 Solution

ID: past-exam-of-the-probability-statistics-and-options-i-course-of-the-university-of-oxford/2010/e/1/a/iii/solution

\overline{A}

is the intersection of all closed subsets of

X

containing

A

. It is the smallest such closed subset. Equivalently,

x \in \overline{A}

exactly when every open neighborhood of

x

meets

A

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