The function is defined as follows:
$$
f(x)=\left\{\begin{aligned}-1, & x < 0 \\ 0, & x=0 \\ 1, & x > 0 \end{aligned}\right.
$$
This means that for all $x$ less than 0, $f(x)$ is -1. For $x$ equal to 0, $f(x)$ is 0. And for all $x$ greater than 0, $f(x)$ is 1.
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