# Differentiation of integrals

In mathematics, the problem of differentiation of integrals is that of determining under what circumstances the mean value integral of a suitable function on a small neighbourhood of a point approximates the value of the function at that point. More formally, given a space X with a measure μ and a metric d, one asks for what functions f : X → R does $\lim _{r\to 0}{\frac {1}{\mu {\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \mu (y)=f(x)$ for all (or at least μ-almost all) x ∈ X? (Here, as in the rest of the article, B$_r$(x) denotes the open ball in X with d-radius r and centre x.) This is a natural question to ask, especially in view of the heuristic construction of the Riemann integral, in which it is almost implicit that f(x) is a "good representative" for the values of f near x.

## Theorems on the differentiation of integrals
### Lebesgue measure

One result on the differentiation of integrals is the Lebesgue differentiation theorem, as proved by Henri Lebesgue in 1910. Consider n-dimensional Lebesgue measure λ$^n$ on n-dimensional Euclidean space R$^n$. Then, for any locally integrable function f : R$^n$ → R, one has $\lim _{r\to 0}{\frac {1}{\lambda ^{n}{\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \lambda ^{n}(y)=f(x)$ for λ$^n$-almost all points x ∈ R$^n$. It is important to note, however, that the measure zero set of "bad" points depends on the function f.

### Borel measures on R$^n$

The result for Lebesgue measure turns out to be a special case of the following result, which is based on the Besicovitch covering theorem: if μ is any locally finite Borel measure on R$^n$ and f : R$^n$ → R is locally integrable with respect to μ, then $\lim _{r\to 0}{\frac {1}{\mu {\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \mu (y)=f(x)$ for μ-almost all points x ∈ R$^n$.

### Gaussian measures

The problem of the differentiation of integrals is much harder in an infinite-dimensional setting. Consider a separable Hilbert space (H, ⟨ , ⟩) equipped with a Gaussian measure γ. As stated in the article on the Vitali covering theorem, the Vitali covering theorem fails for Gaussian measures on infinite-dimensional Hilbert spaces. Two results of David Preiss (1981 and 1983) show the kind of difficulties that one can expect to encounter in this setting:
* There is a Gaussian measure γ on a separable Hilbert space H and a Borel set M ⊆ H so that, for γ-almost all x ∈ H, $\lim _{r\to 0}{\frac {\gamma {\bigM\cap B_{r}(x){\big )}}{\gamma {\bigB_{r}(x){\big )}}}=1.$
* There is a Gaussian measure γ on a separable Hilbert space H and a function f ∈ L$^1$(H, γ; R) such that $\lim _{r\to 0}\inf \left\{\left.{\frac {1}{\gamma {\bigB_{s}(x){\big )}}}\int _{B_{s}(x)}f(y)\,\mathrm {d} \gamma (y)\right|x\in H,0<s<r\right\}=+\infty .$
However, there is some hope if one has good control over the covariance of γ. Let the covariance operator of γ be S : H → H given by $\langle Sx,y\rangle =\int _{H}\langle x,z\rangle \langle y,z\rangle \,\mathrm {d} \gamma (z),$ or, for some countable orthonormal basis (e$_i$)$_{i∈N}$ of H, $Sx=\sum _{i\in \mathbf {N} }\sigma _{i}^{2}\langle x,e_{i}\rangle e_{i}.$
In 1981, Preiss and Jaroslav Tišer showed that if there exists a constant 0 < q < 1 such that $\sigma _{i+1}^{2}\leq q\sigma _{i}^{2},$ then, for all f ∈ L$^1$(H, γ; R), ${\frac {1}{\mu {\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \mu (y){\xrightarrow[{r\to 0}]{\gamma }}f(x),$ where the convergence is convergence in measure with respect to γ. In 1988, Tišer showed that if $\sigma _{i+1}^{2}\leq {\frac {\sigma _{i}^{2}}{i^{\alpha }}}$ for some α > 5 ⁄ 2, then ${\frac {1}{\mu {\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \mu (y){\xrightarrow[{r\to 0}]}f(x),$ for γ-almost all x and all f ∈ L$^p$(H, γ; R), p > 1.
As of 2007, it is still an open question whether there exists an infinite-dimensional Gaussian measure γ on a separable Hilbert space H so that, for all f ∈ L$^1$(H, γ; R), $\lim _{r\to 0}{\frac {1}{\gamma {\bigB_{r}(x){\big )}}}\int _{B_{r}(x)}f(y)\,\mathrm {d} \gamma (y)=f(x)$ for γ-almost all x ∈ H. However, it is conjectured that no such measure exists, since the σ$_i$ would have to decay very rapidly.