Sinusoidal spiral

In geometry, the sinusoidal spirals are a family of curves defined by the equation in polar coordinates

:$r^n\; =\; a^n\; cos(n\; heta),$

where "a" is a nonzero constant and "n" is a rational number other than 0. With a rotation about the origin, this can also be written

:$r^n\; =\; a^n\; sin(n\; heta).,$

The term "spiral" is a misnomer because they not actually spirals and often have a flower-like shape. Many well known curves are sinusoidal spirals including:
* Line ("n" = −1)
* Circle ("n" = 1)
* Equilateral hyperbola ("n" = −2)
* Parabola ("n" = −1/2)
* Cardioid ("n" = 1/2)
* Lemniscate of Bernoulli ("n" = 2)

The curves were first studied by Colin Maclaurin.

Properties

The inverse of a sinusoidal spiral with respect to a circle with center at the origin is another sinusoidal spiral whose value of "n" is the negative of the original curve's value of "n". For example, the inverse of the lemniscate of Bernoulli is a hyperbola.

The isoptic, pedal and negative pedal of a sinusoidal spiral are different sinusoidal spirals.

One path of a particle moving according to a central force proportional to a power of "r" is a sinusoidal spiral.

When "n" is an integer, and "n" points are arranged regularly on a circle of radius "a", then the set of points so that the geometric mean of the distances from the point to the n points is a is a sinusoidal spiral. In this case the sinusoidal spiral is a polynomial lemniscate

References

*Yates, R. C.: "A Handbook on Curves and Their Properties", J. W. Edwards (1952), "Spiral" p. 213–214
* [http://www.2dcurves.com/spiral/spirals.html "Sinusoidal spiral" at www.2dcurves.com]
* [http://www-groups.dcs.st-and.ac.uk/~history/Curves/Sinusoidal.html "Sinusoidal Spirals" at The MacTutor History of Mathematics]
* [http://www.mathcurve.com/courbes2d/spiralesinusoidale/spiralesinusoidale.shtml "Sinusoidal spiral" at Encyclopédie des Formes Mathématiques Remarquables]
*MathWorld |title=Sinusoidal Spiral |urlname=SinusoidalSpiral