Let $f\left(x\right)=max\left\{x,x^3\right\},x\in R$ . The set of points where $f\left(x\right)$ is not differentiable is`

$f\left(x\right)=\left\{\begin{array}{l}x,x\le -1\\ x^3,-1<x\le 0\\ x,0<x\le 1\\ x^3,x>1\end{array}\right.$

$f\left(x\right)$ is continuous for all $x$

$f\left(x\right)$ is differentiable for all  $x$ except $x$ = - 1, 0, 1.