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\begin{document}
\large
\begin{displaymath} 
\sf 
W(\vec{r}) =  \left\{
\begin{array}{ll}
c (E-U(\vec{r}))^{\frac{3N}{2}-1} & \mbox{\hspace{12pt} for $E>U(\vec{r})$} \\
            0 & \mbox{\hspace{12pt} for $E \leq U(\vec{r})$}
\end{array}
\right.
\end{displaymath}\
\end{document}
