Stationary Phase and the Theory of Measurement -- 1/N expansion --
  The measuring process is studied, where a macroscopic number N of particles in the detector interact with the object. The macrovariable accompanies the stationary phase in the path-integral form, which is in one-to-one correspondence with the eigen-value of the object operator O to be measured. When N goes to infinity, the fluctuation of the object between different eigenvalues of O is suppressed, frozen to one the same state while the detector is on. A model is studied which produces the ideal result when N is infinite, and the correction terms are calculated in powers of 1/N. It is identical to the expansion including the fluctuation of the object successively. 
R. Fukuda