On multipole expansions in the theory of
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C.J. Bouwkamp; H.B.G. Casimir
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Article
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1954
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Elsevier Science
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in which Q is the charge density, connected with the current density by the equation of continuity c -1 div i -----ik~. (2) As is well known, eqs (1) admit one and only one solution that is compatible with Sommerfeld's radiation condition at infinity. This unique solution is conveniently expressed