Spectral distortion

Fourier Analysis of following signals reveals distortion of spectral peak due to non stationarity

$$\begin{eqnarray*} \hbox to 2mm { Stat: \hss} && s_1(n) = A\cos(0.2n) \\ \hbox t... ...s}&& s_3(n) = (A-\Delta_An)\cos((0.2+0.0002(n-512))(n-512))) \\ \end{eqnarray*}$$