  Hamiltonian systems with a mixed phase space typically exhibit an algebraic
decay of correlations and of Poincare' recurrences, with numerical experiments
over finite times showing system-dependent power-law exponents. We conjecture
the existence of a universal asymptotic decay based on results for a Markov
tree model with random scaling factors for the transition probabilities.
Numerical simulations for different Hamiltonian systems support this conjecture
and permit the determination of the universal exponent.
