Solution

ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2016/paper-1/4/b/solution

A direct way to avoid the singular transform of is to invert a triangular function. Evenness gives
At the limit is , so there is no singularity in the original signal. Thus
Alternatively, the hint uses the Hadamard finite-part reciprocal-power distribution, whose distributional Fourier transform is ; combining its three translates gives , the same triangle. Treating as an ordinary integrable function would be invalid.
The message spectrum has height at zero, vanishing at . For , the modulated continuous spectrum consists of disjoint triangles of height , centred at , each with half-width one. In addition there is a Dirac delta carrier line of weight at each centre. These lines have specified area, not a finite height; arrows mark them schematically.
Figure 1.
Triangular message spectrum, separated sidebands, and overlapping sidebands with the prescribed carrier-rejecting filter
. Details.

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