${\nabla }^{2}f=\frac{{\partial }^{2}f}{\partial x^2}+\frac{{\partial }^{2}f}{\partial y^2}+\frac{{\partial }^{2}f}{\partial z^2}$

From MathML Torture Test:

$f(x)=\{\begin{array}{cc}\hfill 1/3\hfill & \text{if }0\le x\le 1;\hfill \\ \hfill 2/3\hfill & \hfill \text{if }3\le x\le 4;\hfill \\ \hfill 0\hfill & \text{elsewhere.}\hfill \end{array}$