(t)=E[ƒ*(x)ƒ(x+t )].

Equation (4) is the basis of a statistical theory of spectra.3 This definition only makes sense for processes that are at least wide-sense stationary. The autocorrelation function does not exist for processes that are not stationary, since its value would depend on x as well as t . The variable t is often called the lag, and f*(x)f(x+t ) called a lagged product.