Freeth's Nephroid

Polar equation: r = a(1 + 2sin(theta/2))

Click below to see one of the Associated curves.

Definitions of the Associated curves

Evolute       Involute 1       Involute 2

 Inverse curve wrt origin      Inverse wrt another circle

Pedal curve wrt origin      Pedal wrt another point

Negative pedal curve wrt origin      Negative pedal wrt another point

   Caustic wrt horizontal rays      Caustic curve wrt another point

This is a strophoid of a circle with the pole O at the centre of the circle and the fixed point P on the circumference of the circle.

In the picture above, O is the origin and P is the node where the curve crosses itself three times.

If the line through P parallel to the y-axis cuts the nephroid at A then angle AOP is 3pi/7. This can be used to construct a regular 7 sided figure.

T J Freeth (1819-1904) was an English mathematician. In a paper published by the London Mathematical Society in 1879 he described various strophoids, including the strophoid of a trisectrix.


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JOC/EFR February 1996