Relative errors of potential and forces for the HO (red lines) and CylSP (blue lines) methods for snapshots of an N-body realization of a Ferrers bar with a particle number N_p = 10⁶. From left to right: the results for the HO with (n_max, l_max) = (16,12), (20,20), (30,30) and (40,40), respectively. For the CylSP method, R_min = 0.1, N_R = 40, N_z = 40, z_min = 0.1, R_max = z_max = 100.