Nicomedes conchoid

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A plane algebraic curve of order 4 whose equation in Cartesian rectangular coordinates has the form


and in polar coordinates

$$\rho=\frac{a}{\sin\psi}\pm l.$$

Nicomedes conchoid with parameters (a,l)=(2,3)

Outer branch (see Fig.). Asymptote $x=a$. Two points of inflection, $B$ and $C$.

Inner branch. Asymptote $x=a$. The coordinate origin is a double point whose character depends on the values of $a$ and $l$. For $l<a$ it is an isolated point and, in addition, the curve has two points of inflection, $E$ and $F$; for $l>a$ it is a node; for $l=a$ it is a cusp. The curve is a conchoid of the straight line $x=a$.

The curve is named after Nicomedes (3rd century B.C.), who used it to solve the problem of trisecting an angle.


[1] A.A. Savelov, "Planar curves" , Moscow (1960) (In Russian)
[a1] J.D. Lawrence, "A catalog of special plane curves" , Dover, reprint (1972)
How to Cite This Entry:
Nicomedes conchoid. 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