# Elliptic curves#

## Conductor#

How do you compute the conductor of an elliptic curve (over \(\QQ\)) in Sage?

Once you define an elliptic curve \(E\) in Sage, using the
`EllipticCurve`

command, the conductor is one of several “methods”
associated to \(E\). Here is an example of the syntax
(borrowed from section 2.4 “Modular forms” in the tutorial):

```
sage: E = EllipticCurve([1,2,3,4,5])
sage: E
Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over
Rational Field
sage: E.conductor()
10351
```

## \(j\)-invariant#

How do you compute the \(j\)-invariant of an elliptic curve in Sage?

Other methods associated to the `EllipticCurve`

class are
`j_invariant`

, `discriminant`

, and `weierstrass_model`

. Here is
an example of their syntax.

```
sage: E = EllipticCurve([0, -1, 1, -10, -20])
sage: E
Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field
sage: E.j_invariant()
-122023936/161051
sage: E.short_weierstrass_model()
Elliptic Curve defined by y^2 = x^3 - 13392*x - 1080432 over Rational Field
sage: E.discriminant()
-161051
sage: E = EllipticCurve(GF(5),[0, -1, 1, -10, -20])
sage: E.short_weierstrass_model()
Elliptic Curve defined by y^2 = x^3 + 3*x + 3 over Finite Field of size 5
sage: E.j_invariant()
4
```

## The \(GF(q)\)-rational points on E#

How do you compute the number of points of an elliptic curve over a finite field?

Given an elliptic curve defined over \(\mathbb{F} = GF(q)\), Sage can compute its set of \(\mathbb{F}\)-rational points

```
sage: E = EllipticCurve(GF(5),[0, -1, 1, -10, -20])
sage: E
Elliptic Curve defined by y^2 + y = x^3 + 4*x^2 over Finite Field of size 5
sage: E.points()
[(0 : 1 : 0), (0 : 0 : 1), (0 : 4 : 1), (1 : 0 : 1), (1 : 4 : 1)]
sage: E.cardinality()
5
sage: G = E.abelian_group()
sage: G
Additive abelian group isomorphic to Z/5 embedded in Abelian group of points on Elliptic Curve defined by y^2 + y = x^3 + 4*x^2 over Finite Field of size 5
sage: G.permutation_group()
Permutation Group with generators [(1,2,3,4,5)]
```

## Modular form associated to an elliptic curve over \(\QQ\)#

Let \(E\) be a “nice” elliptic curve whose equation has integer coefficients, let \(N\) be the conductor of \(E\) and, for each \(n\), let \(a_n\) be the number appearing in the Hasse-Weil \(L\)-function of \(E\). The Taniyama-Shimura conjecture (proven by Wiles) states that there exists a modular form of weight two and level \(N\) which is an eigenform under the Hecke operators and has a Fourier series \(\sum_{n = 0}^\infty a_n q^n\). Sage can compute the sequence \(a_n\) associated to \(E\). Here is an example.

```
sage: E = EllipticCurve([0, -1, 1, -10, -20])
sage: E
Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field
sage: E.conductor()
11
sage: E.anlist(20)
[0, 1, -2, -1, 2, 1, 2, -2, 0, -2, -2, 1, -2, 4, 4, -1, -4, -2, 4, 0, 2]
sage: E.analytic_rank()
0
```