An experiment conducted over time T necessarily produces a windowed view of the phenomenon generating the data. It is a useful strategy to regard the windowed data as one period of a periodic function. Then one may write

and seek to calculate the Fourier coefficients c_n from the integral expression

Large values of c_n will be regarded as a quantitative indication of a genuine periodic change in the phenomenon under study. In digital form the relevant equations are

The time interval [0,T] has been marked out in equal steps T/N, and frequency f_n = n/T has been replaced by the dimensionless quantity n/N.