A subset of a topological space is closed if its complement is an open subset, or equivalently if it contains all its limit points. When equipped with the subspace topology, we may call (or its inclusion ) a closed subspace. More abstractly, a subspace of a space is closed if the inclusion map is a closed map.
The collection of closed subsets of a space is closed under arbitrary intersections. If , then the intersection of all closed subsets containing is the smallest closed subset that contains , called the closure of , and variously denoted , , , , etc. It follows that implies and , so that forms a Moore closure operator on the power set .
Since closed subsets are closed with respect to finite unions, we have .
A topological closure operator is a Moore closure operator that preserves finite unions ( and ). It is easy to see that all such closure operators come from a topology whose closed sets are the fixed points of .
This mildly amusing curiosity asks how many set-theoretic operations on a topological space are derivable from closure and complementation and applying finite composition. The answer is that at most 14 operations are so derivable (and there are examples showing this number is achievable). As the proofs below indicate, this bare fact has little to do with topology; it has more to do with general Moore closures and how they interact with complements.
, i.e., . Applying the order-preserving operation to both sides together with the fact that , this gives
Since , we have also . Applying the order-reversing operation to both sides, we obtain
Combining the two displayed inequalities gives , and then multiplying this on the right by , the proposition follows.
Let be the monoid presented by two generators and subject to the relations , , and . Then , called the Kuratowski monoid, has at most 14 elements.
We may apply an obvious reduction algorithm on the set of words in two letters , in which a word is reduced by replacing any substring by and any substring by an empty substring, so that any word which cannot be further reduced must be alternating in . This leads to a list of 14 words
with any further alternating words reducible by replacing a substring by . Thus each element in the monoid is represented by one of these 14 words.
These 14 words actually name distinct set-theoretic operations for a judicious choice of space ; as a corollary, the Kuratowski monoid has exactly 14 elements. For instance (courtesy of Wikipedia), taking with its standard topology, the orbit of the element under the monoid action consists of 14 distinct elements.
At most 7 operations are possible with interior and closure, corresponding to the covariant Kuratowski operations. Thus there is a 7-element submonoid . Spaces for which the topological action is not injective are of some structural interest; for instance, the spaces for which are the extremally disconnected spaces, whereas spaces for which are those where the open sets are equivalence classes for some equivalence relation (partition spaces). Those for which are discrete spaces.
A more manifestly topological consideration is what happens when we throw joins (or meets) into the mix. Briefly, at most 13 subsets can be obtained by starting with a subset and generating new subsets by taking closures, interiors, and unions; the order structure of these 13 subsets coincides with the free cocompletion of the finite ordered monoid with respect to nonempty joins. Here we must use distributivity of over joins.
The idea is that this subspace is the part of which does not involve (hence the notation , or any other notation for a complement), and we may identify with when we are looking only away from .