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Given a topologically valid Feynman diagram, can one always sum over all momenta without regard to the sign of the $k_0$ on internal edges ? In other words is it true that the requirement that only positive energy states propagate into the future does not reduce the connectivity of the diagrams ? I already posed this question specifically in respect of the self-energy of a fermion, but it admits of a more general formulation like the one here.

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