BILLIARDS CORRELATION-FUNCTIONS

We discuss various experiments on the time decay of velocity autocorre lation functions in billiards. We perform new experiments and find res ults which are compatible with an exponential mixing hypothesis first put forward by Friedman and Martin (FM): they do not seem compatible w ith the stretched exponentials believed, in spite of FM and more recen tly of Chernov, to describe the mixing. The analysis leads to several byproducts: we obtain information about the normal diffusive nature of the motion and we consider the probability distribution of the number of collisions in time t(m) (as t(m) --> infinity finding a strong dep endence on some geometric characteristics of the locus of the billiard obstacles.