Real Analysis – Does the Existence of Derivatives in the Average Sense Imply Absolute Continuity?

measure-theoryreal-analysis

Let $f: \mathbb R \to \mathbb R$ be a measurable function. Suppose there exists some integrable function $g$, and a measurable set $E$ of full measure such that

$$\lim_{r \to 0_+} \sup_{x \in E} \left | \frac{1}{r^2} \int_0^r f(x+t) – f(x-t) \, dt – g(x) \right | = 0.$$

Question: Does $f$ admit an absolutely continuous representative with derivative $g$? That is, an absolutely continuous function $\tilde f$ with $\tilde f = f$ a.e., and $\tilde f’ = g$ a.e.

Comments:

  1. A quite remarkable consequence of the above would be the following characterisation of (equivalence classes of) continuously differentiable functions.

Theorem (?): A function $f$ admits a continuously differentiable representative if and only if it satisfies the hypotheses of the original claim and further $g$ is continuous.

  1. The desired result is closely related to the result proven in the post Uniformly Lebesgue differentiable functions.

Best Answer

Let $F(x)=\int_0^x f(t)\, dt$. Then our assumption says that $$ G_r(x)\equiv \frac{F(x+r)+F(x-r)-2F(x)}{r^2} \to g(x) $$ as $r\to 0+$, uniformly on $E$. Thus also $G_r\to g$ in $\mathcal D'$ (= as distributions), by dominated convergence. However, in $\mathcal D'$, these difference quotients are just another way to obtain the distributional derivatives; this is well known in the simpler version for the first distributional derivative $u'$ of a $u\in\mathcal D'$, which we can obtain as $u'=\lim (\tau_h u-u)/h$, with the limit taken in $\mathcal D'$, but the argument is the same here: the operations are moved over to the test functions $\varphi$, and for these we have the required convergence to $\varphi''$ in the topology of $\mathcal D$.

We conclude that $F''=g$ in $\mathcal D'$, but $F'=f$, so we see that $f$ has an integrable distributional derivative and thus is absolutely continuous.