# Differentiable+Not monotone

Is there a real function that is differentiable at any point but nowhere monotone?

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the constant function works, but I assume that example should be disallowed. – Sean Tilson Jan 11 '11 at 14:58
Doesn't "monotone" by default mean weakly, not strictly, monotone? (i.e. monotone increasing means for all $x$, $y$, $x \le y \Rightarrow f(x) \le f(y)$). So constant functions are everywhere monotone. – Chris Eagle Jan 11 '11 at 18:33
See also the MO version of this question for additional details and references. – Andrés E. Caicedo May 16 '14 at 16:34
And see here for details of the Katznelson-Stromberg construction. – Andrés E. Caicedo May 21 '14 at 3:05