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In 1952 Dyson put forward a simple and powerful argument indicating that theperturbative expansions of QED are asymptotic. His argument can be related toChandrasekhar's limit on the mass of a star for stability against gravitationalcollapse. Combining these two arguments we estimate the optimal number of termsof the QED series to be 3.1(137)^{3/2}=5000.

Optimal number of terms in QED series