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2020 Mathematics Subject Classification: Primary: 49Q15 Secondary: 49Q2049Q0528A7532C3058A2558C35 [MSN][ZBL]

An area of analysis concerned with solving geometric problems via measure-theoretic techniques. The canonical motivating physical problem is probably that investigated experimentally by J. Plateau in the nineteenth century: Given a boundary wire, how does one find the (minimal) soap film which spans it? In mathematical terms: Given a boundary curve, find the surface of minimal area spanning it. (Cf. also Plateau problem.) The many different approaches to solving this problem have found utility in most areas of modern mathematics and geometric measure theory is no exception: techniques and ideas from geometric measure theory have been found useful in the study of partial differential equations, the calculus of variations, geometric analysis, harmonic analysis, and fractals.

Many variational problems (cf. also Variational calculus) are solved by enlarging the allowed class of solutions, showing that in this enlarged class a solution exists, and then showing that the solution possesses more regularity than an arbitrary element of the enlarged class. Much of the work in geometric measure theory has been directed towards placing this informal description on a formal footing appropriate for the study of surfaces.

Measure theoretic concepts

One of the central issues of geometric measure theory is to define the concepts of volume, area and length in the uttermost generality. The roots of them are obviously in measure theory. In particular, the concept of outer measure in a metric space plays a fundamental role.

Caratheodory construction

The following is a common construction of metric outer measures in metric spaces $(X, d)$.

Definition 1 If $\mathcal{C}$ is a class of subsets of $X$ containing the empty set, $\nu : \mathcal{C}\to [0, \infty]$ a set function with $\nu (\emptyset) =0$ and $\delta\in ]0, \infty]$, then for every $A\subset X$ we define \[ \mu^\delta (A) := \inf \left\{ \sum_i \nu (E_i) : \{E_i\}_{i\in \mathbb N} \subset \mathcal{C} \mbox{ covers '"`UNIQ-MathJax9-QINU`"' and '"`UNIQ-MathJax10-QINU`"'}\right\}\, \] and \[ \mu (A) := \lim_{\delta\downarrow 0}\; \mu^\delta (A)\, . \]

Observe that the latter limit exists because $\mu^\delta (A)$ is a nonincreasing function of $\delta$. This construction is often called Caratheodory construction. See Section 2.1 of [KP] (cp. also with [Fe]).

Theorem 2 Let $\nu$ and $\mu$ be as in Definition 7. Then $\mu$ is a metric outer measure.

(Cp. with Claims 1,2 and 3 in the proof of Theorem 1 in Section 2.1 of [EG]: although the reference handles the cases of Hausdorff outer measures, the proof extends verbatim to the setting above).

Hausdorff measures

The Caratheodory construction gives several generalizations of the concept of dimension and volume. The most common is due to Hausdorff (cp. with [Ha]).

Definition 3 The Hausdorff outer measure $\mathcal{H}^\alpha$ is given by such $\mu$ as in Definition 1 when we choose $\mathcal{C} = \mathcal{P} (X)$ and $\nu (A) = \omega_\alpha ({\rm diam}\, (A))^\alpha$, where \[ \omega_\alpha = \frac{\pi^{\alpha/2}}{\Gamma \left(\frac{\alpha}{2}+1\right)}\, \] (cp. with Section 2.1 of [EG]).

When $\alpha$ is a (positive) integer $n$, $\omega_n$ equals the Lebesgue measure of the unit ball in $\mathbb R^n$. With this choice the $n$-dimensional Hausdorff outer measure on the euclidean space $\mathbb R^n$ coincides with the Lebesgue measure. It must be noted, however, that some authors prefer to set the constant $\omega_\alpha$ equal to $1$ (see for instance [Ma]).

Hausdorff dimension

The following is a simple consequence of the definition (cp. with Theorem 4.7 of [Ma]).

Theorem 4 For $0\leq s<t<\infty$ and $A\subset X$ we have

• $\mathcal{H}^s (A) < \infty \Rightarrow \mathcal{H}^t (A) = 0$;
• $\mathcal{H}^t (A)>0 \Rightarrow \mathcal{H}^s (A) = \infty$.

The Hausdorff dimension ${\rm dim}_H (A)$ of a subset $A\subset X$ is then defined as

Definition 5 \begin{align*} {\rm dim}_H (A) &= \sup \{s: \mathcal{H}^s (A)> 0\} = \sup \{s: \mathcal{H}^s (A) = \infty\}\\ &=\inf \{t: \mathcal{H}^t (A) = 0\} = \inf \{t: \mathcal{H}^t (A) < \infty\}\, . \end{align*}

Other measures related to the volume

The Caratheodory construction can be used to build other generalizations of the concept of volume, such as

• The spherical $\alpha$-dimensional outer measures on a metric space $(X,d)$, see Section 2.1.2 of [KP].
• The Gross outer measures, the Caratheodory outer measures, the integral-geometric outer measures (see also Favard measure) and the Gillespie outer measures in $\mathbb R^n$, see Sections 2.1.3-2.1.4-2.1.5 of [KP] (cp. also with 2.10.2-2.10.3-2.10.4 of [Fe]).

All these measures coincide on sufficiently regular set (in particular on rectifiable sets, see below), but they are, in general, different.

Differentiation theorem

A fundamental tool in geometric measure theory is the following theorem (cp. with Differentiation of measures), usually credited to Besicovitch and which gives an explicit characterization of the Radon-Nykodim decomposition for locally finite Radon measures on the euclidean space.

Theorem 6 (cp. with Theorem 2.12 of [Ma] and Theorem 2 in Section 1.6 of [EG]) Let $\mu$ and $\nu$ be two locally finite Radon measures on $\mathbb R^n$. Then,

• the limit

\[ f(x) := \lim_{r\downarrow 0} \frac{\nu (B_r (x))}{\mu (B_r (x))} \] exists at $\mu$-a.e. $x$ and defines a $\mu$-measurable map;

• the set

\begin{equation}\label{e:singular} S:= \left\{ x: \lim_{r\downarrow 0} \frac{\nu (B_r (x))}{\mu (B_r (x))} = \infty\right\} \end{equation} is $\nu$-measurable and a $\mu$-null set;

• $\nu$ can be decomposed as $\nu_a + \nu_s$, where

\[ \nu_a (E) = \int_E f\, d\mu \] and \[ \nu_s (E) = \nu (S\cap E)\, . \] Moreover, for $\mu$-a.e. $x$ we have: \begin{equation}\label{e:Lebesgue} \lim_{r\downarrow 0} \frac{1}{\mu (B_r (x))} \int_{B_r (x)} |f(y)-f(x)|\, d\mu (y) = 0\qquad \mbox{and}\qquad \lim_{r\downarrow 0} \frac{\nu_s (B_r (x))}{\mu (B_r (x))}= 0\, . \end{equation}

Covering arguments

Theorem 6 does not hold in general metric spaces. It holds provided the metric space satisfies some properties about covering of sets with balls (cp. with Covering theorems (measure theory)). In fact, aside from their links to the differentiation of measures, both the Vitali and Besicovitch covering Lemmas (see again Covering theorems (measure theory)) and similar arguments play a pivotal role in several fundamental results of geometric measure theory.

Rectifiable sets

Also called countably rectifiable set, rectifiable sets are central objects of study in Geometric measure theory, cp. with Rectifiable set. Rectifiable sets of the euclidean space can be thought as measure-theoretic generalizations of $C^1$ submanifolds. As such they have an integer Hausdorff dimension. In the special case of $1$-dimensional sets of the euclidean space, they were first introduced by Besicovitch, as a suitable generalization of rectifiable curves. In what follows we will use the terminology $m$-dimensional rectifiable set. Some authors prefer the terminology countably $m$-rectifiable set or, briefly, $m$-rectifiable.

Rectifiable subsets of the Euclidean space $\mathbb R^n$ can be defined in several ways, see Rectifiable sets. We adopt here the following one

Definition 7 (cp. with Lemma 11.1 of [Si]) A Borel set $E\subset \mathbb R^n$ is a rectifiable subset of dimension $k$ if it has Hausdorff dimension $k$ and there is a countable family of $C^1$ $k$-dimensional submanifolds of $\mathbb R^n$ which cover $\mathcal{H}^k$-almost all $E$.

The assumption that $E$ is a Borel set might be dropped, but in this case the set might not be $\mathcal{H}^k$-measurable (see Rectifiable set). In what follows we will assume that rectifiable sets $E$ are $\mathcal{H}^k$ measurable: $\sigma$-finite $\mathcal{H}^k$-measurable sets can be decomposed into the union of a Borel set and an $\mathcal{H}^k$-null set.

Besicovitch's works

As already mentioned, the theory of rectifiable $1$-dimensional sets was first laid down by Besicovitch in a series of works (see ??-??-??). In these seminal works Besicovitch considered and solved several questions which became later pivotal for rectifiable sets of higher dimension and also introduced many useful tools (such as the Besicovitch covering theorem, see Covering theorems (measure theory)) which have been widely used in different contexts.

One-dimensional sets

The theory of one-dimensional rectifiable sets is somewhat special since much stronger theorems can be proved which fail for higher dimensions. Perhaps the most useful one is the following (cp. with Theorem 3.14 of [Fa]):

Theorem 8 A continuum, i.e. a compact connected set, $E\subset\mathbb R^n$ of finite $\mathcal{H}^1$ measure is always rectifiable and arcwise connected. Indeed it is always the image of a rectifiable curve.

We refer to [Fa] for a comprehensive account of the theory of rectifiable one-dimensional sets.

References

[EG]	L.C. Evans, R.F. Gariepy, "Measure theory and fine properties of functions" Studies in Advanced Mathematics. CRC Press, Boca Raton, FL, 1992. MR1158660 Zbl 0804.2800
[Fa]	K. J. Falconer. "The geometry of fractal sets". Cambridge Tracts in Mathematics, 85. Cambridge University Press, Cambridge, 1986. MR0867284 Zbl 0587.28004
[Fe]	H. Federer, "Geometric measure theory". Volume 153 of Die Grundlehren der mathematischen Wissenschaften. Springer-Verlag New York Inc., New York, 1969. MR0257325 Zbl 0874.49001
[Ha]	F. Hausdorff, "Dimension and äusseres Mass" Math. Ann. , 79 (1918) pp. 157–179 MR1511917
[KP]	S. G. Krantz, H. Parks, "Geometric Integration Theory", Birkhäuser (2008).
[Ma]	P. Mattila, "Geometry of sets and measures in Euclidean spaces. Fractals and rectifiability". Cambridge Studies in Advanced Mathematics, 44. Cambridge University Press, Cambridge, 1995. MR1333890 Zbl 0911.28005
[Mu]	M. E. Munroe, "Introduction to Measure and Integration". Addison Wesley (1953).