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\begin{eqnarray} 
\sf 
e_1 & = & E(\vec{r}) \nonumber \\
e_2 & = & E(\vec{r}+\frac{1}{2} c \vec{F}) \nonumber \\
e_3 & = & E(\vec{r}+c\vec{F})  \nonumber
\end{eqnarray}
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