## Properties of the interaction elements

Since the interaction is invariant under the interchange of two particles it means for example that we have $$\langle \mu\nu|\hat{v}|\mu\nu\rangle = \langle \nu\mu|\hat{v}|\nu\mu\rangle,$$ or in the more general case $$\langle \mu\nu|\hat{v}|\sigma\tau\rangle = \langle \nu\mu|\hat{v}|\tau\sigma\rangle.$$