Let the function f be: $$f(x)={\left(1-\frac{2|x|}{X}\right)}^{\alpha}, $$ $X$ is just a real parameter.
How can I find the set of $\alpha$ such that $f \in W^{1,4}([-\frac{X}{2},\frac{X}{2}])$ ? I know the definition but I'm new with this kind of question so any help is appreciated.