  We analyze numerically a two-dimensional $\lambda\phi^4$ theory showing that
in the limit of a strong coupling $\lambda\to\infty$ just the homogeneous
solutions for time evolution are relevant in agreement with the duality
principle in perturbation theory as presented in [M.Frasca, Phys. Rev. A {\bf
58}, 3439 (1998)], being negligible the contribution of the spatial varying
parts of the dynamical equations. A consequence is that the Green function
method works for this non-linear problem in the large coupling limit as in a
linear theory. A numerical proof is given for this. With these results at hand,
we built a strongly coupled quantum field theory for a $\lambda\phi^4$
interacting field computing the first order correction to the generating
functional. Mass spectrum of the theory is obtained turning out to be that of a
harmonic oscillator with no dependence on the dimensionality of spacetime. The
agreement with the Lehmann-K\"allen representation of the perturbation series
is then shown at the first order.
