kuratowski's closure-complement problem asks for the largest number of distinct sets obtainable by repeatedly applying the set operations of closure and complement to a given starting subset of a topological space. The answer is 14. This result was first published by Kazimierz Kuratowski in 1922. The problem gained wide exposure three decades later as an exercise in John L. Kelley's classic textbook General Topology.
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Proof
Letting S denote an arbitrary subset of a topological space, write kS for the closure of S, and cS for the complement of S. The following three identities imply that no more than 14 distinct sets are obtainable:
(1) kkS = kS. (The closure operation is idempotent.)
(2) ccS = S. (The complement operation is an involution.)
(3) kckckckcS = kckcS.(Or equivalently kckckckS=kckckckccS=kckS. Using identity (2).)
The first two are trivial. The third follows from the identity kikiS = kiS where iS is the interior of S which is equal to the complement of the closure of the complement of S, iS = ckcS. (The operation ki = kckc is idempotent.)
A subset realizing the maximum of 14 is called a 14-set. The space of real numbers under the usual topology contains 14-sets. Here is one example:
where
Further results
Despite its origin within the context of a topological space, Kuratowski's closure-complement problem is actually more algebraic than topological. A surprising abundance of closely related problems and results have appeared since 1960, many of which have little or nothing to do with point-set topology.