Proof of Furry's theorem

Question

i was wondering if anyone could give an explicit calculation or show a link that shows the proof to Furry's theorem. showing how the vacuum expectation value of any odd number of electromagnetic currents vanishes. in particular the case for $n=3$.

Answer 1

This comes from the fact that the electromagnetic current changes sign under charge conjugation i.e $Cj^{\mu\nu}C^{\dagger} = -j^{\mu\nu}$ and thus the vaccum expectation of all diagrams withodd number of em legs vanishes