# Elliptic curve

#### Background Information

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In mathematics, an **elliptic curve** is a smooth, projective algebraic curve of genus one, on which there is a specified point *O*. An elliptic curve is in fact an abelian variety — that is, it has a multiplication defined algebraically, with respect to which it is a (necessarily commutative) group — and *O* serves as the identity element. Often the curve itself, without *O* specified, is called an elliptic curve.

Any elliptic curve can be written as a plane algebraic curve defined by an equation of the form:

which is non-singular; that is, its graph has no cusps or self-intersections. (When the characteristic of the coefficient field is equal to 2 or 3, the above equation is not quite general enough to comprise all non-singular cubic curves; see below for a more precise definition.) The point *O* is actually the " point at infinity" in the projective plane.

If *y*^{2} = *P*(*x*), where *P* is any polynomial of degree three in *x* with no repeated roots, then we obtain a nonsingular plane curve of genus one, which is thus also an elliptic curve. If *P* has degree four and is squarefree this equation again describes a plane curve of genus one; however, it has no natural choice of identity element. More generally, any algebraic curve of genus one, for example from the intersection of two quadric surfaces embedded in three-dimensional projective space, is called an elliptic curve, provided that it has at least one rational point.

Using the theory of elliptic functions, it can be shown that elliptic curves defined over the complex numbers correspond to embeddings of the torus into the complex projective plane. The torus is also an abelian group, and in fact this correspondence is also a group isomorphism.

Elliptic curves are especially important in number theory, and constitute a major area of current research; for example, they were used in the proof, by Andrew Wiles (assisted by Richard Taylor), of Fermat's Last Theorem. They also find applications in cryptography (see the article elliptic curve cryptography) and integer factorization.

An elliptic curve is *not* an ellipse: see elliptic integral for the origin of the term. Topologically, an elliptic curve is a torus.

## Elliptic curves over the real numbers

Although the formal definition of an elliptic curve is fairly technical and requires some background in algebraic geometry, it is possible to describe some features of elliptic curves over the real numbers using only high school algebra and geometry.

In this context, an elliptic curve is a plane curve defined by an equation of the form

where *a* and *b* are real numbers. This type of equation is called a **Weierstrass equation**.

The definition of elliptic curve also requires that the curve be non-singular. Geometrically, this means that the graph has no cusps, self-intersections, or isolated points. Algebraically, this involves calculating the discriminant

The curve is non-singular if and only if the discriminant is not equal to zero. (Although the factor −16 seems irrelevant here, it turns out to be convenient in a more advanced study of elliptic curves.)

The (real) graph of a non-singular curve has *two* components if its discriminant is positive, and *one* component if it is negative. For example, in the graphs shown in figure to the right, the discriminant in the first case is 64, and in the second case is −368.

## The group law

By adding a "point at infinity", we obtain the projective version of this curve. If *P* and *Q* are two points on the curve, then we can uniquely describe a third point which is the intersection of the curve with the line through *P* and *Q*. If the line is tangent to the curve at a point, then that point is counted twice; and if the line is parallel to the *y*-axis, we define the third point as the point "at infinity". Exactly one of these conditions then holds for any pair of points on an elliptic curve.

It is then possible to introduce a group operation, *+*, on the curve with the following properties: we consider the point at infinity to be 0, the identity of the group; and if a straight line intersects the curve at the points *P*, *Q* and *R*, then we require that *P* + *Q* + *R* = 0 in the group. One can check that this turns the curve into an abelian group, and thus into an abelian variety. It can be shown that the set of *K*- rational points (including the point at infinity) forms a subgroup of this group. If the curve is denoted by *E*, then this subgroup is often written as *E(K)*.

The above group can be described algebraically as well as geometrically. Given the curve *y*^{2} = *x*^{3} − *px* − *q* over the field *K* (whose characteristic we assume to be neither 2 nor 3), and points *P* = (*x _{P}*,

*y*) and

_{P}*Q*= (

*x*,

_{Q}*y*) on the curve, assume first that

_{Q}*x*≠

_{P}*x*. Let

_{Q}*s*be the slope of the line containing

*P*and

*Q*; i.e.,

*s*=

^{(yP − yQ)}⁄

_{(xP − xQ)}. Since

*K*is a field,

*s*is well-defined. Then we can define

*R*=

*P*+

*Q*= (

*x*,

_{R}*−y*) by

_{R}If *x _{P}* =

*x*(third and fourth panes above), then there are two options: if

_{Q}*y*= −

_{P}*y*, including the case where

_{Q}*y*=

_{P}*y*= 0, then the sum is defined as 0; thus, the inverse of each point on the curve is found by reflecting it across the

_{Q}*x*-axis. If

*y*=

_{P}*y*≠ 0 (second pane), then

_{Q}*R*=

*P*+

*P*= 2

*P*= (

*x*

_{R},

*−y*

_{R}) is given by

## Elliptic curves over the complex numbers

The formulation of elliptic curves as the embedding of a torus in the complex projective plane follows naturally from a curious property of Weierstrass's elliptic functions. These functions and their first derivative are related by the formula

Here, *g*_{2} and *g*_{3} are constants; is the Weierstrass elliptic function and its derivative. It should be clear that this relation is in the form of an elliptic curve (over the complex numbers). The Weierstrass functions are doubly periodic; that is, they are periodic with respect to a lattice Λ; in essence, the Weierstrass functions are naturally defined on a torus *T* = **C**/Λ. This torus may be embedded in the complex projective plane by means of the map

This map is a group isomorphism, carrying the natural group structure of the torus into the projective plane. It is also an isomorphism of Riemann surfaces, so topologically, a given elliptic curve looks like a torus. If the lattice Λ is related by multiplication by a non-zero complex number *c* to a lattice *c*Λ , then the corresponding curves are isomorphic. Isomorphism classes of elliptic curves are specified by the j-invariant.

The isomorphism classes can be understood in a simpler way as well. The constants *g*_{2} and *g*_{3}, called the modular invariants, are uniquely determined by the lattice, that is, by the structure of the torus. However, the complex numbers form the splitting field for polynomials with real coefficients, and so the elliptic curve may be written as

One finds that

and

so that the modular discriminant is

Here, λ is sometimes called the modular lambda function.

Note that the uniformization theorem implies that every compact Riemann surface of genus one can be represented as a torus.

This also allows an easy understanding of the torsion points on an elliptic curve: if the lattice Λ is spanned by the fundamental periods ω_{1} and ω_{2}, then the *n*-torsion points are the (equivalence classes of) points of the form , for *a* and *b* integers in the range from 0 to *n*-1.

Over the complex numbers, every elliptic curve has nine inflection points. Every line through two of these points also passes through a third inflection point; the nine points and 12 lines formed in this way form a realization of the Hesse configuration.

## Elliptic curves over the rational numbers

A curve *E* defined over the field of rational numbers is also defined over the field of real numbers, therefore the law of addition (of points with real coordinates) by the tangent and secant method can be applied to *E*. The explicit formulae show that the sum of two points *P* and *Q* with rational coordinates has again rational coordinates, since the line joining *P* and *Q* has rational coefficients. This way, one shows that the set of rational points of *E* forms a subgroup of the group of real points of *E*. As this group, it is an abelian group, that is, *P* + *Q* = *Q* + *P*.

### The structure of rational points

The most important result is that all points can be constructed by the method of tangents and secants starting with a *finite* number of points. More precisely the Mordell-Weil theorem states that the group *E*(**Q**) is a finitely generated (abelian) group. By the fundamental theorem of finitely generated abelian groups it is therefore a finite direct sum of copies of **Z** and finite cyclic groups.

The proof of that theorem rests on two ingredients: first, one shows that for any integer *m* > 1, the quotient group *E*(**Q**)/*mE*(**Q**) is finite (weak Mordell–Weil theorem). Second, introducing a height function *h* on the rational points *E*(**Q**) defined by *h*(*P*_{0}) = 0 and *h*(*P*) = log max(|*p*|, |*q*|) if *P* (unequal to the point at infinity *P*_{0}) has as abscissa the rational number *x* = ^{p}⁄_{q} (with coprime *p* and *q*). This height function *h* has the property that *h*(*mP*) grows roughly like the square of *m*. Moreover, only finitely many rational points with height smaller than any constant exist on *E*.

The proof of the theorem is thus a variant of the method of infinite descent and relies on the repeated application of Euclidean divisions on *E*: let *P* ∈ *E*(**Q**) be a rational point on the curve, writing *P* as the sum 2*P*_{1} + *Q*_{1} where *Q*_{1} is a fixed representant of *P* in *E*(**Q**)/2*E*(**Q**), the height of *P*_{1} is about ^{1}⁄_{4} of the one of *P* (more generally, replacing 2 by any *m* > 1, and ^{1}⁄_{4} by ^{1}⁄_{m2}). Redoing the same with *P*_{1}, that is to say *P*_{1} = 2*P*_{2} + *Q*_{2}, then *P*_{2} = 2*P*_{3} + *Q*_{3}, etc. finally expresses *P* as an integral linear combination of points *Q _{i}* and of points whose height is bounded by a fixed constant chosen in advance: by the weak Mordell–Weil theorem and the second property of the height function

*P*is thus expressed as an integral linear combination of a finite number of fixed points.

So far, the theorem is not effective since there is no known general procedure for determining the representants of *E*(**Q**)/*mE*(**Q**).

The rank of *E*(**Q**), that is the number of copies of **Z** in *E*(**Q**) or, equivalently, the number of independent points of infinite order, is called the *rank* of *E*. The Birch and Swinnerton-Dyer conjecture is concerned with determining the rank. One conjectures that it can be arbitrarily large, even if only examples with relatively small rank are known. The elliptic curve with biggest exactly known rank is

*y*^{2}+*xy*=*x*^{3}− 26175960092705884096311701787701203903556438969515*x*+ 51069381476131486489742177100373772089779103253890567848326.

It has rank 18, found by Noam Elkies in 2006. Curves of rank at least 28 are known, but their rank is not exactly known.

As for the groups constituting the torsion subgroup of *E*(**Q**), the following is known the torsion subgroup of *E*(**Q**) is one of the 15 following groups (a theorem due to Barry Mazur): **Z**/*N***Z** for *N* = 1, 2, …, 10, or 12, or **Z**/2**Z** × **Z**/2*N***Z** with *N* = 1, 2, 3, 4. Examples for every case are known. Moreover, elliptic curves whose Mordell-Weil groups over **Q** have the same torsion groups belong to a parametrized family.

### The Birch and Swinnerton-Dyer conjecture

The *Birch and Swinnerton-Dyer conjecture* (BSD) is one of the Millennium problems of the Clay Mathematics Institute. The conjecture relies on analytic and arithmetic objects defined by the elliptic curve in question.

At the analytic side, an important ingredient is a function of a complex variable, *L*, the Hasse–Weil zeta function of *E* over **Q**. This function is a variant of the Riemann zeta function and Dirichlet L-functions. It is defined as an Euler product, with one factor for every prime number *p*.

For a curve *E* over **Q** given by a minimal equation

with integral coefficients *a _{i}*, reducing the coefficients modulo

*p*defines an elliptic curve over the finite field

**F**

_{p}(except for a finite number of primes

*p*, where the reduced curve has a singularity and thus fails to be elliptic, in which case

*E*is said to be of bad reduction at

*p*).

The zeta function of an elliptic curve over a finite field **F**_{p} is, in some sense, a generating function assembling the information of the number of points of *E* with values in the finite field extensions of **F**_{p}, **F**_{pn}. It is given,

The interior sum of the exponential resembles the development of the logarithm and, in fact, the so-defined zeta function is a rational function:

The Hasse–Weil zeta function of *E* over **Q** is then defined by collecting this information together, for all primes *p*. It is defined by

where ε(*p*) = 1 if *E* has good reduction at *p* and 0 otherwise (in which case *a _{p}* is defined differently than above).

This product converges for only. Hasse's conjecture affirms that the *L*-function admits an analytic continuation to the whole complex plane and satisfies a functional equation relating, for any *s*, *L*(*E*, *s*) to *L*(*E*, 2−*s*). In 1999 this was shown to be a consequence of the proof of the Shimura–Taniyama–Weil conjecture, which asserts that every elliptic curve over *Q* is a modular curve, which implies that its *L*-function is the *L*-function of a modular form whose analytic continuation is known.

One can therefore speak about the values of *L*(*E*, *s*) at any complex number *s*. The Birch-Swinnerton-Dyer conjecture relates the arithmetic of the curve to the behaviour of its *L*-function at *s* = 1. More precisely, it affirms that the order of the *L*-function at *s* = 1 equals the rank of *E* and predicts the leading term of the Laurent series of *L*(*E*, *s*) at that point in terms of several quantities attached to the elliptic curve.

Much like the Riemann hypothesis, this conjecture has multiple consequences, including the following two:

- Let
*n*be an odd square-free integer. Assuming the Birch and Swinnerton-Dyer conjecture,*n*is the area of a right triangle with rational side lengths (a congruent number) if and only if the number of triplets of integers (*x*,*y*,*z*) satisfying is twice the number of triples satisfying . This statement, due to Tunnell, is related to the fact that*n*is a congruent number if and only if the elliptic curve has a rational point of infinite order (thus, under the Birch and Swinnerton-Dyer conjecture, its*L*-function has a zero at 1). The interest in this statement is that the condition is easily verified. - In a different direction, certain analytic methods allow for an estimation of the order of zero in the centre of the critical strip of families of
*L*-functions. Admitting the BSD conjecture, these estimations correspond to information about the rank of families of elliptic curves in question. For example,: suppose the generalized Riemann hypothesis and the BSD conjecture, the average rank of curves given by is smaller than 2.

### The modularity theorem and its application to Fermat's Last Theorem

The modularity theorem, once known as the Taniyama–Shimura–Weil conjecture, states that every elliptic curve *E* over **Q** is a modular curve, that is to say, its Hasse–Weil zeta function is the *L*-function of a modular form of weight 2 and level *N*, where *N* is the conductor of *E* (an integer divisible by the same prime numbers as the discriminant of *E*, Δ(*E*).) In other words, if, for , one writes the *L*-function in the form

the expression , where *q* = exp(2π*iz*) defines a parabolic modular newform of weight 2 and level *N*. For prime numbers ℓ not dividing *N*, the coefficient of the form equals ℓ – the number of solutions of the minimal equation of the curve modulo ℓ.

For example, to the elliptic curve with discriminant (and conductor) 37, is associated the form , where . For prime numbers ℓ distinct of 37, one can verify the property about the coefficients. Thus, for ℓ = 3, the solutions of the equation modulo 3 are (0, 0), (0, 1), (2, 0), (1, 0), (1, 1), (2, 1), as and .

The conjecture, going back to the fifties, has been completely shown in 1999 using ideas of Andrew Wiles, who already proved it in 1994 for a large family of elliptic curves.

There are several formulations of the conjecture. Showing that they are equivalent is difficult and was a main topic of number theory in the second half of the 20th century. The modularity of an elliptic curve *E* of conductor *N* can be expressed also by saying that there is a non-constant rational map defined over **Q**, from the modular curve to *E*. In particular, the points of *E* can be parametrized by modular functions.

For example, a modular parametrization of the curve is given by

where, as above, *q* = exp(2π*iz*). The functions *x(z)* and *y(z)* are modular of weight 0 and level 37; in other words they are meromorphic, defined on the upper half-plane and satisfy and likewise for *y(z)* for all integers *a, b, c, d* with *ad* - *bc* = 1 and 37|*c*.

Another formulation depends on the comparison of Galois representations attached on the one hand to elliptic curves, and on the other hand to modular forms. The latter formulation has been used in the proof the conjecture. Dealing with the level of the forms (and the connection to the conductor of the curve) is particularly delicate.

The most spectacular application of the conjecture is the proof of Fermat's Last Theorem (FLT). Suppose that for a prime *p* > 5, the Fermat equation

has a solution with non-zero integers, hence a counter-example to FLT. Then the elliptic curve

of discriminant can not be modular. Thus, the proof of the Taniyama–Shimura–Weil conjecture for this family of elliptic curves (called Hellegouarch–Frey curves) implies the FLT. The proof of the link between these two statements, based on an idea of Gerhard Frey (1985), is difficult and technical. It was established by Kenneth Ribet in 1987.

### Integral points

This section is concerned with points *P* = (*x, y*) of *E* such that *x* is an integer. The following theorem is due to C. L. Siegel: the set of points *P* = (*x, y*) of *E*(**Q**) such that *x* is integral is finite. This theorem can be generalized to points whose *x* coordinate has a denominator divisible only by a fixed finite set of prime numbers.

The theorem can be formulated effectively. For example, if the Weierstrass equation of *E* has integer coefficients bounded by a constant *H*, the coordinates (*x, y*) of a point of *E* with both *x* and *y* integer satisfy:

For example, the equation has eight integral solutions with *y* > 0 :

- (
*x*,*y*) = (−1,4), (−2,3), (2,5), (4,9), (8,23), (43,282), (52,375), (5234,378661).

As another example, Ljunggren's equation, a curve whose Weierstrass form is *y*^{2} = *x*^{3} − 2*x*, has only four solutions with *y* ≥ 0 :

- (
*x*,*y*) = (0,0), (−1,1), (2, 2), (338,6214).

### Generalization to number fields

Many of the preceding results remain valid when the field of definition of *E* is a number field, that is to say, a finite field extension of **Q**. In particular, the group *E(K)* of *K*-rational points of an elliptic curve *E* defined over *K* is finitely generated, which generalizes the Mordell–Weil theorem above. A theorem due to Loïc Merel shows that for a given integer *d*, there are ( up to isomorphism) only finitely many groups that can occur as the torsion groups of *E*(*K*) for an elliptic curve defined over a number field *K* of degree *d*. More precisely, there is a number *B*(*d*) such that for any elliptic curve *E* defined over a number field *K* of degree *d*, any torsion point of *E*(*K*) is of order less than *B*(*d*). The theorem is effective: for *d* > 1, if a torsion point is of order *p*, with *p* prime, then .

As for the integral points, Siegel's theorem generalizes to the following: let *E* be an elliptic curve defined over a number field *K*, *x* and *y* the Weierstrass coordinates. Then the points of *E(K)* whose *x*-coordinate is in the ring of integers *O*_{K} is finite.

The properties of the Hasse–Weil zeta function and the Birch and Swinnerton-Dyer conjecture can also be extended to this more general situation.

## Elliptic curves over a general field

Elliptic curves can be defined over any field *K*; the formal definition of an elliptic curve is a non-singular projective algebraic curve over *K* with genus 1 with a given point defined over *K*.

If the characteristic of *K* is neither 2 nor 3, then every elliptic curve over *K* can be written in the form

where *p* and *q* are elements of *K* such that the right hand side polynomial *x*^{3} − *px* − *q* does not have any double roots. If the characteristic is 2 or 3, then more terms need to be kept: in characteristic 3, the most general equation is of the form

for arbitrary constants such that the polynomial on the right-hand side has distinct roots (the notation is chosen for historical reasons). In characteristic 2, even this much is not possible, and the most general equation is

provided that the variety it defines is non-singular. If characteristic were not an obstruction, each equation would reduce to the previous ones by a suitable change of variables.

One typically takes the curve to be the set of all points (*x*,*y*) which satisfy the above equation and such that both *x* and *y* are elements of the algebraic closure of *K*. Points of the curve whose coordinates both belong to *K* are called ** K-rational points**.

## Isogeny

Let *E* and *D* be elliptic curves over a field *k*. An *isogeny* between *E* and *D* is a finite morphism *f*: *E* → *D* of varieties that preserves basepoints (in other words, maps the given point on *E* to that on *D*).

The two curves are called *isogenous* if there is an isogeny between them. This is an equivalence relation, symmetry being due to the existence of the dual isogeny. Every isogeny is an algebraic homomorphism and thus induces homomorphisms of the groups of the elliptic curves for *k*-valued points.

## Elliptic curves over finite fields

Let *K* = **F**_{q} be the finite field with *q* elements and *E* an elliptic curve defined over *K*. While the precise number of rational points of an elliptic curve *E* over *K* is in general rather difficult to compute, Hasse's theorem on elliptic curves gives us, including the point at infinity, the following estimate:

In other words, the number of points of the curve grows roughly as the number of elements in the field. This fact can be understood and proven with the help of some general theory; see local zeta function, Étale cohomology.

The set of points *E*(**F**_{q}) is a finite abelian group. It is always cyclic or the product of two cyclic groups. For example, the curve defined by

over **F**_{71} has 72 points (71 affine points including (0,0) and one point at infinity) over this field, whose group structure is given by **Z**/2**Z** × **Z**/36**Z**. The number of points on a specific curve can be computed with Schoof's algorithm.

Studying the curve over the field extensions of **F**_{q} is facilitated by the introduction of the local zeta function of *E* over **F**_{q}, defined by a generating series (also see above)

where the field *K _{n}* is the (unique) extension of

*K*=

**F**

_{q}of degree

*n*(that is, ). The zeta function is a rational function in

*T*. There is an integer

*a*such that

Moreover,

with complex numbers α, β of absolute value . This result are a special case of the Weil conjectures. For example, the zeta function of over the field **F**_{2} is given by since the curve has () points over if *r* is odd (even, respectively).

The Sato–Tate conjecture is a statement about how the error term in Hasse's theorem varies with the different primes *q*, if you take an elliptic curve E over **Q** and reduce it modulo q. It was proven (for almost all such curves) in 2006 due to the results of Taylor, Harris and Shepherd-Barron, and says that the error terms are equidistributed.

Elliptic curves over finite fields are notably applied in cryptography and for the factorization of large integers. These algorithms often make use of the group structure on the points of *E*. Algorithms that are applicable to general groups, for example the group of invertible elements in finite fields, , can thus be applied to the group of points on an elliptic curve. For example, the discrete logarithm is such an algorithm. The interest in this is that choosing an elliptic curve allows for more flexibility than choosing *q* (and thus the group of units in **F**_{q}). Also, the group structure of elliptic curves is generally more complicated.

## Algorithms that use elliptic curves

Elliptic curves over finite fields are used in some cryptographic applications as well as for integer factorization. Typically, the general idea in these applications is that a known algorithm which makes use of certain finite groups is rewritten to use the groups of rational points of elliptic curves. For more see also:

- Elliptic curve cryptography
- Elliptic Curve DSA
- Lenstra elliptic curve factorization
- Elliptic curve primality proving

## Alternative representations of elliptic curves

- Hessian curve
- Edwards curve
- Twisted curve
- Twisted Hessian curve
- Twisted Edwards curve
- Doubling-oriented Doche–Icart–Kohel curve
- Tripling-oriented Doche–Icart–Kohel curve
- Jacobian curve
- Montgomery curve