General analysis of polarization phenomena in e^++e^-to N+bar N for axial parametrization of two--photon exchange

General properties of the differential cross section and of the polarization observables are derived for the $e^++e^-\to N+\bar N$ reaction, in presence of two--photon exchange. Polarization effects are investigated for longitudinally polarized electron and polarized antinucleon and/or polarized nucleon in the final state. The single--spin asymmetry induced by the transverse polarization of the electron beam is also discussed.