  Electrons interact strongly with their environment. The result of these
interactions is, most of the time, encoded in an effective mass. In
non-relativistic systems, as in condensed matter, the electrons plus
interactions form a quasiparticle with an effective mass. From the side of
relativistic systems, the fermions also acquire an effective mass due to the
interactions with the surrounding medium. We employ a non-perturbative method
to calculate the effective mass of relativistic and non-relativistic fermions,
in various situations. We find the effective masses up to second order of the
iteration method. The results can be of interest in current studies on fermion
systems.
