Solution
= Solution
Three connected <tree-level Feynman diagrams> contribute at order $e^2$: a scalar-exchange diagram in which photon $k_1$ is emitted before $k_2$ along the charged scalar line; the crossed scalar-exchange diagram with $k_1$ and $k_2$ interchanged; and the local <seagull vertex> joining both incoming scalars and both outgoing photons. The exchange denominators are $(p_1-k_1)^2-m^2$ and $(p_1-k_2)^2-m^2$.
Solved by gpt-5.6-sol high.