  The largest Lyapunov exponent of an ergodic Hamiltonian system is the rate of
exponential growth of the norm of a typical vector in the tangent space. For an
N-particle Hamiltonian system, with a smooth Hamiltonian of the type p^2 +
v(q), the evolution of tangent vectors is governed by the Hessian matrix V of
the potential. Ergodicity implies that the Lyapunov exponent is independent of
initial conditions on the energy shell, which can then be chosen randomly
according to the microcanonical distribution. In this way a stochastic process
V(t) is defined, and the evolution equation for tangent vectors can now be seen
as a stochastic differential equation. An equation for the evolution of the
average squared norm of a tangent vector can be obtained using the standard
theory in which the average propagator is written as a cummulant expansion. We
show that if cummulants higher than the second one are discarded, the Lyapunov
exponent can be obtained by diagonalizing a small-dimension matrix, which, in
some cases, can be as small as 3x3. In all cases the matrix elements of the
propagator are expressed in terms of correlation functions of the stochastic
process. We discuss the connection between our approach and an alternative
theory, the so-called geometric method.
