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
points.
Topological Closure
See also
Closed Set, Sequence, Set Closure, TopologyThis entry contributed by Todd Rowland
Explore with Wolfram|Alpha
Cite this as:
Rowland, Todd. "Topological Closure." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/TopologicalClosure.html