On the Approximation in the Hermitian Treatment of Dyson Boson Expansion   Theory
  We discuss about the Hermitian treatment of Dyson-type boson expansion theory. We show that the basic assumption of the conventional treatment does not hold in general and the method is only approximately valid. We also show that the approximation is the same order as that of truncation of the expansion usually done in the Hermitian type boson expansion theory. 
Atsushi Kajiyama, Kimikazu Taniguchi and Yoshinao Miyanishi