The function y(t) in this example is the signal equal to the periodic continuous-time integral of cos(x) such that
$ y(t) = \int_{-\infty}^{t} cos(x)\, dx = -sin(t) $
where its Fourier series coefficients are described by the equation
$ \left ( \frac{1}{jk\omega_0} \right )a_k = \left ( \frac{1}{jk \left (2\pi/T \right)} \right )a_k $
