  In this paper we continue our study of polynomial diffeomorphisms of C^2. Let
us recall that there is an invariant measure $\mu$, which is the pluri-complex
version of the harmonic measure of the Julia set for polynomial maps of C. In
this paper we give an integral formula for the Lyapunov exponents of a
polynomial automorphism with respect to $\mu$ analogous to the Brolin-Manning
formula polynomial maps of C. Our formula relates the Lyapunov exponents to the
value of a Green function at a type of critical point which we define in this
paper. We show that these critical points have a very natural dynamical
interpretation.
