Ribaucour curve

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A planar curve with curvature radius $R$ at an arbitrary point $M$ proportional to the length of the segment of the normal $MP$ (see Fig.).

Figure: r081760a

The equation for the Ribaucour curve in Cartesian orthogonal coordinates is


where $n=MP/R$. If $n=1/h$ ($h$ is any integer), then a parametric equation for the Ribaucour curve is

$$x=(m+1)C\int\limits_0^t\sin^{m+1}tdt,\quad y=C\sin^{m+1}t,$$

where $m=-(n+1)n$. When $m=0$, the Ribaucour curve is a circle; when $m=1$, it is a cycloid; when $m=-2$, it is a catenary; and when $m=-3$, it is a parabola.

The length of an arc of the curve is


and the curvature radius is


This curve was studied by A. Ribaucour in 1880.


[1] A.A. Savelov, "Planar curves" , Moscow (1960) (In Russian)
[2] P.K. Rashevskii, "A course of differential geometry" , Moscow (1956) (In Russian)



[a1] F. Gomes Teixeira, "Traité des courbes" , 1–3 , Chelsea, reprint (1971)
How to Cite This Entry:
Ribaucour curve. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by D.D. Sokolov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article