In the theory of the Earth's normal modes of oscillation it is customary to use Rayleigh's principle in the determination of perturbations in the eigenperiods due to perturbations in the structure of an Earth model. When internal boundaries are perturbed the usual methods have been found to yield incorrect numerical results and this paper is directed towards a correct determination of the perturbations in this case.
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John Woodhouse (1976) studied this question.