Thinking about the (probably) well-known fallacy about approaching a unit square diagonal with staircase functions and thus concluding the diagonal length be $2$ instead of $\sqrt 2$ led me to an interesting question:
Given a sequence $(f_k)_{k\in\mathbb N}$ of differentiable functions converging towards a differentiable limit function $f$, when does the limit of derivatives coincide with the derivative of the limit function, that is, when do we have $f'(x)=\lim_{k\to\infty}f_k'(x)$ for all $x$ in the function's domain? And what about second or $n$-th derivatives, supposing all the functions $f_k$ as well as $f$ are twice or $n$ times differentiable?
No need to tell me staircase functions aren't differentiable – this is supposed to be a more general question about necessary and sufficient conditions for the limit of $n$-th derivatives to coincide with the $n$-th derivative of the limit.
Best Answer
You can have weaker hypotheses. Namely: