ABSTRACT

Up to now we have written approximation for G by relying on the propagator interpretations of G and of the G 2 that appears in the equations of motion for G. We have thus been able to write a few simple approximations for G 2 in terms of the processes that we wish to consider. However, $ {\text{However,}} $ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315196596/a397f31e-8107-49bb-8694-b978d38168d6/content/inline-math6_1.tif"/> physical intuition can take us just so far. The use of purely imaginary times makes a direct interpretation of these equations difficult. Furthermore, it is hard to find physical ways of determining the numerical factors that appear in front of the various terms in the expansion of G 2. We, therefore, seek a systematic way of deriving approximations for G.