In this paper https://arxiv.org/abs/math-ph/9901018 the author (on page 4) mentions : "...a standard argument for the absolute convergence of the Dyson series for [the evolution operator] $\hat{U}_{\tau}$ ..." .
Could you please give me a hint of what is a standard way of proving the absolute convergence of the Dyson series for the evolution operator? References are welcome.
(I am not a mathematician, but a physicist)