 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, Césaro curve, Minkowski's question mark function, the Lévy C curve, the blancmange curve and the Koch curve are all special cases of the general de Rham curve.
Contents
Construction
Consider some metric space (M,d) (generally ^{2} with the usual euclidean distance), and a pair of contracting maps on M:
By the Banach fixed point theorem, these have fixed points p_{0} and p_{1} respectively. Let x be a real number in the interval [0,1], having binary expansion
where each b_{k} is 0 or 1. Consider the map
defined by
where denotes function composition. It can be shown that each c_{x} will map the common basin of attraction of d_{0} and d_{1} to a single point p_{x} in M. The collection of points 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
 d_{0}(p_{1}) = d_{1}(p_{0})
then it may be shown that the resulting curve 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 selfsimilar, since p(x) = d_{0}(p(2x)) for and p(x) = d_{1}(p(2x − 1)) for .
The selfsymmetries 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 socalled perioddoubling monoid is a subset of the modular group.
The image of the curve, i.e. the set of points , can be obtained by an Iterated function system using the set of contraction mappings . 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.
Classification and examples
Césaro curves
Césaro curves (or CésaroFaber curves) are De Rham curves generated by affine transformations conserving orientation, with fixed points p_{0} = 0 and p_{1} = 1.
Because of these constraints, Césaro curves are uniquely determined by a complex number a such that  a  < 1 and  1 − a  < 1.
The contraction mappings d_{0} and d_{1} are then defined as complex functions in the complex plane by:
 d_{0}(z) = az
 d_{1}(z) = a + (1 − a)z
For the value of a = (1 + i) / 2, the resulting curve is the Lévy C curve.
Koch–Peano curves
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 p_{0} = 0 and p_{1} = 1.
These mappings are expressed in the complex plane as a function of , the complex conjugate of z:
The name of the family comes from its two most famous members. The Koch curve is obtained by setting:
while the Peano curve corresponds to:
General affine maps
The CésaroFaber and PeanoKoch 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
and
Being affine transforms, these transforms act on a point (u,v) of the 2D plane by acting on the vector
The midpoint of the curve can be seen to be located at (u,v) = (α,β); the other four parameters may be varied to create a large variety of curves.
The blancmange curve of parameter w can be obtained by setting , δ = ζ = 0 and η = w. That is:
and
Since the blancmange curve of parameter w = 1 / 4 is the parabola of equation 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
and
Generalizations
It is easy to generalize the definition by using more than two contraction mappings. If one uses n mappings, then the nary decomposition of x has to be used instead of the binary expansion of real numbers. The continuity condition has to be generalized in:
 d_{i}(p_{(n − 1)}) = d_{(i + 1)}(p_{0}), for .
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.
See also
References
 Georges de Rham, On Some Curves Defined by Functional Equations (1957), reprinted in Classics on Fractals, ed. Gerald A. Edgar (AddisonWesley, 1993), pp. 285–298.
 Linas Vepstas, A Gallery of de Rham curves, (2006).
 Linas Vepstas, Symmetries of PeriodDoubling Maps, (2006). (A general exploration of the modular group symmetry in fractal curves.)
Categories: Fractal curves
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