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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
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.
A transformed bicorn with a = 1
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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
J. Dennis Lawrence (1972). A catalog of special plane curves. Dover Publications. pp. 147–149. ISBN 0-486-60288-5.
"Bicorn" at The MacTutor History of Mathematics archive
Weisstein, Eric W., "Bicorn" from MathWorld.
"Bicorne" at Encyclopédie des Formes Mathématiques Remarquables
The Collected Mathematical Papers of James Joseph Sylvester. Vol. II Cambridge (1908) p. 468 (online)
Undergraduate Texts in Mathematics
Graduate Studies in Mathematics
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