  We have recently studied in the delta-resonance--nucleon-hole model the
dependence of the pion spectral function in hot dense asymmetric nuclear matter
on the charge of the pion due to the pion p-wave interaction in nuclear medium.
In a thermal model, this isospin-dependent effect enhances the ratio of
negatively charged to positively charged pions in neutron-rich nuclear matter,
and the effect is comparable to that due to the uncertainties in the
theoretically predicted stiffness of nuclear symmetry energy at high densities.
This effect is, however, reversed if we also take into account the s-wave
interaction of the pion in nuclear medium as given by chiral perturbation
theory, resulting instead in a slightly reduced ratio of negatively charged to
positively charged pions. Relevance of our results to the determination of the
nuclear symmetry energy from the ratio of negatively to positively charged
pions produced in heavy ion collisions is discussed.
