An incident plane wave e 0 E 0 exp[i(k 0 r t)] scatters from a target with amplitude f(k). One can prove that

An incident plane wave ê₀ E₀ exp[i(k₀ · r − ωt)] scatters from a target with amplitude f(k). One can prove that f(k₀) · ê^∗₀/k² is a causal response function of the sort discussed in Section 18.7. Use this information to prove the wavelength sum rule,

lim Re [f(x, ko). ] = 0 1 7 St ' (2').