 Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2012/e/e2/a/solution

B

is a family of open subsets such that every open set is a union of members of

B

. Equivalently, for every

U \in T

and

x \in U

, some

B \in B

satisfies

x \in B \subseteq U .

The empty open set is represented by the empty union.

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