  In the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck
transport model, sensitive regions of some nucleon observables to the nuclear
symmetry energy are studied. It is found that the symmetry energy sensitive
observable n/p ratio in the $^{132}$Sn+$^{124}$Sn reaction at 0.3 GeV/nucleon
in fact just probes the density-dependent symmetry energy below the density of
$1.5\rho_0$ and effectively probes the density-dependent symmetry energy around
or somewhat below the saturation density. Nucleon elliptic flow can probe the
symmetry energy from the low-density region to the high-density region when
changing the incident beam energies from 0.3 to 0.6 GeV/nucleon in the
semi-central $^{132}$Sn+$^{124}$Sn reaction. And nucleon transverse and
elliptic flows in the semi-central $^{197}$Au+$^{197}$Au reaction at 0.6
GeV/nucleon are more sensitive to the high-density behavior of the nuclear
symmetry energy. One thus concludes that nucleon observables in the heavy
reaction system and with higher incident beam energy are more suitable to be
used to probe the high-density behavior of the symmetry energy. The present
study may help one to get more specific information about the density-dependent
symmetry energy from nucleon flow observable in heavy-ion collisions at
intermediate energies.
