https://peeterjoot.com/2018/10/22/phy2403h-quantum-field-theory-lecture-12-klein-gordon-greens-function-feynman-propagator-path-deformation-weightmann-function-retarded-greens-function-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 12: Klein-Gordon Green's function, Feynman propagator path deformation, Weightmann function, Retarded Green's function. Taught by Prof. Erich Poppitz