Freeth's Nephroid
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. Freethdescribed it and various other MacTutor Archive). If the line through Parallelto the y-Axis cuts the Nephroid at 
, then Angle 
is 
, so this curve can be used toconstruct a regular Heptagon. The Polar equation is
See also Strophoid
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.
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