The utility of this frequency domain function is rooted in the Poisson summation formula. Let X(f) be the Fourier transform of any function, x(t), whose samples at some interval, T (seconds), are equal (or proportional to) the x[n] sequence, i.e. Tcdot x(nT)=x[n]. Then the periodic function represented by the Fourier series is a periodic summation of X(f). In terms of frequency scriptstyle f in hertz (cycles/sec):