Is a C^1-function in a bounded domain $\Omega\subset R^n;$ an element of the Sobolev space $W^{2,\infty}(\Omega)$ ?



Derivable functions & Sobolev spaces
No. Consider $x^{3/2}$ on $(-1,1)$. It has Holder continuous but not Lipschitz derivative. See Gilbarg-Trudinger Ch. 4 for some related exercises (e.g. a C^1 function with derivative continuous but not Holder continuous for any $\alpha$).


