A Strophoid of a Circle with the Pole at the Center of the Circle and the fixed point
on the Circumference of the Circle. In a paper published by the London Mathematical Society in 1879, T. J. Freeth
described it and various other Strophoids (MacTutor Archive). If the line through Parallel
to the *y*-Axis cuts the Nephroid at , then Angle is , so this curve can be used to
construct a regular Heptagon. The Polar equation is

**References**

Lawrence, J. D. *A Catalog of Special Plane Curves.* New York: Dover, pp. 175 and 177-178, 1972.

MacTutor History of Mathematics Archive. ``Freeth's Nephroid.'' http://www-groups.dcs.st-and.ac.uk/~history/Curves/Freeths.html.

© 1996-9

1999-05-26