Closure (English)
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Definition (undergraduate level)
The closure of A , written [A] is the intersection of all closed sets containing A ; a point b ∈ [A] iff every neighbourhood of b contains points of A . The closure of a set is closed. We say that a set X0 is dense in X if [(X0)]=X . This means that any point in X can be approximated arbitrarily closely by elements of X0 .
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