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I am a beginner in learning QFT. In homework problems in QED I often meet questions that asks one to calculate Feynman rules from some given lagrangian density. Some simple idea is to "read off" the original term. For instance, for a :

$L_I=-g\bar\psi \gamma^\mu\Gamma\psi A_\mu$

the vertex term is given as $-ig\gamma^\mu\Gamma$.

But for a more complex interaction, for instance between a scalar field and the $A_\mu$ field:

$L_I=C* \phi A_\mu A^\mu$

where C is a constant, how does one calculate the rules for, say, Feynman vertices?

Or, in general, given a certain $L$, may I ask what would be the general "train-of-thought" for deriving the Feynmann rules?

Thank you!

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