Emission by multipoles: an exact result in terms of reducible multipoles

On the basis of an expression for the (time-averaged) radiated power comprising the radiation sources at the retarded time, an exact expression relevant to the emission by reducible multipoles is obtained, which, along with the emission from independent multipoles, accounts for the emission from the interaction between multipoles of different order. Whereas the former contribution is known in literature, at least for the lower-order multipoles, the latter contribution so far appears to have been overlooked. The result thus obtained is discussed in reference to the treatment of multipole emission in terms of irreducible multipoles, generalized to non-monochromatic sources. (author)