TeX source:

\begin{aligned} &\mathbf{D}(|\mathrm{x}|) = \left\{\begin{array}{lll} +1 & \text { if } \mathrm{x} > 0 \\ \text { undefined } & \quad \text { if } \mathrm{x} = 0 \\ -1 & \text { if } \mathrm{x} < 0 \end{array}\right. \end{aligned}