ABSTRACT
UntilnowwehavelimitedourselvestoaformalstudyofGreen's functions:wehavenomethodforcalculatingthem.Wearegoingtofill thisgapbydevelopinga"perturbation"technique,basedonanexpansioninpowersoftheinteractionbetweenparticles.Thisisaverydifficult proceduretojustify,astheconvergenceoftheseriesobtainedisindoubt. Asamatteroffact,weareincapableofcalculatingallthetermsinthe series.Thus,wecannotdrawanyconclusionsconcerningitsconvergence, andwearethereforereducedtoassumingthemathematicalvalidityof theprocedureused.
