\[\left\langle f_{\omega} f_{\omega^{\prime}}^{*}\right\rangle=\frac{1}{(2 \pi)^{2}} \int_{-\infty}^{+\infty} d t \int_{-\infty}^{+\infty} d \tau K_{f}(\tau) e^{i\left(\omega-\omega^{\prime}\right) t}...⟨fωf∗ω′⟩=1(2π)2∫+∞−∞dt∫+∞−∞dτKf(τ)ei(ω−ω′)teiω′τ≡1(2π)2∫+∞−∞Kf(τ)eiω′τdτ∫+∞−∞ei(ω−ω′)tdt.
