circular algebraic curve
{{Short description|Plane algebraic curve}}
{{No footnotes|date=October 2015}}
In geometry, a circular algebraic curve is a type of plane algebraic curve determined by an equation F(x, y) = 0, where F is a polynomial with real coefficients and the highest-order terms of F form a polynomial divisible by x2 + y2. More precisely, if
F = Fn + Fn−1 + ... + F1 + F0, where each Fi is homogeneous of degree i, then the curve F(x, y) = 0 is circular if and only if Fn is divisible by x2 + y2.
Equivalently, if the curve is determined in homogeneous coordinates by G(x, y, z) = 0, where G is a homogeneous polynomial, then the curve is circular if and only if G(1, i, 0) = G(1, −i, 0) = 0. In other words, the curve is circular if it contains the circular points at infinity, (1, i, 0) and (1, −i, 0), when considered as a curve in the complex projective plane.
Multicircular algebraic curves
An algebraic curve is called p-circular if it contains the points (1, i, 0) and (1, −i, 0) when considered as a curve in the complex projective plane, and these points are singularities of order at least p. The terms bicircular, tricircular, etc. apply when p = 2, 3, etc. In terms of the polynomial F given above, the curve F(x, y) = 0 is p-circular if Fn−i is divisible by (x2 + y2)p−i when i < p. When p = 1 this reduces to the definition of a circular curve. The set of p-circular curves is invariant under Euclidean transformations. Note that a p-circular curve must have degree at least 2p.
{{cn-span|date=September 2020|The set of p-circular curves of degree p + k, where p may vary but k is a fixed positive integer, is invariant under inversion.}} When k is 1 this says that the set of lines (0-circular curves of degree 1) together with the set of circles (1-circular curves of degree 2) form a set which is invariant under inversion.
Examples
- The circle is the only circular conic.
- Conchoids of de Sluze (which include several well-known cubic curves) are circular cubics.
- Cassini ovals (including the lemniscate of Bernoulli), toric sections and limaçons (including the cardioid) are bicircular quartics.
- Watt's curve is a tricircular sextic.
Footnotes
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References
- {{in lang|fr}} [http://www.mathcurve.com/courbes2d/circulaire/circulaire.shtml "Courbe Algébrique Circulaire" at Encyclopédie des Formes Mathématiques Remarquables]
- {{in lang|fr}} [http://www.mathcurve.com/courbes2d/multicirculaire/multicirculaire.shtml "Courbe Algébrique Multicirculaire" at Encyclopédie des Formes Mathématiques Remarquables]
- [http://www.2dcurves.com/algebraic.html Definition at 2dcurves.com]