THE DELTA-EXPANSION AND THE PRINCIPLE OF MINIMAL SENSITIVITY

The delta-expansion is a nonperturbative approach for field theoretic models which combines the techniques of perturbation theory and the va riational principle. Different ways of implementing the principle of m inimal sensitivity to the delta-expansion produce in general different results for observables. For illustration we use the Nambu-Jona-Lasin io model for chiral symmetry restoration at finite density and compare results with those obtained with the Hartree-Fock approximation.