2

$f'(x)$, $f''(x)$, $f^{(3)}(x)$, ..., $f^{(n)}(x)$ all exist $\forall x \in I$, an interval in $\mathbb{R}$ but $f^{(n+1)}(x)$ does not exist for any $x \in I$

Can such a function exist?

In words: Does there exist a function $f$ that is $n$-times differentiable on an interval $I$ but is not $(n+1)$-times differentiable anywhere on $I$ (i.e. $(n+1)$-differentiable nowhere on $I$)?

I think it can't but can't think of the reason why. I can construct a function with countably infinite points in I where the function is only differentiable n times, but, it seems impossible to have every point in the interval, I, have this property.

BCLC
  • 13,459
futurebird
  • 6,248

2 Answers2

4

Yes. Take the Weierstrass function $f$. It is differentiable nowhere. If $F$ is a primitive of $f$ (which exists, since $f$ is continuous), then $F'$ exists, but $F''$ exists nowhere.

José Carlos Santos
  • 427,504
2

Take any continuous, nowhere differentiable function (e.g. Weierstrass function). Then integrate it $n$ times. First time you can do it, as the function is continuous. Later, you are integrating differentiable, so continuous, functions.

Paweł Czyż
  • 3,238
  • How do I know the first anti-derivative of the Weierstrass function is also continuous? I understand why I can integrate it once, but am still trying to understand doing it more than once. – futurebird Apr 18 '18 at 11:35
  • 1
    The integral of a continuous function is differentiable, so is continuous and hence integrable. – B. Mehta Apr 18 '18 at 11:44
  • 1
    If $F(x) = \int_0^x f(t);dt$, then $F'(x) = f(x)$ provided $f$ is continuous at the point $x$. Perhaps your calculus textbook contains this fundamental theorem. – GEdgar Apr 18 '18 at 11:49
  • @GEdgar , Paweł Czyż , Does this same example disprove if I were to assume $f$ is continuously $n$-times differentiable instead of just $n$-times differentiable? this is my question here – BCLC Oct 13 '21 at 17:27