Abstract
The momentum distribution ϱ(k) of a normal Fermi liquid exhibits a discontinuity at the Fermi momentum kF. According to the Migdal-Luttinger theorem, the magnitude of this discontinuity is equal to the quasiparticle strength at kF. These properties together with particle number conservation are used as criteria to assess the validity of the following theoretical methods for calculating the momentum distribution. (a) The “derivative expansion” which is a new approximation scheme that we have developed. (b) The “spectral function approach” in which ϱ(k) is expressed as an integral over the energy of the spectral function. (c) The “linked cluster expansion” in which ϱ(k) is expanded in powers of the strength of the two-body interaction.
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