GENERALIZATION OF FRÖHLICH'S OPERATOR FOR THE ELECTRON-PHONON COUPLING IN POLAR CRYSTALS U. Schröder, M. Weiss, R. Reiger et J.B. Page J. Phys. Colloques, 42 C6 (1981) C6-496-C6-498 DOI: 10.1051/jphyscol:19816145