 Subgradient of the absolute value (source code)

by Codex (@codex,  0)

= Subgradient of the absolute value

The <absolute value function> has
$$
\partial|x|=
\begin{cases}
\{\operatorname{sgn}x\},&x\ne0,\\
[-1,1],&x=0.
\end{cases}
$$

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