In mathematics, a Caccioppoli set is a set whose boundary is measurable and has a (at least locally) finite measure. A synonym is set of (locally) finite perimeter. Basically, a set is a Caccioppoli set if its characteristic function is a function of bounded variation.
Contents
History
The basic concept of a Caccioppoli set was firstly introduced by the Italian mathematician Renato Caccioppoli in the paper (Caccioppoli 1927): considering a plane set or a surface defined on an open set in the plane, he defined their measure or area as the total variation in the sense of Tonelli of their defining functions, i.e. of their parametric equations, provided this quantity was bounded. The measure of the boundary of a set was defined as a functional, precisely a set function, for the first time: also, being defined on open sets, it can be defined on all Borel sets and its value can be approximated by the values it takes on an increasing net of subsets. Another clearly stated (and demonstrated) property of this functional was its lower semi-continuity.
In the paper (Caccioppoli 1928), he precised by using a triangular mesh as an increasing net approximating the open domain, defining positive and negative variations whose sum is the total variation, i.e. the area functional. His inspiring point of view, as he explicitly admitted, was those of Giuseppe Peano, as expressed by the Peano-Jordan Measure: to associate to every portion of a surface an oriented plane area in a similar way as an approximating chord is associated to a curve. Also, another theme found in this theory was the extension of a functional from a subspace to the whole ambient space: the use of theorems generalizing the Hahn–Banach theorem is frequently encountered in Caccioppoli research. However, the restricted meaning of total variation in the sense of Tonelli added much complication to the formal development of the theory, and the use of a parametric description of the sets restricted its scope.
Lamberto Cesari introduced the "right" generalization of functions of bounded variation to the case of several variables only in 1936: perhaps, this was one of the reasons that induced Caccioppoli to present an improved version of his theory only nearly 24 years later, in the talk (Caccioppoli 1953) at the IV UMI Congress in October 1951, followed by five notes published in the Rendiconti of the Accademia Nazionale dei Lincei. These notes were sharply criticized by Laurence Chisholm Young in the Mathematical Reviews.
In 1952 Ennio de Giorgi presented his first results, developing the ideas of Caccioppoli, on the definition of the measure of boundaries of sets at the Salzburg Congress of the Austrian Mathematical Society: he obtained this results by using a smoothing operator, analogous to a mollifier, constructed from the Gaussian function, independently proving some results of Caccioppoli. Probably he was led to study this theory by his teacher and friend Mauro Picone, who had also been the teacher of Caccioppoli and was likewise his friend. De Giorgi met Caccioppoli in 1953 for the first time: during their meeting, Caccioppoli expressed a profound appreciation of his work, starting their lifelong friendship. The same year he published his first paper on the topic i.e. (De Giorgi 1953): however, this paper and the closely following one did not attracted much interest from the mathematical community. It was only with the paper (De Giorgi 1954), reviewed again by Laurence Chisholm Young in the Mathematical Reviews, that his approach to sets of finite perimeter became widely known and appreciated: also, in the review, Young revised his previous criticism on the work of Caccioppoli.
The last paper of De Giorgi on the theory of perimeters was published in 1958: in 1959, after the death of Caccioppoli, he started to call sets of finite perimeter "Caccioppoli sets". Two years later Herbert Federer and Wendell Fleming published their paper (Federer & Fleming 1960), changing the approach to the theory. Basically they introduced two new kind of currents, respectively normal currents and integral currents: in a subsequent series of papers and in his famous treatise, Federer showed that Caccioppoli sets are normal currents of dimension
Formal definition
In what follows, the definition and properties of functions of bounded variation in the
Caccioppoli definition
Definition 1. Let
where
Definition 2. The Borel set
Therefore, a Caccioppoli set has a characteristic function whose total variation is locally bounded. From the theory of functions of bounded variation it is known that this implies the existence of a vector-valued Radon measure
As noted for the case of general functions of bounded variation, this vector measure
De Giorgi definition
In his papers (De Giorgi 1953) and (De Giorgi 1954), Ennio de Giorgi introduces the following smoothing operator, analogous to the Weierstrass transform in the one-dimensional case
As one can easily prove,
also, its gradient is everywhere well defined, and so is its absolute value
Having defined this function, De Giorgi gives the following definition of perimeter:
Definition 3. Let
Actually De Giorgi considered the case
Basic properties
The following properties are the ordinary properties which the general notion of a perimeter is supposed to have:
Notions of boundary
For any given Caccioppoli set
is the perimeter within any open set
The topological boundary
It is natural to try to understand the relationship between the objects
Lemma. The support of the vector-valued Radon measure
Proof. To see this choose
Likewise, if
With
The reduced boundary
The topological boundary
representing a square together with a line segment sticking out on the left has perimeter
has one-dimensional Hausdorff measure
The "correct" boundary should therefore be a subset of
Definition 4. The reduced boundary of a Caccioppoli set
exists and has length equal to one, i.e.
One can remark that by the Radon-Nikodym Theorem the reduced boundary
The inclusions above are not necessarily equalities as the previous example shows. In that example,
De Giorgi's theorem
For convenience, in this section we treat only the case where
i.e. that its Hausdorff measure equals the perimeter of the set. The statement of the theorem is quite long because it interrelates various geometric notions in one fell swoop.
Theorem. Suppose
for every continuous, compactly supported
defined previously. This unit vector also satisfies
locally in
In other words, up to
A Gauss–Green formula
From the definition of the vector Radon measure
This is one version of the divergence theorem for domains with non smooth boundary. De Giorgi's theorem can be used to formulate the same identity in terms of the reduced boundary