Calculate subgradient of the function $f(x)=|x-3|+|x+1|$ at point $x=-1$ and $x=3$.
At $x=-1$ for all $y\in R$ following should be satisfied: $|y-3|+|y+1|\ge4+k(y+1)$, where $k$ is subgradient. But how do I solve that? How to find $k$?
Same for $x=3$ I get $|y-3|+|y+1|\ge4+k(y-3)$.
