In algebraic geometry, an algebraic curve is an algebraic variety of dimension one. The theory of these curves in general was quite fully developed in the nineteenth century, after many particular examples had been considered, starting with circles and other conic sections.

Plane algebraic curves

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An algebraic curve defined over a field F may be considered as the locus of points in Fⁿ determined by n-1 independent polynomial functions in n variables with coefficients in F, g_i(x₁, ..., x_n), where the curve is definied by setting each g_i=0.

Using the resultant, we can eliminate all but two of the varibles and reduce the curve to a birationally equivalent plane curve, f(x,y) = 0, still with coefficients in F, but usually of higher degree, and often possessing additional singularities. For example, eliminating z between the two equations x²+y²-z²=0 and x+2y+3z-1=0, which defines an intersection of a cone and a plane in three dimensions, we obtain the conic section 8x² + 5y²-4xy+2x+4y-1=0, which in this case is an ellipse. If we eliminate z between 4x²+y²-z² = 1 and z=x², we obtain y²=x⁴ - 4x² +1, which is the equation of an hyperelliptic curve.

Projective curves

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It is often desirable to consider that curves are a locus of points in projective space. In the set of equations g_i=0, we can replace each x_k with x_k/x₀, and multiply by x₀ⁿ, where n is the degree of g_i. In this way we obtain n-1 homogeneous polynomial functions, which define the corresponding curve in projective space. For a plane algebraic curve we have a single equation f(z,y,z)=0, where f is homogeneous; for example, the Fermat curve xⁿ+y^{n+zn=0 is a projective curve.}

Algebraic function fields

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The study of algebraic curves can be reduced to the study of irreducible algbebraic curves. Up to birational equivalence, these are categorically equivalent to algebraic function fields. An algebraic function field is a field of algebraic functions in one variable K defined over a given field F. This means there exists an element x of K which is transcendental over F, and such that K is a finite algebraic extension of F(x), which is the field of rational functions in the indeterminate x over F.

For example, consider the field C of complex numbers, over which we may define the field C(x) of rational functions in C. If y² = x³-x-1, then the field C(x,y) is an elliptic function field. The element x is not uniquely determined; the field can also be regarded, for instance, as an extension of C(y). The algebraic curve corresponding to the function field is simply the set of points (x,y) in C² satisfying y² = x³-x-1.

If the field F is not algebraically closed, the point of view of function fields is a little more general than that of considering the locus of points, since we include, for instance, "curves" with no points on them. If the base field F is the field R of real numbers, then x²+y²=-1 defines an algebraic extension field of R(x), but the corresponding curve considered as a locus has no points in R. However, it does have points defined over the algebraic closure C of R.

Complex curves and real surfaces

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A complex projective algebraic curve resides in n-dimensional complex projective space CPⁿ. This has complex dimension n, but topological dimension, as a real manifold, 2n, and is compact, connected, and orientable. An algebraic curve likewise has topological dimension two; in other words, it is a surface. A nonsingular n-dimensional complex projective algebraic curve will then be a smooth orientable surface as a real manifold, embedded in a compact real manifold of dimension 2n which is CPⁿ regarded as a real manifold. The topological genus of this surface, that is the number of handles or donut holes, is the genus of the curve. By considering the complex analytic structure induced on this compact surface we are led to the theory of compact Riemann surfaces.

Compact Riemann surfaces

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A Riemann surface is a connected complex analytic manifold of one complex dimension, which makes it a connected real manifold of two dimensions. It is compact if it is compact as a topological space.

There is a triple equivalence of categories between the category of irreducible projective algebraic curves over the complex numbers, the category of compact Riemann surfaces, and the category of complex algebraic function fields, so that in studying these subjects we are in a sense studying the same thing. This allows complex analytic methods to be used in algebraic geometry, and algebraic-geometric methods in complex analysis, and field-theoretic methods to be used in both, which is characteristic of a much wider class of problems than simply curves and Riemann surfaces.

Singularities

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Using the intrinsic concept of tangent space, points P on an algebraic curve C are classified as smooth or non-singular, or else singular. Given n-1 homogeneous polynomial functions in n+1 variables, we may find the Jacobian matrix as the (n-1)x(n+1) matrix of partial derivatives. If the rank of this matrix at a point P on the curve has the maximal value of n-1, then the point is a smooth point. In particular, if the curve is a plane projective algebraic curve, defined by a single homogeneous polynomial equation f(x, y, z)=0, then the singular points are precisely the points P where the rank of the 1 by n+1 matrix is zero, that is, where

$\backslash frac\{\; \backslash partial\; f\; \}\{\; \backslash partial\; x\; \}(P)=\backslash frac\{\; \backslash partial\; f\; \}\{\; \backslash partial\; y\; \}(P)=\backslash frac\{\; \backslash partial\; f\; \}\{\; \backslash partial\; z\; \}(P)=0.$.

Since f is a polynomial, this definition is purely algebraic and makes no assumption about the nature of the field F, which in particular need not be the real or complex numbers. It should of course be recalled that (0,0,0) is not a point of the curve and hence not a singular point.

The singularities of a curve are not birational invariants. However, locating and classifying the singularities of a curve is one way of computing the genus, which is a birational invariant. For this to work, we should consider the curve projectively and require F to be algebraically closed, so that all the singularities which belong to the curve are considered.

Classification of singularities

Singular points include multiple points where the curve crosses over itself, and also various types of cusp, for example that shown by the curve with equation x³ = y² at (0,0).

A curve C has at most a finite number of singular points. If it has none, it can be called smooth or non-singular. For this definition to be correct, we must use an algebraically closed field and a curve C in projective space (i.e. complete in the sense of algebraic geometry). If for example we simply look at a curve in the real affine plane there might be singular points 'at infinity', or that needed complex number co-ordinates for their expression.

Given a curve, we can put it into projective form, and move any singularity to the origin. For an irreducible algebraic curve defined over the real or complex numbers, we can define a real 3-sphere of radius ε around the origin by |x|²+|y|² = ε for complex values x and y. The complex curve, which defines a real surface, intersects this 3-sphere in an embedded curve. If ε is small enough, then this curve is smooth, and consists of a finite number r of linked components. The curve is a link, with the number of components r being the number of analytic branches, defined by Puiseux series, passing through the origin. If r=1, so that we have only one component, then the link is a knot.

The delta invariant of the singularity, δ, is the unknotting number or Gordian number of the link, which is the minimum number of times one needs to pass the link through itself to completely unlink and unknot it. In purely algebreo-geometric terms, it can be described as the dimension of the integral closure of the stalk at the point P modulo the stalk, or alternatively as the sum of m(m-1)/2, where m is the multiplicity, over all infinitely near singular points Q lying over the singular point P. Intutively, a singular point with delta invariant δ concentrates δ ordinary double points at P.

The Milnor number μ of the singularity is the degree of the mapping grad f(x,y)/|grad f(x,y)| on the small sphere of radius ε, in the sense of the topological degree of a continuous mapping, where grad f is the (complex) gradient vectorfield of f. It is related to δ and r by μ = 2δ-r+1. Another singularity invariant of note is the multiplicity m, defined as the maximum integer such that

$\backslash frac\{\; \backslash partial^m\; f\; \}\{\; \backslash partial\; x\; \}(P)=\backslash frac\{\; \backslash partial^m\; f\; \}\{\; \backslash partial\; y\; \}(P)=\backslash frac\{\; \backslash partial^m\; f\; \}\{\; \backslash partial\; z\; \}(P)=0.$.

Computing the delta invariants of all of the singularities allows the genus g of the curve to be determined; if d is the degree, then

$g\; =\; \backslash frac\{1\}\{2\}(d-1)(d-2)\; -\; \backslash sum\_P\; \backslash delta\_P,$

where the sum is taken over all singular points P of the complex projective plane curve.

Singularities may be classified by the triple δ, r, where m is the multiplicity, δ is the delta-invariant, and r is the branching number. In these terms, an ordinary cusp is a point with invariants and an ordinary double point is a point with invariants [2,1,2. An ordinary n-multiple point may be defined as one having invariants n(n-1)/2, n.

Examples of curves

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Rational curves

A rational curve, also called a unicursal curve, is any curve which is birationally equivalent to a line, which we may take to be a projective line and identify with the field of rational functions in one indeterminate F(x). If F is algebraically closed, this is equivalent to a curve of genus zero; however the field R(x,y) with x²+y²=-1 is a field of genus zero which is not a rational function field.

Concretely, a rational curve of dimension n over F can be parametrized (except for isolated exceptional points) by means of n rational functions defined in terms of a single parameter t; by clearing denominators we can turn this into n+1 polynomial functions in projective space. An example would be the rational normal curve.

Any conic section defined over F with a rational point in F is a rational curve. It can be parametrized by drawing a line with slope t through the rational point, and intersection with the plane quadradic curve; this gives a polynomial with F-rational coefficients and one F-rational root, hence the other root is F-rational (ie, belongs to F) also. For example, consider the ellipse x² + xy + y² = 1, where (-1, 0) is a rational point. Drawing a line with slope t from (-1,0), y=t(x+1), substituting it in the equation of the ellipse, factoring, and solving for x, we obtain

$x\; =\; \backslash frac\{1-t^2\}\{1+t+t^2\}.$

We then have that the equation for y is

$y=t(x+1)=\backslash frac\{t(t+2)\}\{1+t+t^2\}$

which defines a rational parametrization of the ellipse and hence shows the ellipse is a a rational curve. All points of the ellipse are given, except for (-1,1), which corresponds to $t=\backslash infty$; the entire curve is parametrized therefore by the real projective line.

Viewing rational parametrizations with rational coefficients projectively, we can view them as giving number theoretical information about homogeneous equations defined over the integers. For example from the above, we obtain

$X=1-t^2,\; Y=t(t+2),\; Z=t^2+t+1$

for which

$X^2+XY+Y^2=Z^2$

is true for integer X, Y and Z if t is an integer. Hence we obtain triangles with integer length sides, such as sides of length 3, 7, and 8, where one of the angles is 60 degrees, from relationships such as $8^2-3\; \backslash cdot\; 8\; +\; 3^2=7^2$.

Many of the curves on Wikipedia's list of curves are rational, and hence have similar rational parametrizations.

Elliptic curves

An elliptic curve may be defined as any curve of genus one, but often it is further required that it be a nonsingular cubic curve, which sufficies to model any genus one curve. A further restriction that it have an inflection point at infinity is also often imposed; this amounts to requiring that it can be written in Tate-Weierstrass form, which in its projective version is

$y^2z\; +\; a\_1\; xyz\; +\; a\_3\; yz^2\; =\; x^3\; +\; a\_2\; x^2z\; +\; a\_4\; xz^2\; +\; a\_6\; z^3.$

Elliptic curves carry the structure of an abelian group, which in terms of curves defined over the complex numbers is the additive group of the complex plane modulo the period lattice of the corresponding elliptic functions.

Curves of genus greater than one

Curves of genus greater than one differ markedly from both rational and elliptic curves. Such curves defined over the rational numbers, by Faltings' theorem, can have only a finite number of rational points, and they may be viewed as having a hyperbolic geometry structure. Examples are the hyperelliptic curves, the Klein quartic curve, and the Fermat curve xⁿ+yⁿ=zⁿ when n is greater than three.

See also

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• Bézout's theorem
• Birational geometry
• Conic section
• Cubic plane curve
• Curve
• Elliptic curve
• Faltings' theorem
• Fractional ideal
• Function field of an algebraic variety
• Function field (scheme theory)
• Genus (mathematics)
• Hilbert's sixteenth problem
• Hyperelliptic curve
• Jacobian variety
• Klein quartic
• List of curves
• Quartic plane curve
• Rational normal curve
• Riemann-Hurwitz formula
• Riemann–Roch theorem
• Riemann surface
• Weber's theorem

References

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• Egbert Brieskorn and Horst Knörrer, Plane Algebraic Curves, John Stillwell, trans., Birkhäuser, 1986
• Claude Chevalley, Introduction to the Theory of Algebraic Functions of One Variable, American Mathematical Society, Mathematical Surveys Number VI, 1951
• Hershel M. Farkas and Irwin Kra, Riemann Surfaces, Springer, 1980
• Phillip A. Griffiths, Introduction to Algebraic Curves, Kuniko Weltin, trans., American Mathematical Society, Translation of Mathematical Monographs volume 70, 1985 revision
• Robin Hartshorne, Algebraic Geometry, Springer, 1977
• Shigeru Iitaka, Algebraic Geometry: An Introduction to the Birational Geometry of Algebraic Varieties, Springer, 1982
• John Milnor, Singular Points of Complex Hypersurfaces, Princeton University Press, 1968
• George Salmon, Higher Plane Curves, Third Edition, G. E. Stechert & Co., 1934
• Jean-Pierre Serre, Algebraic Groups and Class Fields, Springer, 1988

Curves | Algebraic curves | Birational geometry

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