The fluctuation–dissipation theorem relates the two-sided power spectrum (i.e. both positive and negative frequencies) of x to the imaginary part of the Fourier transform χ χ ^ ^ ( ω ω ) of the susceptibility χ χ ( t ) : S x ( ω ω ) = − − 2 k B T ω ω Im ⁡ ⁡ χ χ ^ ^ ( ω ω ) .