  We show that Green function methods can be straightforwardly applied to
nonlinear equations appearing as the leading order of a short time expansion.
Higher order corrections can be then computed giving a satisfactory agreement
with numerical results. The relevance of these results relies on the
possibility of fully exploiting a gradient expansion in both classical and
quantum field theory granting the existence of a strong coupling expansion.
Having a Green function in this regime in quantum field theory amounts to
obtain the corresponding spectrum of the theory.
