# Derivable functions & Sobolev spaces

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

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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 $\alpha$).