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## Main Question or Discussion Point

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.

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.

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