卡西尼卵形線

喺幾何上，卡西尼卵形線（hippopede ，嚟自古希臘文ἱπποπέδη，意思係「horse fetter」）係一條平面曲線，由呢個方程定義

$(x^2+y^2)^2=cx^2+dy^2$,

where it is assumed that c>0 and c>d since the remaining cases either reduce to a single point or can be put into the given form with a rotation. Hippopedes are bicircular rational algebraic curves of degree 4 and symmetric with respect to both the x and y axes. When d>0 the curve has an oval form and is often known as an oval of Booth, and when d<0 the curve resembles a sideways figure eight, or lemniscate, and is often known as a lemniscate of Booth, after James Booth (1810–1878) who studied them. Hippopedes were also investigated by Proclus (for whom they are sometimes called Hippopedes of Proclus) and Eudoxus. For d = −c, the hippopede corresponds to the lemniscate of Bernoulli.

作為環面嘅截面

Hippopedes with a = 1, b = 0.1, 0.2, 0.5, 1.0, 1.5, and 2.0.

Hippopedes with b = 1, a = 0.1, 0.2, 0.5, 1.0, 1.5, and 2.0.

Hippopedes can be defined as the curve formed by the intersection of a torus and a plane, where the plane is parallel to the axis of the torus and tangent to it on the interior circle. Thus it is a spiric section which in turn is a type of toric section.

If a circle with radius a is rotated about an axis at distance b from its center, then the equation of the resulting hippopede in polar coordinates

$r^2 = 4 b (a- b \sin^{2} \theta)\,$

or in Cartesian coordinates

$(x^2+y^2)^2+4b(b-a)(x^2+y^2)=4b^2x^2$.

Note that when a>b the torus intersects itself, so it does not resemble the usual picture of a torus.

參考

• Lawrence JD. (1972) Catalog of Special Plane Curves, Dover. Pp. 145–146.
• Booth J. A Treatise on Some New Geometrical Methods, Longmans, Green, Reader, and Dyer, London, Vol. I (1873) and Vol. II (1877).
• Template:MathWorld
• "Hippopede" at 2dcurves.com
• "Courbes de Booth" at Encyclopédie des Formes Mathématiques Remarquables

睇埋

• 曲線一覽

出面網頁

• "The Hippopede of Proclus" at The National Curve Bank