{{Short description|Mathematics concept}} {{About|rectifiable sets in measure theory|rectifiable curves|Arc length}} In mathematics, a '''rectifiable set''' is a set that is smooth in a certain measure-theoretic sense. It is an extension of the idea of a rectifiable curve to higher dimensions; loosely speaking, a rectifiable set is a rigorous formulation of a piece-wise smooth set. As such, it has many of the desirable properties of smooth manifolds, including tangent spaces that are defined almost everywhere. Rectifiable sets are the underlying object of study in geometric measure theory.
==Definition== A Borel subset <math>E</math> of Euclidean space <math>\mathbb{R}^n</math> is said to be '''<math>m</math>-rectifiable''' set if <math>E</math> is of Hausdorff dimension <math>m</math>, and there exist a countable collection <math>\{f_i\}</math> of continuously differentiable maps
:<math>f_i:\mathbb{R}^m \to \mathbb{R}^n</math>
such that the <math>m</math>-Hausdorff measure <math>\mathcal{H}^m</math> of
:<math>E\setminus \bigcup_{i=0}^\infty f_i\left(\mathbb{R}^m\right)</math>
is zero. The backslash here denotes the set difference. Equivalently, the <math>f_i</math> may be taken to be Lipschitz continuous without altering the definition.<ref>{{harvnb|Simon|1984|p=58}}, calls this definition "countably ''m''-rectifiable".</ref><ref>{{SpringerEOM|title=Rectifiable set|id=Rectifiable_set&oldid=29261}}</ref><ref>{{MathWorld|title=Rectifiable Set|id=RectifiableSet|access-date=2020-04-17}}</ref> Other authors have different definitions, for example, not requiring <math>E</math> to be <math>m</math>-dimensional, but instead requiring that <math>E</math> is a countable union of sets which are the image of a Lipschitz map from some bounded subset of <math>\mathbb{R}^m</math>.<ref>{{harvtxt|Federer|1969|pp=3.2.14}}</ref>
A set <math>E</math> is said to be '''purely <math>m</math>-unrectifiable''' if for ''every'' (continuous, differentiable) <math>f:\mathbb{R}^m \to \mathbb{R}^n</math>, one has
:<math>\mathcal{H}^m \left(E \cap f\left(\mathbb{R}^m\right)\right)=0.</math>
A standard example of a purely-1-unrectifiable set in two dimensions is the Cartesian product of the Smith–Volterra–Cantor set times itself.
=== Rectifiable sets in metric spaces ===
{{harvtxt|Federer|1969|pp=251–252}} gives the following terminology for ''m''-rectifiable sets ''E'' in a general metric space ''X''. # ''E'' is '''<math>m</math> rectifiable''' when there exists a Lipschitz map <math>f:K \to E</math> for some bounded subset <math>K</math> of <math>\mathbb{R}^m</math> onto <math>E</math>. # ''E'' is '''countably <math>m</math> rectifiable''' when ''E'' equals the union of a countable family of <math>m</math> rectifiable sets. # ''E'' is '''countably <math>(\phi,m)</math> rectifiable''' when <math>\phi</math> is a measure on ''X'' and there is a countably <math>m</math> rectifiable set ''F'' such that <math>\phi(E\setminus F)=0</math>. # ''E'' is '''<math>(\phi,m)</math> rectifiable''' when ''E'' is countably <math>(\phi,m)</math> rectifiable and <math>\phi(E)<\infty</math> # ''E'' is '''purely <math>(\phi,m)</math> unrectifiable''' when <math>\phi</math> is a measure on ''X'' and ''E'' includes no <math>m</math> rectifiable set ''F'' with <math>\phi(F)>0</math>.
Definition 3 with <math>\phi=\mathcal{H}^m</math> and <math>X=\mathbb{R}^n</math> comes closest to the above definition for subsets of Euclidean spaces.
==Notes== {{reflist}}
==References== * {{citation|last = Federer | first = Herbert | authorlink = Herbert Federer | title = Geometric measure theory| publisher = Springer-Verlag | location = New York | year = 1969 | pages = xiv+676 | isbn = 978-3-540-60656-7 | mr= 0257325 | series = Die Grundlehren der mathematischen Wissenschaften|volume=153}} * {{springer|author=T.C.O'Neil|id=G/g130040|title=Geometric measure theory}} * {{Citation| last = Simon | first = Leon | author-link =Leon Simon | title = Lectures on Geometric Measure Theory | place = Canberra | publisher = Centre for Mathematics and its Applications (CMA), Australian National University | series = Proceedings of the Centre for Mathematical Analysis | volume = 3 | year = 1984 | pages =VII+272 (loose errata) | isbn = 0-86784-429-9 | zbl = 0546.49019 }}
==External links== * [https://www.encyclopediaofmath.org/index.php/Rectifiable_set Rectifiable set] at [http://www.encyclopediaofmath.org/ Encyclopedia of Mathematics]
Category:Measure theory