Covariant pion-nucleus optical potential

We present a detailed evaluation of the leading term (triangle diagram) of the pion-nucleus optical potential. The development uses a covariant multiple scattering theory introduced previously. It is shown how recent phenomenological studies of the pion-nucleon (off-shell) scattering amplitudes may be incorporated into this theory. Further, various problems associated with the transformation of the pion-nucleon scattering amplitudes from the center of mass of the $\pi -N$ system to the center of mass of the $\pi$-nucleus system are resolved in this approach.