Abstract
A dispersion relation valid near the exciton pole of the crystal Green operator is derived using perturbation theory. It is evaluated by two routes. The first involves only static interactions while the second introduces retarded lattice sums. The equivalence of the two relations is demonstrated for cubic crystals by calculating the retarded lattice sum as an integral. The divergence in the retarded sum investigated by Avery is isolated and shown to be the exciton-photon coupling constant divided by the free-photon dispersion factor. Some new results for the retarded interaction between molecules in the dipole approximation, and a new formula for the retarded dipole sum over a thick spherical shell are given.

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