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In mathematics, a projective line is a one-dimensional projective space. The projective line over a field K, denoted P1(K), may be defined as the set of one-dimensional subspaces of the two-dimensional vector space K2 (it does carry other geometric structures).
An arbitrary point in the projective line P1(K) may be given in homogeneous coordinates by a pair
of points in K which are not both zero. Two such pairs are equal if they differ by an overall (nonzero) factor λ:
Line extended by a point at infinity
The projective line may be identified with the line K extended by a point at infinity. More precisely, the line K may be identified with the subset of P1(K) given by
This subset covers all points in P1(K) except one, which is called the point at infinity:
This allows to extend the arithmetic on K to P1(K) by the formulas
Translating this arithmetic in term of homogeneous coordinates gives, when both sides are defined:
Real projective line
The projective line over the real numbers is called the real projective line. It may also be thought of as the line K together with an idealised point at infinity ∞ ; the point connects to both ends of K creating a closed loop or topological circle.
Compare the extended real number line, which distinguishes ∞ and −∞.
Complex projective line: the Riemann sphere
Adding a point at infinity to the complex plane results in a space that is topologically a sphere. Hence the complex projective line is also known as the Riemann sphere (or sometimes the Gauss sphere). It is in constant use in complex analysis, algebraic geometry and complex manifold theory, as the simplest example of a compact Riemann surface.
For a finite field
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The case of K a finite field F is also simple to understand. In this case if F has q elements, the projective line has
- q + 1
elements. We can write all but one of the subspaces as
- y = ax
with a in F; this leaves out only the case of the line x = 0. For a finite field there is a definite loss if the projective line is taken to be this set, rather than an algebraic curve — one should at least see the underlying infinite set of points in an algebraic closure as potentially on the line.
Quite generally, the group of homographies with coefficients in K acts on the projective line P1(K). This group action is transitive, so that P1(K) is a homogeneous space for the group, often written PGL2(K) to emphasise the projective nature of these transformations. Transitivity says that any point Q may be transformed to any other point R by a homography. The point at infinity on P1(K) is therefore an artifact of choice of coordinates: homogeneous coordinates
- [X:Y] = [tX:tY]
express a one-dimensional subspace by a single non-zero point (X,Y) lying in it, but the symmetries of the projective line can move the point ∞ = [1:0] to any other, and it is in no way distinguished.
Much more is true, in that some transformation can take any given distinct points Qi for i = 1,2,3 to any other 3-tuple Ri of distinct points (triple transitivity). This amount of specification 'uses up' the three dimensions of PGL2(K); in other words, the group action is sharply 3-transitive. The computational aspect of this is the cross-ratio. Indeed, a generalized converse is true: a sharply 3-transitive group action is always (isomorphic to) a generalized form of a PGL2(K) action on a projective line, replacing "field" by "KT-field" (generalizing the inverse to a weaker kind of involution), and "PGL" by a corresponding generalization of projective linear maps.
As algebraic curve
The projective line is a fundamental example of an algebraic curve. From the point of view of algebraic geometry, P1(K) is a non-singular curve of genus 0. If K is algebraically closed, it is the unique such curve over K, up to isomorphism. In general (non-singular) curves of genus 0 are isomorphic over K to a conic C, which is the projective line if and only if C has a point defined over K; geometrically such a point P can be used as origin to make clear the correspondence using lines through P.
The function field of the projective line is the field K(T) of rational functions over K, in a single indeterminate T. The field automorphisms of K(T) over K are precisely the group PGL2(K) discussed above.
One reason for the great importance of the projective line is that any function field K(V) of an algebraic variety V over K, other than a single point, will have a subfield isomorphic with K(T). From the point of view of birational geometry, this means that there will be a rational map from V to P1(K), that is not constant. The image will omit only finitely many points of P1(K), and the inverse image of a typical point P will be of dimension dim V − 1. This is the beginning of methods in algebraic geometry that are inductive on dimension. The rational maps play a role analogous to the meromorphic functions of complex analysis, and indeed in the case of compact Riemann surfaces the two concepts coincide.
If V is now taken to be of dimension 1, we get a picture of a typical algebraic curve C presented 'over' P1(K). Assuming C is non-singular (which is no loss of generality starting with K(C)), it can be shown that such a rational map from C to P1(K) will in fact be everywhere defined. (That is not the case if there are singularities, since for example a double point where a curve crosses itself may give an indeterminate result after a rational map.) This gives a picture in which the main geometric feature is ramification.
Many curves, for example hyperelliptic curves, are best presented abstractly, as ramified covers of the projective line. According to the Riemann–Hurwitz formula, the genus then depends only on the type of ramification.
A rational curve is a curve of genus 0, so any curve in the birational class of the projective line (see rational variety). A rational normal curve in projective space Pn is a rational curve that lies in no proper linear subspace; it is known that there is essentially one example, given parametrically in homogeneous coordinates as
See twisted cubic for the first interesting case.
- Action of PGL(2) on Projective Space – see comment and cited paper.