Twisted cubic

In mathematics, a twisted cubic is a smooth, rational curve C of degree three in projective 3-space P³. It is a fundamental example of a skew curve. It is essentially unique, up to projective transformation (the twisted cubic, therefore). In algebraic geometry, the twisted cubic is a simple example of a projective variety that is not linear or a hypersurface, in fact not a complete intersection. It is the three-dimensional case of the rational normal curve, and is the image of a Veronese map of degree three on the projective line.

Definition

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The twisted cubic is most easily given parametrically as the image of the map

:<math>\nu:\mathbf{P}^1\to\mathbf{P}^3</math>

which assigns to the homogeneous coordinate <math>[S:T]</math> the value

:<math>\nu:[S:T] \mapsto [S^3:S^2T:ST^2:T^3].</math>

In one coordinate patch of projective space, the map is simply the moment curve

:<math>\nu:x \mapsto (x,x^2,x^3)</math>

That is, it is the closure by a single point at infinity of the affine curve <math>(x,x^2,x^3)</math>.

The twisted cubic is a projective variety, defined as the intersection of three quadrics. In homogeneous coordinates <math>[X:Y:Z:W]</math> on P³, the twisted cubic is the closed subscheme defined by the vanishing of the three homogeneous polynomials

:<math>F_0 = XZ - Y^2</math>

:<math>F_1 = YW - Z^2</math>

:<math>F_2 = XW - YZ.</math>

It may be checked that these three quadratic forms vanish identically when using the explicit parameterization above; that is, substitute x³ for X, and so on.

More strongly, the homogeneous ideal of the twisted cubic C is generated by these three homogeneous polynomials of degree 2.

Properties

The twisted cubic has the following properties:

• It is the set-theoretic complete intersection of <math>XZ - Y^2</math> and <math>Z(YW-Z^2)-W(XW-YZ)</math>, but not a scheme-theoretic or ideal-theoretic complete intersection; meaning to say that the ideal of the variety cannot be generated by only 2 polynomials; a minimum of 3 are needed. (An attempt to use only two polynomials make the resulting ideal not radical, since <math>(YW-Z^2)^2</math> is in it, but <math>YW-Z^2</math> is not).
• Any four points on C span P³.
• Given six points in P³ with no four coplanar, there is a unique twisted cubic passing through them.
• The union of the tangent and secant lines (the secant variety) of a twisted cubic C fill up P³ and the lines are pairwise disjoint, except at points of the curve itself. In fact, the union of the tangent and secant lines of any non-planar smooth algebraic curve is three-dimensional. Further, any smooth algebraic variety with the property that every length four subscheme spans P³ has the property that the tangent and secant lines are pairwise disjoint, except at points of the variety itself.
• The projection of C onto a plane from a point on a tangent line of C yields a cuspidal cubic.
• The projection from a point on a secant line of C yields a nodal cubic.
• The projection from a point on C yields a conic section.

References

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