Welcome! Some math code copy-pasted for reference. - Ramprax (talk) 11:42, 4 February 2020 (UTC)Reply

be the Dirichlet kernel.

This is clearly symmetric about zero, that is,


The second term vanishes as R goes to infinity. As for the first integral, one can use one version of the Sokhotski–Plemelj theorem for integrals over the real line: for a complex-valued function f defined and continuously differentiable on the real line and real constants and with one finds

where denotes the Cauchy principal value. Back to the above original calculation, one can write

Ramprax (talk) 11:42, 4 February 2020 (UTC)Reply