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\begin{equation*} 
U_\gamma ={\bf T} e^ {-i \int_0^T \cal{U}(\lambda)\,H_0 \, \cal{U}^\dagger(\lambda)
  dt}= {\bf T} \!\!
\lim_{N\rightarrow \infty} e^ {-i \sum_{i=1}^N \cal{U}_i\,H_0 \,\cal{U}^\dagger_i \Delta
t} \\
=  {\bf T} \lim_{N\rightarrow \infty} \prod_{i=1}^N \cal{U}_i e^{-iH_0 \Delta t} \cal{U}_i^\dagger 
\end{equation*}
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