“…In the effective and quasi-crystalline approximations, it has been shown that in the low frequency limit, i.e., p → 0, we have M pL (p) = M pT (p) = M p , so that the mass operator is of the form M p (p ) = M p I [11] . Consequently, k 2 pT (0) = k 2 pL (0) , and the second integral in the expression for G (R ) vanishes. From Eq.…”