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N^{*}→N+photon

If we want to calculate the amplitude with the following interaction Lagrangian:

(http://arxiv.org/abs/nucl-th/0205052)

If we use functional method,the field operator is not polynomial,how to use "center formula"to bring functional derivative in? Or we must drop functional method,and use expension of field operator into annihilation and creation operator to get amplitude ?

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# From Effective Lagrangian to Feynman Rule

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