Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2020/ii/paper-4/40e/a/solution

Termwise differentiation of a Fourier series multiplies a mode by its wavenumber, so
The convolution theorem turns pointwise multiplication into discrete convolution:
so
The assumed real analyticity justifies these termwise operations through rapid decay of the Fourier coefficients.

New to topics? Read the docs here!