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Dyson series : ウィキペディア英語版
Dyson series
In scattering theory, a part of mathematical physics, the Dyson series, formulated by Freeman Dyson, is a perturbative series, and each term is represented by Feynman diagrams. This series diverges asymptotically, but in quantum electrodynamics (QED) at the second order the difference from experimental data is in the order of 10−10. This close agreement holds because the coupling constant (also known as the fine structure constant) of QED is much less than 1. Notice that in this article Planck units are used, so that ''ħ'' = 1 (where ''ħ'' is the reduced Planck constant).
== The Dyson operator ==

Suppose that we have a Hamiltonian , which we split into a ''free'' part 0 and an ''interacting part'' , i.e. ''H'' = ''H''0 + ''V''.
We will work in the interaction picture here and assume units such that the reduced Planck constant is 1.
In the interaction picture, the evolution operator defined by the equation
:\Psi(t) = U(t,t_0) \Psi(t_0)
is called the Dyson operator.
We have
:U(t,t) = I,
:U(t,t_0) = U(t,t_1) U(t_1,t_0),
:U^(t,t_0) = U(t_0,t),
and hence the Tomonaga–Schwinger equation,
:i\frac d U(t,t_0)\Psi(t_0) = V(t) U(t,t_0)\Psi(t_0).
Consequently,
:U(t,t_0)=1 - i \int_^t.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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