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Freeth’s nephroid
T.J. Freeth, 1878
Each line through the centre O of a circle crosses the circle at a point K; mark on it the two points whose distance from K equals the distance from K to a fixed point A of the circle. These marks make Freeth’s nephroid, one of the strophoids Freeth described in 1878. It can be used to construct the regular heptagon.
r = a[1 + 2 sin(θ/2)]
On mathcurve.com as Néphroïde de Freeth