  Based on an isospin- and momentum-dependent hadronic transport model IBUU04,
effects of the nuclear symmetry energy on the single and double
pi^{-}/pi^{+}ratios in central reactions of ^{132}Sn+^{124}Sn and
^{112}Sn+^{112}Sn at a beam energy of 400 MeV/nucleon are studied. It is found
that around the Coulomb peak of the single pi^{-}/pi^{+} ratio the double
pi^{-}/pi^{+} ratio taken from the two isotopic reactions retains about the
same sensitivity to the density dependence of nuclear symmetry energy. Because
the double pi^{-}/pi^{+}ratio can reduce significantly the systematic errors,
it is thus a more effective probe for the high-density behavior of the nuclear
symmetry energy.
