In addition to proposals of the acoustic-plasmon [1] and the exciton [2] mechanisms specific to multicomponent electronic systems as well as the mediation of magnetic excitations [3,4], Takada has suggested the plasmon mechanism of superconductivity for the first time in the one-component electron gas [5,6].  
  This suggestion has invoked world-wide intensive studies of this mechanism mainly because this is considered as an indispensable step to develop microscopic theories of superconducting mechanisms. Based on the studies including the vertex corrections in terms of either the EPX method [7] or the loca-field correction [8], Takada has reached the conclusion that the pairing occurs in the strongly-correlated electron liquid in the vicinity of the Wigner-lattice phase [9].  
  The plasmon mechanism can be validated, if the valley degeneracy is introduced to widen the applicability of RPA [10,11]. Specifically, superconductivity in the electron gas is reduced to that in the charged boson gas in the limit of infinite valley degeneracy.