ABSTRACT
There is a problem to associate the particle states in the presence of interactions with the ones we have derived from the noninteracting theory. The problem is that particles can have self-interactions long before and after the different particles have scattered off each other. Unitarity of an S-matrix is guaranteed as soon as the Hamiltonian is Hermitian. The Feynman rules in momentum space for computing the reduced matrix elements will obviously have to be modified for the external lines to a factor and an overall factor. In computing the reduced matrix elements, the symmetry factors are determined without allowing for permutations on the external lines.
