PONNO A.,GALGANI L., GUERRA F. 
An analytical estimate of stochasticity thresholds in Fermi-Pasta-Ulam 
and $\phi^4$ models.
(327K, PostScript)

ABSTRACT.  We consider an infinitely extended FPU model, and we show that the slow 
modulating amplitude of a narrow wave packet asymptotically satisfies 
the Nonlinear Schrodinger equation (NLS). It is well known that NLS 
presents a threshold below which the packet width remains narrow. 
We give an analytical estimate of such a threshold; we also make a 
comparison with the numerical results known to us, and show they are 
in remarkable agreement with our estimate. Analogous results are found 
for the $\phi^4$ model.