1. The problem statement, all variables and given/known data Problem with the ordering of integrals in the derivation of the Lehmann-Kaller form of the exact propagator in Srednicki's book. We start with the definition of the exact propagator in terms of the 2-point correlation function and introduce the complete set of momentum eigenstates and then define a certain spectral density in terms of a delta function. But the spectral density is also a function of 'k', so we cannot take the spectral density outside the integral over 'k'. Since that is not possible, the subsequent manipulations fail too. 2. Relevant equations In Srednicki's book : Equation 13.11 and 13.12 If that is incorrect, the use of 13.15 to get 13.16 is not possible. 3. The attempt at a solution I don't see how it is possibe to derive the equation without that interchange. I'd appreciate any clarifications on this issue. Am I missing some trivial thing?