The above equation uses the fact that the spherical Bessel functions have closed analytic forms for l = 0 represented as j0(kr) = sin(kr)/(kr) and y0(kr) = -cos kr/(kr). In order to calculate the total impurity-induced electron density change at position ⃗ r from the impurity, one has to account for all possible states within the energy region E ∈ [0, EF ] (at the temperature T = 0 up to the Fermi energy EF , and it starts from 0 due to the lack of other external potentials), thus arriving at equation (13), where kF is the Fermi wavevector roughly describing the radius of the Fermi ball in the reciprocal space and it is related to the Fermi energy via the following relation