Abstract
The strength of ground state correlations calculated from the RPA and quasiboson approximation is known to differ by a factor of two from that, calculated by various other methods. This contradiction is investigated in detail in an extended Lipkin-Meshkov-Glick model. It is proved that the RPA + quasiboson approximation formula gives the correct ground state correlations if only collective states are included, as suggested by Rowe. The notion of a collective state is defined quantitatively.
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