1. function ${\displaystyle f(t)}$ which is a measure of the similarity of two deterministic functions ${\displaystyle f_1(t)}$ and ${\displaystyle f_2(t)}$, defined by

   ${\displaystyle f(t)={\displaystyle {\int }_{-\infty }^{+\infty }{f}_1}(\tau )f_2(t+\tau )\mathrm{d}\tau }$

2. function ${\displaystyle f(t)}$ which is a measure of the similarity of two stationary random functions ${\displaystyle f_1(t)}$ and ${\displaystyle f_2(t)}$, defined by

   ${\displaystyle f(t)=\underset{T\to \infty }{\text{lim}}\frac{1}{2T}{\displaystyle {\int }_{-T}^{+T}{f}_1(\tau )}{f}_2(t+\tau )\mathrm{d}\tau }$

Note 1 to entry: The Fourier transform of ${\displaystyle f(t)}$ is equal to the product of the conjugate of the Fourier transform of ${\displaystyle f_1(t)}$ and the Fourier transform of ${\displaystyle f_2(t)}$:

${\displaystyle F(\omega )=F_1^{\ast }\left(\omega \right)F_2(\omega )}$