and if we additionally assume the spherical symmetry of the E( ⃗ k) = E(k) function, then we obtain (31) and for large enough distances r the sin(kr) oscillates so fast that the only contribution comes from the neighborhood of k(ω) (such k that the equation ω -E(k) = 0 =⇒ k(ω) is satisfied for a particular ω), henceforth we may apply the Sokhotski-Plemelj theorem 124