  The polarization operator (tensor) for planar charged fermions in constant
uniform magnetic field is calculated in the one-loop approximation of the 2+1
dimensional quantum electrodynamics (QED$_{2+1}$) with a nonzero fermion
density. We construct the Green function of the Dirac equation with a constant
uniform external magnetic field in the QED$_{2+1}$ at the finite chemical
potential, find the imaginary part of this Green function and then obtain the
polarization tensor related to the combined contribution from real particles
occupying the finite number of energy levels and magnetic field. We expect that
some physical effects under consideration seem to be likely to be revealed in a
monolayer graphene sample in the presence of external constant uniform magnetic
field $B$ perpendicular to it.
