Bicorn

For the hat, see Bicorne.

For the mythical beast, see Bicorn (legendary creature).

File:Bicorn.svg

Bicorn

In geometry, the bicorn, also known as a cocked hat curve due to its resemblance to a bicorne, is a rational quartic curve defined by the equation

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

It has two cusps and is symmetric about the y-axis.

History

In 1864, James Joseph Sylvester studied the curve

$y^4-xy^3-8xy^2+36x^2y+16x^2-27x^3=0$

in connection with the classification of quintic equations; he named the curve a bicorn because it has two cusps. This curve was further studied by Arthur Cayley in 1867.

Properties

File:Bicorn-inf.jpg

A transformed bicorn with a = 1

The bicorn is a plane algebraic curve of degree four and genus zero. It has two cusp singularities in the real plane, and a double point in the complex projective plane at x=0, z=0 . If we move x=0 and z=0 to the origin substituting and perform an imaginary rotation on x bu substituting ix/z for x and 1/z for y in the bicorn curve, we obtain

$(x^2-2az+a^2z^2)^2 = x^2+a^2z^2.\,$

This curve, a limaçon, has an ordinary double point at the origin, and two nodes in the complex plane, at x = ± i and z=1.

The parametric equations of a bicorn curve are:

$x = a \sin(\theta)$ and $y = \frac{\cos^2(\theta) \left(2+\cos(\theta)\right)}{3+\sin^2(\theta)}$ with $-\pi\le\theta\le\pi$

See also

• List of curves

References

• "Bicorn" at The MacTutor History of Mathematics archive
• Weisstein, Eric W., "Bicorn", MathWorld.
• "Bicorne" at Encyclopédie des Formes Mathématiques Remarquables
• The Collected Mathematical Papers of James Joseph Sylvester. Vol. II Cambridge (1908) p. 468 (online)