  Through molecular dynamics, we study the $d=2,3$ classical model of $N$
coupled rotators (inertial XY model) assuming a coupling constant which decays
with distance as $r_{ij}^{-\alpha}$ ($\alpha \ge 0$). The total energy $<H>$ is
asymptotically $\propto N {\tilde N}$ with ${\tilde N} \equiv
[N^{1-\alpha/d}-(\alpha/d)]/[1-\alpha/d]$, hence the model is thermodynamically
extensive if $\alpha/d>1$ and nonextensive otherwise. We numerically show that,
for energies above some threshold, the (appropriately scaled) maximum Lyapunov
exponent is $\propto N^{-\kappa}$ where $\kappa$ is an {\it universal} (one and
the same for $d=1,2$ and 3, and all energies) function of $\alpha/d$, which
monotonically decreases from 1/3 to zero when $\alpha/d$ increases from 0 to 1,
and identically vanishes above 1. These features are consistent with the
nonextensive statistical mechanics scenario, where thermodynamic extensivity is
associated with {\it exponential} mixing in phase space, whereas {\it weaker}
(possibly {\it power-law} in the present case) mixing emerges at the $N \to
\infty$ limit whenever nonextensivity is observed.
