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$  S=\sum_{n=0}^{\infty}\frac{(ie)^{n}}{n!}\int{...}\int{d^{4}x_{1}...{d^{4}x_{n}T\huge\lbrace{N[\bar{\psi}(\not{A} + \not{A_{e}})\psi]_{x_{1}} ...{ N[\bar{\psi}( \not{A} +\not{A _{e}})\psi]_{x_{n}}\rbrace $

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