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## Homework Statement

I have in the picture attached a screenshot from Peskin's textbook.

I don't understand how did they get that for the two last diagrams that ##D=-2##.

The question is from pages 316-317 of Peskin's textbook.

## Homework Equations

$$D=4-N_{\gamma}-3/2N_e$$

where ##N_e##=number of external electron lines;

##N_\gamma##=number of external photon lines.

## The Attempt at a Solution

It seems that for the two last diagrams, we have: ##N_{\gamma}=N_e=2##, which means that ##D=4-2-2*3/2=2-3=-1##, why is it that ##D=-2##?

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