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.
Perturbation series in quantum field theory often exhibit divergence, as Freeman Dyson observed. According to the Lipatov method, -th order contribution of perturbation theory into any quantity can be approximated at large  using the saddle-point approximation for functional integrals and is influenced by instanton configurations.