  Friedel oscillations (FO) of electron density caused by a delta-like neutral
impurity in two-dimensional (2D) systems in a magnetic field are calculated.
Three 2D cases are considered: free electron gas, monolayer graphene and
group-VI dichalcogenides. An exact form of the renormalized Green's function is
used in the calculations, as obtained by a summation of the infinite Dyson
series and regularization procedure. Final results are valid for large ranges
of potential strengths $V_0$, electron densities $n_e$, magnetic fields $B$ and
distances from the impurity $r$. Realistic models for the impurities are used.
The first FO of induced density in WS$_2$ are described by the relation $\Delta
n(\vec{r}) \propto \sin(2\pi r/T_{FO})/r^2$, where $T_{FO} \propto
1/\sqrt{E_F}$. For weak impurity potentials, the amplitudes of FO are
proportional to $V_0$. For attractive potentials and high fields the total
electron density remains positive for all $r$. On the other hand, for low
fields, repulsive potentials and small $r$, the total electron density may
become negative, so that many-body effects should be taken into account.
