  We present a numerical study of the application of the Shannon entropy
technique to the planar restricted three-body problem in the vicinity of
first-order interior mean-motion resonances with the perturber. We estimate the
diffusion coefficient for a series of initial conditions and compare the
results with calculations obtained from the time evolution of the variance in
the semimajor-axis and eccentricity plane. Adopting adequate normalization
factors, both methods yield comparable results, although much shorter
integration times are required for entropy calculations. A second advantage of
the use of entropy is that it is possible to obtain reliable results even
without the use of ensembles or analysis restricted to surfaces of section or
representative planes. This allows for a much more numerically efficient tool
that may be incorporated into a working N-body code and applied to numerous
dynamical problems in planetary dynamics. Finally, we estimate instability
times for a series of initial conditions in the 2/1 and 3/2 mean-motion
resonances and compare them with times of escape obtained from directed N-body
simulations. We find very good agreement in all cases, not only with respect to
average values but also in their dispersion for near-by trajectories
