For which values of $\alpha, k,p,d$ is $$ \|x\|^\alpha \in \textrm{W}^{k,p} (B(0,1)) \quad ? $$
where $\displaystyle{ \textrm{B}(0,1) = \{x \in \mathbb R^d : \|x\|<1\}}$
This is an exercise from the book of J.Jost "Postmodern Analysis". It is in Chapter 20 ,page 285. In the examples of this chapter proves that $\|x\|^\alpha \in \textrm{W}^{1,p} (B(0,1))$ for $(\alpha -1)p +d >0$. But in this problem I don't know even how to start.
Any help would be appreciated.