Is there an epsilon-delta definition of the second derivative?

Is there an epsilon-delta definition for the second derivative?

I know that there is such a definition for the first derivate $f'(x)$ which can be derived from the limit $f'(x) = \lim_{y\rightarrow x} \frac{f(y)-f(x)}{y-x}$ for a function $f:D\rightarrow \mathbb{R}$:

$$\forall \epsilon > 0\, \exists \delta > 0\, \forall y \in D\setminus \{x\}:|y-x|<\delta \Rightarrow \left|\frac{f(y)-f(x)}{y-x}-f'(x)\right|<\epsilon$$

So $f'(x)$ can be described as the number which fulfills the above statement. Is there a similar statement for the second derivative?

Update: This MSE thread shows that there are different definitions for the derivative (and thus for the second derivative). So I want to make my question more concrete:

My definition of derivation: Let be $f:D\rightarrow\mathbb{R}$ with $D\subseteq\mathbb{R}$ arbitrary. Let $D^*$ be the set off all points $x\in D$ for which there is at least one sequence $(x_n)$ in $D\setminus\{x\}$ with $\lim_{n\rightarrow\infty} x_n=x$. I define the limit $\lim_{y\rightarrow x\ ,y\in D\setminus\{x\}} {f(y)-f(x) \over y-x}$ as the first derivation for a given $x\in D^*$ (if the limit exists).

My definition of the second derivative: Let be $f:D\rightarrow\mathbb{R}$ with $D\subseteq\mathbb{R}$ arbitrary. We call $f''(x)$ the second derivative if there exists an open interval $x\in O\subseteq \mathbb{R}$ so that $f$ is differentiable on $O\cap D$ and $f''(x)$ is the first derivative of the function $f': (O\cap D)\rightarrow\mathbb{R}:x\mapsto f'(x)$ at the point $x$ (which also means that $x\in(O\cap D)^*$).

My question: Is there a statement $\forall \epsilon > 0: \exists \delta > 0: A(\epsilon, \delta, f, x, c)$ for $f:D\rightarrow \mathbb{R}$ ($D\subseteq \mathbb{R}$) and $c,x\in\mathbb{R}$ which is equivalent to the statement that $f$ is differentiable on a set $x\in O\cap D$ where $O$ is an open interval and that $c$ is the second derivative of $f$ at $x$?

I also will accept answers where you need more restrictions to the question. For example you might want to use the value of the first derivative $f'(x)$ (at the same point where you want to define the second derivative) in your statement or you want to restrict $f$ on functions with open domains or domains which are intervals. In this case I will accept your answer and open a new thread asking for a more general solution.

Please notice that there is a community wiki post where I want to collect all the progress we made so far.

Solution 1:

I'm not sure, but I think there are two problems with the formula you used to approximate $f''(x_0)$:

• it uses the same discretization step for the approximation of the first and second derivative (it's like computing one directional derivative for a function of two variables: it might exist, but that does not imply that the differential exists).

• it's a centered finite difference formula, which therefore vanishes for a function that is odd around $x_0$ (or even gives infinity if $f$ is odd but $f(x_0)\neq0$, in which case $f$ is for sure discontinuous).

But I think the idea would work if the increments used in the approximation of the first and second derivative were different and the discretization formulas were not centered. Namely

$$f''(x)\simeq \frac{f'(x+h)-f'(x)}{h}$$

and then

$$f'(x+h)\simeq \frac{f(x+h+k)-f(x+h)}{k}$$ $$f'(x)\simeq \frac{f(x+k)-f(x)}{k}$$ which gives

$$f''(x)\simeq \frac{\dfrac{f(x+h+k)-f(x+h)}{k}-\dfrac{f(x+k)-f(x)}{k}}{h}=$$

$$f''(x)\simeq \frac{f(x+h+k)-f(x+h)-f(x+k)+f(x)}{hk}$$

Now, for the example reported in the link you gave, this formula does not give a finite result as $h,k$ go to $0$ independently.

To summarize, I would say that $f''$ exists and it's equal to $f''(x)$ if

$$\forall \varepsilon>0, \exists \delta>0 : \forall \underline{h}\in\mathbb{R}^2\cap \mathcal{B}(\underline{0},\delta)\setminus\underline{0}$$ $$\left|\frac{f(x+h_1+h_2)-f(x+h_1)-f(x+h_2)+f(x)}{h_1h_2}-f''(x)\right|<\varepsilon.$$

I'm not $100\%$ sure of this statement (in particular of the fact that the two increments have to independent), but it looks right to me. For sure you need not centered schemes though.

Solution 2:

If we are not allowed to talk about $f'(x)$ for $x\ne x_0$ it is not possible to talk about $f''(x_0)$ in the proper sense. One could, however, approach the idea of $f''(x_0)$ via the Taylor expansion of $f$ at $x_0$:

The function $f$, defined in a neighborhood of $x_0$ has second derivative $b$ at $x_0$ if there is an $a\in{\mathbb R}$ such that $$\lim_{h\to 0}{f(x_0+h)-f(x_0)- a h \over h^2}={b\over2}\ .$$ This $\lim$-condition can obviously be expanded into $\epsilon$-$\delta$-language.

Note, however, that the function $f(x):=x^3$ $(x\in{\mathbb Q})$ and $:=0$ $(x\notin{\mathbb Q})$ would have $f''(0)=0$ according to this definition.