Cm. Bender et Ka. Milton, NONPERTURBATIVE CALCULATION OF SYMMETRY-BREAKING IN QUANTUM-FIELD THEORY, Physical review. D. Particles and fields, 55(6), 1997, pp. 3255-3259
A new version of the delta expansion is presented, which, unlike the c
onventional delta expansion, can be used to do nonperturbative calcula
tions in a self-interacting scalar quantum field theory having broken
symmetry. We calculate the expectation value of the scalar field to fi
rst order in delta, where delta is a measure of the degree of nonlinea
rity in the interaction term.