Strange attractors

In this lecture, I have briefly discussed a few selected points in the theory of stochastic motion with a few degrees of freedom. I have made no reference to the very important subject of systems with a quasiperiodic behavior. Many fundamental results have been recently obtained in this field. The bifurcation theory of the diffeomorphisms of the circle (which can be seen as the P. map for a flow on T2) culminate in the thesis of M. Hermann. One of the most impressive result of Hermann is the fact that a property that is not structurally stable, as the unlocking of frequencies for a non linear dissipative forced oscillator may be nevertheless 'physically' relevant, since it occurs for a (Cantor) set of values of the parameters with a non zero Lebesgue measure