Periodic observations of 3C 84, 3C 273, 3C 279, and 3C 454.3 were made at 9.55-mm wavelength from the middle of 1966 to the beginning of 1968. Variations of the flux density indicate that for 3C 84, 3C 273, and 3C 454.3 repeated injections of electrons into the source region take place, follo*ed by an expansion of the electron cloud
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Hobbs et al. (1969) studied this question.