N-curve

N-curve

We take the functional theoretic algebra C[0, 1] of curves. For each loop γ at 1, and each positive integer n, we define a curve γn called n-curve. The n-curves are interesting in two ways.

  1. Their f-products, sums and differences give rise to many beautiful curves.
  2. Using the n-curves, we can define a transformation of curves, called n-curving.

Contents

Multiplicative inverse of a curve

A curve γ in the functional theoretic algebra C[0, 1], is invertible, i.e.

\gamma^{-1} \,

exists if

\gamma(0)\gamma(1) \neq 0. \,

If γ * = (γ(0) + γ(1))e − γ, where e(t)=1, \forall t \in [0, 1], then

\gamma^{-1}= \frac{\gamma^{*}}{\gamma(0)\gamma(1)}.

The set G of invertible curves is a non-commutative group under multiplication. Also the set H of loops at 1 is an Abelian subgroup of G. If \gamma \in H, then the mapping \alpha \to \gamma^{-1}\cdot \alpha\cdot\gamma is an inner automorphism of the group G.

We use these concepts to define n-curves and n-curving.

n-Curves and their products

If x is a real number and [x] denotes the greatest integer not greater than x, then  x-[x] \in [0, 1].

If \gamma \in H and n is a positive integer, then define a curve γn by

\gamma_n (t)=\gamma(nt - [nt]). \,

γn is also a loop at 1 and we call it an n-curve. Note that every curve in H is a 1-curve.

Suppose \alpha, \beta \in H. Then, since \alpha(0)=\beta(1)=1, \mbox{ the f-product } \alpha \cdot \beta = \beta + \alpha -e.

Example 1: Product of the astroid with the n-curve of the unit circle

Let us take u, the unit circle centered at the origin and α, the astroid. The n-curve of “u” is given by,

u_n(t) = \cos(2\pi nt)+ i \sin(2\pi nt) \,

and the astroid is

\alpha(t)=\cos^{3}(2\pi t)+ i \sin^{3}(2\pi t), 0\leq t \leq 1

The parametric equations of their product  \alpha \cdot u_{n} are

x = cos 3(2πt) + cos(2πnt) − 1,
y = sin 3(2πt) + sin(2πnt)

See the figure.

Since both α and un are loops at 1, so is the product.

N-curve with N = 53.
Animation of N-curve for N values from 0 to 50.

Example 2: Product of the unit circle and its n-curve

The unit circle is

 u(t) = \cos(2\pi t)+ i \sin(2\pi t) \,

and its n-curve is

 u_n(t) = \cos(2\pi nt)+ i \sin(2\pi nt) \,

The parametric equations of their product

u \cdot u_{n}

are

x = cos(2πnt) + cos(2πt) − 1,
y = sin(2πnt) + sin(2πt)

See the figure.


Unit Circle with n-Circle.jpg

Example 3: n-Curve of the Rhodonea minus the Rhodonea curve

Let us take the Rhodonea Curve

r = cos(3θ)

If ρ denotes the curve,

 \rho(t) = \cos(6\pi t)[\cos(2\pi t) + i\sin(2\pi t)], 0 \leq t \leq 1

The parametric equations of ρn − ρ are


x = cos(6πnt)cos(2πnt) − cos(6πt)cos(2πt),
 y =  \cos(6\pi nt)\sin(2\pi nt)-\cos(6\pi t)\sin(2\pi t), 0 \leq t \leq 1

Rhodonea Curve.jpgRhodonea-nRhodonea Curve.jpg

n-Curving

If \gamma \in H, then, as mentioned above, the n-curve \gamma_{n} \mbox{ also } \in H. Therefore the mapping \alpha \to \gamma_n^{-1}\cdot \alpha\cdot\gamma_n is an inner automorphism of the group G. We extend this map to the whole of C[0, 1], denote it by \phi_{\gamma_n,e} and call it n-curving with γ. It can be verified that

\phi_{\gamma_n ,e}(\alpha)=\alpha + [\alpha(1)-\alpha(0)](\gamma_{n}-1)e. \

This new curve has the same initial and end points as α.

Example 1 of n-curving

Let ρ denote the Rhodonea curve r = cos(2θ), which is a loop at 1. Its parametric equations are

x = cos(4πt)cos(2πt),
 y = \cos(4\pi t)\sin(2\pi t), 0\leq t \leq 1

With the loop ρ we shall n-curve the cosine curve

c(t)=2\pi t + i \cos(2\pi t),\quad 0 \leq t \leq 1. \,

The curve \phi_{\rho_{n},e}(c) has the parametric equations

x=2\pi[t-1+\cos(4\pi nt)\cos(2\pi nt)], \quad y=\cos(2\pi t)+ 2\pi \cos(4\pi nt)\sin(2\pi nt)


See the figure.

It is a curve that starts at the point (0, 1) and ends at (2π, 1).

Notice how the curve starts with a cosine curve at N=0. Please note that the parametric equation was modified to center the curve at origin.

Example 2 of n-curving

Let χ denote the Cosine Curve

 \chi(t) = 2\pi t +i\cos(2\pi t), 0\leq t \leq 1

With another Rhodonea Curve

ρ = cos(3θ)

we shall n-curve the cosine curve.

The rhodonea curve can also be given as

 \rho(t) = \cos(6\pi t)[\cos (2\pi t)+ i\sin(2\pi t)], 0\leq t \leq 1

The curve \phi_{\rho_{n},e}(\chi) has the parametric equations

x = 2πt + 2π[cos(6πnt)cos(2πnt) − 1],
 y=\cos(2\pi t) + 2\pi \cos( 6\pi nt)\sin(2 \pi nt), 0\leq t \leq 1

See the figure for n = 15.

CosineRhodonea.jpg

Generalized n-Curving

In the FTA C[0, 1] of curves, instead of e we shall take an arbitrary curve β, a loop at 1. This is justified since

L1(β) = L2(β) = 1


Then, for a curve γ in C[0, 1],

γ * = (γ(0) + γ(1))β − γ

and

\gamma^{-1}= \frac{\gamma^{*}}{\gamma(0)\gamma(1)}.

If \alpha \in H, the mapping

\phi_{\alpha_n,\beta}

given by

\phi_{\alpha_n,\beta}(\gamma) = \alpha_n^{-1}\cdot \gamma \cdot \alpha_n
is the n-curving. 

We get the formula

\phi_{\alpha_n ,\beta}(\gamma)=\gamma + [\gamma(1)-\gamma(0)](\alpha_{n}-\beta).

Thus given any two loops α and β at 1, we get a transformation of curve

γ given by the above formula.

This we shall call generalized n-curving.

Example 1

Let us take α and β as the unit circle ``u.’’ and γ as the cosine curve

 \gamma (t) = 4\pi t + i\cos(4\pi t) 0 \leq t \leq 1

Note that γ(1) − γ(0) = 4π

For the transformed curve for n = 40, see the figure.

The transformed curve  \phi_{u_n, u}( \gamma ) has the parametric equations



N-curved cosine.jpg

Example 2

Denote the curve called Crooked Egg by η whose polar equation is

r = cos 3θ + sin 3θ

Its parametric equations are

x = cos(2πt)(cos 3t + sin 3t),
y = sin(2πt)(cos 3t + sin 3t)

Let us take α = η and β = u,

where u is the unit circle.

The n-curved Archimedean spiral has the parametric equations

x = 2πtcos(2πt) + 2π[(cos 3nt + sin 3nt)cos(2πnt) − cos(2πt)],
y = 2πtsin(2πt) + 2π[(cos 3nt) + sin 3nt)sin(2πnt) − sin(2πt)]

See the figures, the Crooked Egg and the transformed Spiral for n = 20.


Crooked Egg.jpg Crooked Spiral.jpg


See also


References

  • Sebastian Vattamattam, "Transforming Curves by n-Curving", in Bulletin of Kerala Mathematics Association, Vol. 5, No. 1, December 2008

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