Physical Review LettersGeneral Theorem for Multiple-Photon ProcessesNorah V. Cohan, Hendrik F. HamekaPublished Mar 14, 1966• 10 citationsPublisherAbstractReceived 10 February 1966DOI:https://doi.org/10.1103/PhysRevLett.16.478©1966 American Physical SocietyDOI10.1103/physrevlett.16.478