Consider a system that is driven by a source f ( t ) that has a power spectrum S ~ ~ ( ω ω ) . The latter is defined as the Fourier transform of ⟨ ⟨ f ( t ) f ( 0 ) ⟩ ⟩ . In linear response theory (LRT) the driving source induces a steady state which is only slightly different from the equilibrium state. In such circumstances the response ( G ) is a linear functional of the power spectrum: