Complex sentence: More usefully, and historically, the theorem applies to wide-sense-stationary random processes, signals whose Fourier transforms do not exist, using the definition of autocorrelation function in terms of expected value rather than an infinite integral.
Split into simpler sentences: More usefully, and historically, the theorem applies to wide-sense-stationary random processes, signals whose Fourier transforms do not exist. This uses the definition of autocorrelation function in terms of the expected value function rather than an infinite integral.