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

 

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

${\displaystyle \nu :\mathbf {P} ^{1}\to \mathbf {P} ^{3}}$ 

which assigns to the homogeneous coordinate ${\displaystyle [S:T]}$  the value

${\displaystyle \nu :[S:T]\mapsto [S^{3}:S^{2}T:ST^{2}:T^{3}].}$ 

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

${\displaystyle \nu :x\mapsto (x,x^{2},x^{3})}$ 

That is, it is the closure by a single point at infinity of the affine curve ${\displaystyle (x,x^{2},x^{3})}$ .

The twisted cubic is a projective variety, defined as the intersection of three quadrics. In homogeneous coordinates ${\displaystyle [X:Y:Z:W]}$  on P³, the twisted cubic is the closed subscheme defined by the vanishing of the three homogeneous polynomials

${\displaystyle F_{0}=XZ-Y^{2}}$ 

${\displaystyle F_{1}=YW-Z^{2}}$ 

${\displaystyle F_{2}=XW-YZ.}$ 

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 ${\displaystyle XZ-Y^{2}}$  and ${\displaystyle Z(YW-Z^{2})-W(XW-YZ)}$ , 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 ${\displaystyle (YW-Z^{2})^{2}}$  is in it, but ${\displaystyle YW-Z^{2}}$  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

• Harris, Joe (1992), Algebraic Geometry, A First Course, New York: Springer-Verlag, ISBN 0-387-97716-3.