Let f\left( x \right) = \left\{ {\matrix{
{\max \left\{ {\left| x \right|,{x^2}} \right\}} & {\left| x \right| \le 2} \cr
{8 - 2\left| x \right|} & {2 < \left| x \right| \le 4} \cr
} } \right.
Let S be the set of points in the interval (– 4, 4) at which f is not differentiable. Then S