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We reflect across the plane through (), (), and the north pole, forming a closed curve containing antipodal points , with length () = (). A curve connecting ± p {\displaystyle \pm p} has length at least π {\displaystyle \pi } , which is the length of the great semicircle between ± p {\displaystyle \pm p} .
The + and invariants keep track of how curves change under these transformations and deformations. The + invariant increases by 2 when a direct self-tangency move creates new self-intersection points (and decreases by 2 when such points are eliminated), while decreases by 2 when an inverse self-tangency move creates new intersections (and increases by 2 when they are eliminated).
Two Dimensional Curves; Visual Dictionary of Special Plane Curves; Curves and Surfaces Index (Harvey Mudd College) National Curve Bank; An elementary treatise on cubic and quartic curves by Alfred Barnard Basset (1901) online at Google Books
A plane curve is the image of any continuous function from an interval to the Euclidean plane.Intuitively, it is a set of points that could be traced out by a moving point. More specifically, smooth curves generally at least require that the function from the interval to the plane be continuously differentiable, and in some contexts are defined to require higher derivative
Both are algebraic curves of degree 2. For any number n of foci, the n-ellipse is a closed, convex curve. [2]: (p. 90) The curve is smooth unless it goes through a focus. [5]: p.7 The n-ellipse is in general a subset of the points satisfying a particular algebraic equation. [5]:
A Jordan curve or a simple closed curve in the plane R 2 is the image C of an injective continuous map of a circle into the plane, φ: S 1 → R 2. A Jordan arc in the plane is the image of an injective continuous map of a closed and bounded interval [a, b] into the plane. It is a plane curve that is not necessarily smooth nor algebraic.
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A plane curve can often be represented in Cartesian coordinates by an implicit equation of the form (,) = for some specific function f.If this equation can be solved explicitly for y or x – that is, rewritten as = or = for specific function g or h – then this provides an alternative, explicit, form of the representation.