Recently i am reading A.Zee's Quantum Field Theory in a Nutshell 2nd Edition. there is a equation that i can not derive by myself. I suspect its correctness. <k1k2|exp(-iHT)|k3k4>=<k1k2|exp(i∫dxL(x))|k3k4>, where the L(x) on the RHS is an operator function of space-time. This equation appears on page 64, chapter I.8, under the section 'Scattering Amplitude'. Zee used this equation to calculate the corresponding scattering amplitude in λψ^4 theory and thus justified the Feynman Rules given in previous chapters, so i think this equation is of crucial importance in Zee's book. Can someone tell me how to derive it? Thanks!