Just to expand on this; the series is usually called a Dyson series (https://en.wikipedia.org/wiki/Dyson_series https://en.wikipedia.org/wiki/Dyson_series). Dys

Just to expand on this; the series is usually called a Dyson series (https://en.wikipedia.org/wiki/Dyson_series https://en.wikipedia.org/wiki/Dyson_series). Dyson showing the connection between perturbation theory and Feynman's formulation was very important.