We discuss the evaluation of the stability, position and width of reso
nances in four-dimensional symplectic mappings. The approach presented
in this paper is based on the computation of the resonant perturbativ
e series carried out by the program ARES. This piece of code allows to
perform the evaluation of the perturbative series and of the interpol
ating Hamiltonian. Given a symplectic map in the neighbourhood of an e
lliptic fixed point, the new code NERO performs the analysis of the or
bits of the interpolating Hamiltonian both in the nonresonant and the
resonant case. For each resonant normal form, the interpolating Hamilt
onian is computed and the position and the stability of the resonant o
rbits and the width of the islands are evaluated; this analysis is car
ried out through the direct inspection of the coefficients of the inte
rpolating Hamiltonian. All the computations are carried out at an arbi
trary order; the first significant perturbative order is taken as a fi
rst guess, and a Newton method is used to evaluate the higher orders e
ffect. (C) 1998 IMACS/Elsevier Science B.V.