Suppose that we want to use numerical methods to compute the fourier transform X(f) of an energy signal x(t) that lasts for T seconds (Figure 2.19). The first step in this computation is to form a periodic sampled x_(s_p ) (t) of x(t) as shown in figure 2.19. The periodic sampled version is obtained from samples of x(t) padded with zeros. Now, if the sampling rate f_x→∞ and T_0→∞, then the transform of x_(s_p ) (t)→x(f), except for a constant of proportionality.