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Concept
Rational normal curve
Résumé
In mathematics, the rational normal curve is a smooth, rational curve C of degree n in projective n-space Pn. It is a simple example of a projective variety; formally, it is the Veronese variety when the domain is the projective line. For n = 2 it is the plane conic Z0Z2 = Z, and for n = 3 it is the twisted cubic. The term "normal" refers to projective normality, not normal schemes. The intersection of the rational normal curve with an affine space is called the moment curve.
The rational normal curve may be given parametrically as the image of the map
which assigns to the homogeneous coordinates [S : T] the value
In the affine coordinates of the chart x0 ≠ 0 the map is simply
That is, the rational normal curve is the closure by a single point at infinity of the affine curve
Equivalently, rational normal curve may be understood to be a projective variety, defined as the common zero locus of the homogeneous polynomials
where are the homogeneous coordinates on Pn. The full set of these polynomials is not needed; it is sufficient to pick n of these to specify the curve.
Let be n + 1 distinct points in P1. Then the polynomial
is a homogeneous polynomial of degree n + 1 with distinct roots. The polynomials
are then a basis for the space of homogeneous polynomials of degree n. The map
or, equivalently, dividing by G(S, T)
is a rational normal curve. That this is a rational normal curve may be understood by noting that the monomials
are just one possible basis for the space of degree n homogeneous polynomials. In fact, any basis will do. This is just an application of the statement that any two projective varieties are projectively equivalent if they are congruent modulo the projective linear group PGLn + 1(K) (with K the field over which the projective space is defined).
This rational curve sends the zeros of G to each of the coordinate points of Pn; that is, all but one of the Hi vanish for a zero of G. Conversely, any rational normal curve passing through the n + 1 coordinate points may be written parametrically in this way.
Source officielle
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