Signum

The signum function (also called the sign function) is the function $$\mathrm{sgn}(x)=\left\{ \begin{array}{ll} 1 &; x > 0 \\ 0 &; x = 0 \\ -1 &; x < 0 \end{array} \right.$$

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  Graph of $\mathrm{sgn}$.