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\begin{equation*}
{\bf T} \lim_{N \rightarrow
\infty} \cal{U}_N \left( \leavevmode\hbox{\small1\kern-3.8pt\normalsize1} -i H_0 \, N\,\cdot \Delta t + \sum_{i=1}^{N-1}
\vec{A}_i \cdot \Delta \vec{\lambda}_i \right) \cal{U}_1^\dagger. 
\end{equation*}
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