Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/ib/paper-4/12e/b/iii/solution

The space is not compact because it is not complete. For , define the continuous function
These functions converge in the L1 norm to the step function , because is supported on an interval of length and bounded in absolute value by one. They are consequently Cauchy for the metric , since for sufficiently close pairs the outer minimum with does nothing.
If converged in this metric to some , it would also converge to in . Uniqueness of an limit would give almost everywhere. Continuity would then force on and on , which is impossible at . Thus this Cauchy sequence has no limit in , while every compact metric space is complete.

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