I am reading section 8.5.1 of this book (page 442 of the book, page 465 of the pdf). The author claims the discontinuous jump in the occupation number is of size Z, where [itex]Z = |\langle 0,N+1|a^\dagger |0,N\rangle|^2 = |\langle 0,N-1|a |0,N\rangle|^2[/itex] I.e. Z is the probability that the system will be in an N+1 (N-1) ground state after an electron is added (removed). They say this follows from equation 8.123 but I don't find it obvious how. Is there an easy logical path from equation 8.123 to the above relation?