1. The problem statement, all variables and given/known data Basically I wanted to see if anyone would be willing to give me the solution to the 4th problem of the Weinberg textbook on quantum field theory. The exact question in the book is "Derive the perturbation expansion (3.5.8) directly from the expansion (3.5.3) of old-fashioned perturbation theory." 2. Relevant equations The relevant equations include the fourier representation of energy, the dyson series for the s-matrix, etc. 3. The attempt at a solution So I'm new to physicsforums so I have no idea how to do physics equations on the computer using what I think is called latex. Also, my work spans quite a few pages but I don't think I've gotten anywhere that would be all that useful in solving the problem. Thank you in advance to those that are willing to help!