  The characteristics of the resonant disturbing function for an asteroid
perturbed by a planet in circular orbit are discussed. The location of the
libration centers and their dependence with the orbital elements of the
resonant orbit are analyzed. A proposed numerical method (Gallardo 2006a) for
computing the strengths of the resonances is revised and applied to the region
of the main belt of asteroids showing the relevance of several mean motion
resonances (MMR) with several planets.
