Perturbation series in quantum field theory are usually divergent as was firstly indicated by Freeman Dyson. According to the Lipatov method, -th order contribution of perturbation theory into any quantity can be evaluated at large  in the saddle-point approximation for functional integrals and is determined by instanton configurations.
As Freeman Dyson first noted, perturbation series in quantum field theory are usually divergent. Using the Lipatov method, -th order contribution of perturbation theory to any quantity can be evaluated at large  through the saddle-point approximation for functional integrals and is determined by instanton configurations.