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Notes: http://www.damtp.cam.ac.uk/user/tong/qft/three.pdf
On page 59, in deriving 3.48, from the first step to the second step, why can we just insert |0><0| like that?
On page 59, in deriving 3.48, from the first step to the second step, why can we just insert |0><0| like that?