  Within the framework of Dyson-Schwinger equations (DSEs), we discuss the
equation of state (EOS) and quark number densities of 2+1 flavors, that is to
say, $u$, $d$, and $s$ quarks. The chemical equilibrium and electric charge
neutrality conditions are used to constrain the chemical potential of different
quarks. The EOS in the cases of 2 flavors and 2+1 flavors are discussed, and
the quark number densities, the pressure, and energy density per baryon are
also studied. The results show that there is a critical chemical potential for
each flavor of quark, at which the quark number density turns to nonzero from
0; and furthermore, the system with 2+1 flavors of quarks is more stable than
that with 2 flavors in the system. These discussion may provide some useful
information to some research fields, such as the studies related to the QCD
phase transitions or compact stars.
