  Strong discontinuities in solutions of the gas dynamic equations under
isentropic conditions, i.e., with continuity of entropy at the discontinuity,
are examined. Solutions for a standard shock wave with continuity of energy at
the discontinuity are compared with those for an isentropic 'shock wave'. It is
shown that numerical simulation of astrophysical problems in which
high-amplitude shock waves are encountered (supernova explosions, modelling of
jets) with conservation of entropy, rather than of energy, leads to large
errors in the shock calculations. The isentropic equations of gas dynamics can
be used only when there are no strong discontinuities in the solution or when
the intensity of the shocks is not high and they do not significantly affect
the flow.
