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\[
  W\left( r,h \right) = \frac{1}{\pi h^3} \begin{cases} 1-\frac{3}{2} q^2 + \frac{3}{4} q^3 & 0 \leq q < 1 \\
                                                        \frac{1}{4} \left( 2-q \right)^3    & 1 \leq q < 2 \\
                                                        0 & 2 \leq q
                                          \end{cases}
\]  

\end{document}
