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This review consists of three parts: Various properties of the quantized neutrino fields are summarized in part I from the viewpoint that a Dirac neutrino consists of two Majorana neutrinos with a degenerate mass but with opposite CP sings. It is shown why the Dirac neutrino has a freedom of the phase transformation to guarantee the lepton number conservation, while the Majorana neutrino does not. Part II is devoted to the theoretical investigation of the double beta decay and the analysis on the experimental data. Various corrections due to the vanishing terms in the closure approximation and the higher spherical waves of leptons are estimated. The constraints on the effective neutrino mass and the effective right-handed weak interaction parameters are derived from the present experimental data. In part III, the above bounds are compared with those from other processes like the 3H decay, the neutrino oscillation, the π-µ-e decay, etc. The possible mixing schemes of the neutrino system are explored.
Doi et al. (Tue,) studied this question.