  In Quantum Field Theory, scattering amplitudes are computed from propagators
which, for internal lines, are built upon spin/polarization-sum relationships.
In turn, these are normally constructed upon plane-wave solutions of the free
field equations. A question that may now arise is whether such
spin/polarization-sums can be generalized. In the past, there has been a first
attempt at generalizing spin sums for fermionic fields in terms of the
Michel-Wightman identities. In this paper, we aim to find the most general spin
sums for fermionic fields within the range of QFT.
