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In scattering theory, a part of mathematical physics, the Dyson series, formulated by Freeman Dyson, is a perturbative expansion of the time evolution operator in the interaction picture. Each term can be represented by a sum of Feynman diagrams.
This series diverges asymptotically, but in quantum electrodynamics at the second order the difference from experimental data is in the order of 10. This close agreement holds because the coupling constant of QED is much less than 1.
Notice that in this article Planck units are used, so that ħ = 1.
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