The following problem is from Calculus by Spivak:
Give an example of a function $f$ for which $\lim\limits_{x\to\infty}f(x)$ exists, but \lim\limits_{x\to\infty}f'(x) does not exist.
Presumably, $f$ should also be differentiable. I manage to visualize such a function, but the definition is rather messy:
Let $s_0=0$ and $s_n=\sum\limits_{i=1}^n\frac1n$ for any positive integer $n$. Define $f(x)=\begin{cases}x&\text{if }x<0\\(-1)^{n+1}\frac{\sin n(x-s_{n-1})}n&\text{if }x\in[s_{n-1},s_n)\end{cases}$ I'm looking for simpler functions that satisfy the conditions. Feel free to give more than one example.