 Solution

ID: past-exam-of-the-probability-statistics-and-options-ii-course-of-the-university-of-oxford/2009/e/e3/b/iv/alpha/solution

a \in / A

, every point of

A

is isolated by (ii). The set

A

is the union of these open singletons, so

A is open in (X, d_{a})

, including the empty case.

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