The closure of a set
is the smallest closed set containing . Closed sets are closed under arbitrary
intersection, so it is also the intersection of all closed sets containing . Typically, it is just with all of its accumulation
The term "closure" is also used to refer to a "closed" version of a given set. The closure of a set can be defined in several
equivalent ways, including
1. The set plus its limit points, also called "boundary" points, the union of which is also called the "frontier."