Dyson series
= Dyson series
{c}
{wiki}
The Dyson series expands the interaction-picture time-evolution operator as
$$
S=T\exp\!\left(-i\int d^4x\,\mathcal H_{\mathrm{int}}(x)\right)
=\sum_{n\geq0}\frac{(-i)^n}{n!}\int d^4x_1\cdots d^4x_n\,T\!\left\{\prod_{j=1}^n\mathcal H_{\mathrm{int}}(x_j)\right\}.
$$