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\begin{equation*} 
U_{\gamma}={\bf T}
\lim_{N \rightarrow \infty} \left( {\bf 1} + \sum_{i=1}^{N-1}
\vec{A}_i \cdot \Delta \vec{\lambda}_i \right) = {\bf P} \exp \oint_\gamma A \,\, ,
\nonumber
\end{equation*}
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