  The maximum Lyapunov exponent (referred to the mean half-period of phase
libration) of the motion in the chaotic layer of a nonlinear resonance subject
to symmetric periodic perturbation, in the limit of infinitely high frequency
of the perturbation, has been numerically estimated by two independent methods.
The newly derived value of this constant is 0.80, with precision presumably
better than 0.01.
