# De Rham curve

In mathematics, a de Rham curve is a certain type of fractal curve named in honor of Georges de Rham.

The Cantor function, Cesàro curve, Minkowski's question mark function, the Lévy C curve, the blancmange curve the Koch curve and the Osgood curve are all special cases of the general de Rham curve.

## Construction

Consider some complete metric space ${\displaystyle (M,d)}$ (generally ${\displaystyle \mathbb {R} }$2 with the usual euclidean distance), and a pair of contracting maps on M:

${\displaystyle d_{0}:\ M\to M}$
${\displaystyle d_{1}:\ M\to M.}$

By the Banach fixed-point theorem, these have fixed points ${\displaystyle p_{0}}$ and ${\displaystyle p_{1}}$ respectively. Let x be a real number in the interval ${\displaystyle [0,1]}$, having binary expansion

${\displaystyle x=\sum _{k=1}^{\infty }{\frac {b_{k}}{2^{k}}},}$

where each ${\displaystyle b_{k}}$ is 0 or 1. Consider the map

${\displaystyle c_{x}:\ M\to M}$

defined by

${\displaystyle c_{x}=d_{b_{1}}\circ d_{b_{2}}\circ \cdots \circ d_{b_{k}}\circ \cdots ,}$

where ${\displaystyle \circ }$ denotes function composition. It can be shown that each ${\displaystyle c_{x}}$ will map the common basin of attraction of ${\displaystyle d_{0}}$ and ${\displaystyle d_{1}}$ to a single point ${\displaystyle p_{x}}$ in ${\displaystyle M}$. The collection of points ${\displaystyle p_{x}}$, parameterized by a single real parameter x, is known as the de Rham curve.

## Continuity condition

When the fixed points are paired such that

${\displaystyle d_{0}(p_{1})=d_{1}(p_{0})}$

then it may be shown that the resulting curve ${\displaystyle p_{x}}$ is a continuous function of x. When the curve is continuous, it is not in general differentiable.

In the remaining of this page, we will assume the curves are continuous.

## Properties

De Rham curves are by construction self-similar, since

${\displaystyle p(x)=d_{0}(p(2x))}$ for ${\displaystyle x\in [0,1/2]}$ and
${\displaystyle p(x)=d_{1}(p(2x-1))}$ for ${\displaystyle x\in [1/2,1].}$

The self-symmetries of all of the de Rham curves are given by the monoid that describes the symmetries of the infinite binary tree or Cantor set. This so-called period-doubling monoid is a subset of the modular group.

The image of the curve, i.e. the set of points ${\displaystyle \{p(x),x\in [0,1]\}}$, can be obtained by an Iterated function system using the set of contraction mappings ${\displaystyle \{d_{0},\ d_{1}\}}$. But the result of an iterated function system with two contraction mappings is a de Rham curve if and only if the contraction mappings satisfy the continuity condition.

Detailed, worked examples of the self-similarities can be found in the articles on the Cantor function and on Minkowski's question-mark function. Precisely the same monoid of self-similarities, the dyadic monoid, apply to every de Rham curve.

## Classification and examples

### Cesàro curves

Cesàro curve for a = 0.3 + i 0.3
Cesàro curve for a = 0.5 + i 0.5. This is the Lévy C curve.

Cesàro curves (or Cesàro–Faber curves) are De Rham curves generated by affine transformations conserving orientation, with fixed points ${\displaystyle p_{0}=0}$ and ${\displaystyle p_{1}=1}$.

Because of these constraints, Cesàro curves are uniquely determined by a complex number ${\displaystyle a}$ such that ${\displaystyle |a|<1}$ and ${\displaystyle |1-a|<1}$.

The contraction mappings ${\displaystyle d_{0}}$ and ${\displaystyle d_{1}}$ are then defined as complex functions in the complex plane by:

${\displaystyle d_{0}(z)=az}$
${\displaystyle d_{1}(z)=a+(1-a)z.}$

For the value of ${\displaystyle a=(1+i)/2}$, the resulting curve is the Lévy C curve.

### Koch–Peano curves

Koch–Peano curve for a = 0.6 + i 0.37. This is close to, but not quite the Koch curve.
Koch–Peano curve for a = 0.6 + i 0.45. This is the Osgood curve.

In a similar way, we can define the Koch–Peano family of curves as the set of De Rham curves generated by affine transformations reversing orientation, with fixed points ${\displaystyle p_{0}=0}$ and ${\displaystyle p_{1}=1}$.

These mappings are expressed in the complex plane as a function of ${\displaystyle {\overline {z}}}$, the complex conjugate of ${\displaystyle z}$:

${\displaystyle d_{0}(z)=a{\overline {z}}}$
${\displaystyle d_{1}(z)=a+(1-a){\overline {z}}.}$

The name of the family comes from its two most famous members. The Koch curve is obtained by setting:

${\displaystyle a_{\text{Koch}}={\frac {1}{2}}+i{\frac {\sqrt {3}}{6}},}$

while the Peano curve corresponds to:

${\displaystyle a_{\text{Peano}}={\frac {(1+i)}{2}}.}$

### General affine maps

Generic affine de Rham curve
Generic affine de Rham curve
Generic affine de Rham curve
Generic affine de Rham curve

The Cesàro–Faber and Peano–Koch curves are both special cases of the general case of a pair of affine linear transformations on the complex plane. By fixing one endpoint of the curve at 0 and the other at one, the general case is obtained by iterating on the two transforms

${\displaystyle d_{0}={\begin{pmatrix}1&0&0\\0&\alpha &\delta \\0&\beta &\varepsilon \end{pmatrix}}}$

and

${\displaystyle d_{1}={\begin{pmatrix}1&0&0\\\alpha &1-\alpha &\zeta \\\beta &-\beta &\eta \end{pmatrix}}.}$

Being affine transforms, these transforms act on a point ${\displaystyle (u,v)}$ of the 2-D plane by acting on the vector

${\displaystyle {\begin{pmatrix}1\\u\\v\end{pmatrix}}.}$

The midpoint of the curve can be seen to be located at ${\displaystyle (u,v)=(\alpha ,\beta )}$; the other four parameters may be varied to create a large variety of curves.

The blancmange curve of parameter ${\displaystyle w}$ can be obtained by setting ${\displaystyle \alpha =\beta =1/2}$, ${\displaystyle \delta =\zeta =0}$ and ${\displaystyle \varepsilon =\eta =w}$. That is:

${\displaystyle d_{0}={\begin{pmatrix}1&0&0\\0&1/2&0\\0&1/2&w\end{pmatrix}}}$

and

${\displaystyle d_{1}={\begin{pmatrix}1&0&0\\1/2&1/2&0\\1/2&-1/2&w\end{pmatrix}}.}$

Since the blancmange curve of parameter ${\displaystyle w=1/4}$ is the parabola of equation ${\displaystyle f(x)=4x(1-x)}$, this illustrate the fact that in some occasion, de Rham curves can be smooth.

### Minkowski's question mark function

Minkowski's question mark function is generated by the pair of maps

${\displaystyle d_{0}(z)={\frac {z}{z+1}}}$

and

${\displaystyle d_{1}(z)={\frac {1}{z+1}}.}$

## Generalizations

It is easy to generalize the definition by using more than two contraction mappings. If one uses n mappings, then the n-ary decomposition of x has to be used instead of the binary expansion of real numbers. The continuity condition has to be generalized in:

${\displaystyle d_{i}(p_{n-1})=d_{i+1}(p_{0})}$, for ${\displaystyle i=0\ldots n-2.}$

This continuity condition can be understood with the following example. Suppose on is working in base-10. Then one has (famously) that 0.999...= 1.000... which is a continuity equation that must be enforced at every such gap. That is, given the decimal digits ${\displaystyle b_{1},b_{2},\cdots ,b_{k}}$ with ${\displaystyle b_{k}\neq 9}$, one has

${\displaystyle b_{1},b_{2},\cdots ,b_{k},9,9,9,\cdots =b_{1},b_{2},\cdots ,b_{k}+1,0,0,0,\cdots }$

Such a generalization allows, for example, to produce the Sierpiński arrowhead curve (whose image is the Sierpiński triangle), by using the contraction mappings of an iterated function system that produces the Sierpiński triangle.

## Multifractal curves

Ornstein and others describe a multifractal system, where instead of working in a fixed base, one works in a variable base.

Consider the product space of variable base-${\displaystyle m_{n}}$ discrete spaces

${\displaystyle \Omega =\prod _{n\in \mathbb {N} }A_{n}}$

for ${\displaystyle A_{n}=\mathbb {Z} /m_{n}\mathbb {Z} =\{0,1,\cdots ,m_{n}-1\}}$ the cyclic group, for ${\displaystyle m_{n}\geq 2}$ an integer. Any real number in the unit interval can be expanded in a sequence ${\displaystyle (a_{1},a_{2},a_{3},\cdots )}$ such that each ${\displaystyle a_{n}\in A_{n}}$. More precisely, a real number ${\displaystyle 0\leq x\leq 1}$ is written as

${\displaystyle x=\sum _{n=1}^{\infty }{\frac {a_{n}}{\prod _{k=1}^{n}m_{k}}}}$

This expansion is not unique, if all ${\displaystyle a_{n}=0}$ past some point ${\displaystyle K. In this case, one has that

${\displaystyle a_{1},a_{2},\cdots ,a_{K},0,0,\cdots =a_{1},a_{2},\cdots ,a_{K}-1,m_{K+1}-1,m_{K+2}-1,\cdots }$

Such points are analogous to the dyadic rationals in the dyadic expansion, and the continuity equations on the curve must be applied at these points.

For each ${\displaystyle A_{n}}$, one must specify two things: a set of two points ${\displaystyle p_{0}^{(n)}}$ and ${\displaystyle p_{1}^{(n)}}$ and a set of ${\displaystyle m_{n}}$ functions ${\displaystyle d_{j}^{(n)}(z)}$ (with ${\displaystyle j\in A_{n}}$). The continuity condition is then as above,

${\displaystyle d_{j}^{(n)}(p_{1}^{(n+1)})=d_{j+1}^{(n)}(p_{0}^{(n+1)})}$, for ${\displaystyle j=0,\cdots ,m_{n}-2.}$

Ornstein's original example used

${\displaystyle \Omega =\left(\mathbb {Z} /2\mathbb {Z} \right)\times \left(\mathbb {Z} /3\mathbb {Z} \right)\times \left(\mathbb {Z} /4\mathbb {Z} \right)\times \cdots }$