- for a deterministic function, correlation function of the function and a time-delayed replica
- for a stationary random function, mathematical expectation of the product of the function and a time-delayed replica:
NOTE 1 The autocorrelation function of a deterministic function or a stationary random function is the inverse Fourier transform of its power spectral density. NOTE 2 When a stationary random function can be considered as ergodic, its autocorrelation function can be calculated from a particular sample:
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