I am currently studying QFT with 'An Introduction to Quantum Field Theory' by peskin. In part 2 (renormalization) of the book, he introduces counterterms and shows how to compute scattering amplitude with them. Below are counterterms of [itex]\phi^4[/itex] theory: Then he calculates a 2-2 scattering process to the second order: Here I have a few questions: 1. Why the fifth diagram in the second line of the second picture is of the same order to the previous three diagrams? 2. Now that [itex]\delta_m,\delta_Z,\delta_\lambda[/itex] are infinite numbers, how can they use perturbation theory and what is the meaning of order in renormalization perturbation theory? Edit: should this thread be moved to Homework section? It just cannot be fit into the question structure there.