  The covariant propagator of a fermion with intrinsic magnetic moment
interacting with a uniform external magnetic field is presented for finite
temperature and baryonic density. The case of a scalar boson is also
considered. The final expressions are given in terms of a four-dimensional
momentum representation. These results, which take account of the full effect
of the magnetic field, are used to evaluate the modification of the pion mass
at zero temperature as a function of the density and the magnetic intensity.
For this purpose a self-consistent calculation, including one- and two-pion
vertices, is employed.
