Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ii/paper-2/11f/c/solution

For rational and positive integer , define
Evaluation of the th derivative at is continuous in the smooth-function metric, so is open.
It is also dense. Given and a metric tolerance , choose so that the contribution of all derivatives of order at least is below , and choose . For large , perturb by
For every , , so . On the other hand,
For sufficiently large , , proving density.
The space is complete by part (b). The Baire category theorem therefore makes
dense. Put . Each complement is closed and nowhere dense, so is a meagre set, or a set of first category. Every has the required derivative growth. This is the generic superfactorial derivative growth at rational points.

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