When a classical particle is weakly coupled to a radiation field, so that the radiative damping can be neglected, it will emit radiation in a pattern that repeats itself every orbital period. The frequencies that make up the outgoing wave are then integer multiples of the orbital frequency, and this is a reflection of the fact that X (t) is periodic, so that its Fourier representation has frequencies 2π n / T only. X ( t ) = ∑ ∑ n = − − ∞ ∞ ∞ ∞ e 2 π π i n t / T X n . The coefficients X n are complex numbers. The ones with negative frequencies must be the complex conjugates of the ones with positive frequencies, so that X (t) will always be real, X n = X − − n ∗ ∗ .