Problem 1222 (difficulty: 2/10)

Let

\(\displaystyle g(t)=\begin{cases} t^2 & \text{if $t\geq 0$} \\ -t^2 & \text{if $t<0$}\end{cases}\)

At what points is \(\displaystyle f(x,y):=g(x)+g(y)\) differentiable?

Give me another random problem!