ABSTRACT
Intheprecedingchapter,wehaveconsideredonlyasingleelementaryexcitation,quasi-particleorquasi-hole.Wenowextendthefieldof thisstudybyconsideringtheinteractionbetweentwoelementaryexcitations.Itisnolongerthesingle-particleGreen'sfunctionGwhichinterests us,butthetwo-particlefunctionK,definedby(3-39).Weimmediately discardthefreepartofK,whichyieldsnonewinformation,andweconsideronlytheinteractionoperatorr(pi),givenby(3-50).Weshalluse indiscriminatelythecompleteinteractionoperator1definedby(3-43)or (3-49)andthereducedoperatorr,withtheafunctionsremoved.The choiceofoneortheotherofthesequantitiesisjustaquestionofconvenience.
