[QUE/QFT-14001]

   Prove that
  \(\begin{equation}
    \int_{t_0}^ t \, dt_1 \int_{t_0}^ {t_1} dt_2\, H^\prime_I(t_1)
      H^\prime_I(t_2) = \frac{1}{2}      \int_{t_0}^ t \, dt_1 \int_{t_0}^ t \,
       dt_2  T\big( H^\prime_I(t_1) H^\prime_I(t_2)\big)
       \end{equation}\)