Massimiliano Berti, Luca Biasco and Philippe Bolle
Drift in phase space: a new variational mechanism with optimal diffusion time
(763K, PS)

ABSTRACT.  We consider non-isochronous, nearly integrable,   
a-priori unstable Hamiltonian systems
with a (trigonometric polynomial) $O(\mu)$-perturbation 
which does not preserve the unperturbed tori. 
We prove the existence of Arnold diffusion with diffusion time 
$ T_d = O((1/ \mu) \log (1/ \mu ))$ by a variational method  
which does not require the existence
of ``transition chains of tori'' provided by KAM theory.
We also prove that our estimate of the diffusion time $T_d $  
is optimal as a consequence of a general stability result
derived from classical perturbation theory.