  We theoretically investigate the effect of various symmetries of the
superconducting order parameter Delta(omega) on the normalized tunneling
conductance of SIN junctions by directly solving the real-axis Eliashberg
equations (EEs) for a half-filled infinite band, with the simplifying
assumption mu*=0. We analyze six different symmetries of the order parameter:
s, d, s+id, s+d, extended s and anisotropic s, by assuming that the spectral
function alpha^{2}F(Omega) contains an isotropic part alpha^{2}F(Omega)_{is}
and an anisotropic one, alpha^{2}F(Omega)_{an}, such that
alpha^{2}F(Omega)_{an} = g alpha^{2}F(Omega)_{is}, where g is a constant.
  We compare the resulting conductance curves at T=2 K to those obtained by
analytical continuation of the imaginary-axis solution of the EEs, and we show
that the agreement is not equally good for all symmetries. Then, we discuss the
effect of non-magnetic impurities on the theoretical tunneling conductance
curves at T=4 K for all the symmetries considered.
  Finally, as an example, we apply our calculations to the case of
optimally-doped high-T_{c} superconductors (HTSC). Surprisingly, although the
possibility of explaining the very complex phenomenology of HTSC is probably
beyond the limits of the Eliashberg theory, the comparison of the theoretical
curves calculated at T=4 K with the experimental ones obtained in various
optimally-doped copper-oxides gives fairly good results.
