- #1

MathematicalPhysicist

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## Main Question or Discussion Point

I don't understand from Peskin when can I use Ward Identity?

I mean I can see that this identity isn't always valid to use, but when it is?

Take for example equation (16.10) page 508 of Peskin's and Schroeder's.

I mean I can see that this identity isn't always valid to use, but when it is?

Take for example equation (16.10) page 508 of Peskin's and Schroeder's.