The electromagnetic field tensor is given by Fμν=∂μAν−∂νAμ, and it appears in the Lagrangian as L=−14F2μν−AμJμ. Schwartz's QFT textbook says that (chapter 8, page 116) F2μν=2(∂μAν)2−2(∂μAμ)2, but I do not see how that is correct.
The first step of the expansion gives F2μν=(∂μAν)2−∂μAν∂νAμ−∂νAμ∂μAν+(∂νAμ)2, and I see that the first and last term add up to produce 2(∂μAν)2. However, the second term ∂μAν∂νAμ when expanded has a term like ∂0A1∂1A0, and I don't see how this is present in (∂μAμ)2=(∂0A0+∂1A1+∂2A2+∂3A3)2.
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