# Properties

 Label 2.2.21.1-21.1-a2 Base field $$\Q(\sqrt{21})$$ Conductor $$(-2 a + 1)$$ Conductor norm $$21$$ CM no Base change yes: 147.a6,63.a6 Q-curve yes Torsion order $$4$$ Rank $$0$$

# Related objects

Show commands for: Magma / Pari/GP / SageMath

## Base field$$\Q(\sqrt{21})$$

Generator $$a$$, with minimal polynomial $$x^{2} - x - 5$$; class number $$1$$.

sage: x = polygen(QQ); K.<a> = NumberField(x^2 - x - 5)

gp: K = nfinit(a^2 - a - 5);

magma: R<x> := PolynomialRing(Rationals()); K<a> := NumberField(R![-5, -1, 1]);

## Weierstrass equation

$$y^2+axy+ay=x^{3}+x^{2}+\left(-5a+13\right)x-5a+13$$
sage: E = EllipticCurve(K, [a, 1, a, -5*a + 13, -5*a + 13])

gp: E = ellinit([a, 1, a, -5*a + 13, -5*a + 13],K)

magma: E := ChangeRing(EllipticCurve([a, 1, a, -5*a + 13, -5*a + 13]),K);

This is a global minimal model.

sage: E.is_global_minimal_model()

## Invariants

 Conductor: $$(-2 a + 1)$$ = $$\left(-a + 2\right) \cdot \left(a + 3\right)$$ sage: E.conductor()  magma: Conductor(E); Conductor norm: $$21$$ = $$3 \cdot 7$$ sage: E.conductor().norm()  magma: Norm(Conductor(E)); Discriminant: $$(63)$$ = $$\left(-a + 2\right)^{4} \cdot \left(a + 3\right)^{2}$$ sage: E.discriminant()  gp: E.disc  magma: Discriminant(E); Discriminant norm: $$3969$$ = $$3^{4} \cdot 7^{2}$$ sage: E.discriminant().norm()  gp: norm(E.disc)  magma: Norm(Discriminant(E)); j-invariant: $$\frac{103823}{63}$$ sage: E.j_invariant()  gp: E.j  magma: jInvariant(E); Endomorphism ring: $$\Z$$ Geometric endomorphism ring: $$\Z$$ (no potential complex multiplication) sage: E.has_cm(), E.cm_discriminant()  magma: HasComplexMultiplication(E); Sato-Tate group: $\mathrm{SU}(2)$

## Mordell-Weil group

 Rank: $$0$$ Torsion structure: $$\Z/4\Z$$ sage: T = E.torsion_subgroup(); T.invariants()  gp: T = elltors(E); T[2]  magma: T,piT := TorsionSubgroup(E); Invariants(T); Torsion generator: $\left(-a + 2 : 2 a - 6 : 1\right)$ sage: T.gens()  gp: T[3]  magma: [piT(P) : P in Generators(T)];

## BSD invariants

 Analytic rank: $$0$$ sage: E.rank()  magma: Rank(E); Mordell-Weil rank: $$0$$ Regulator: $$1$$ Period: $$14.6075277685517$$ Tamagawa product: $$4$$  =  $$2\cdot2$$ Torsion order: $$4$$ Leading coefficient: $$0.796905972804256$$ Analytic order of Ш: $$1$$ (rounded)

## Local data at primes of bad reduction

sage: E.local_data()

magma: LocalInformation(E);

prime Norm Tamagawa number Kodaira symbol Reduction type Root number ord($$\mathfrak{N}$$) ord($$\mathfrak{D}$$) ord$$(j)_{-}$$
$$\left(-a + 2\right)$$ $$3$$ $$2$$ $$I_{4}$$ Non-split multiplicative $$1$$ $$1$$ $$4$$ $$4$$
$$\left(a + 3\right)$$ $$7$$ $$2$$ $$I_{2}$$ Non-split multiplicative $$1$$ $$1$$ $$2$$ $$2$$

## Galois Representations

The mod $$p$$ Galois Representation has maximal image for all primes $$p < 1000$$ except those listed.

prime Image of Galois Representation
$$2$$ 2B

## Isogenies and isogeny class

This curve has non-trivial cyclic isogenies of degree $$d$$ for $$d=$$ 2, 4 and 8.
Its isogeny class 21.1-a consists of curves linked by isogenies of degrees dividing 8.

## Base change

This curve is the base change of elliptic curves 147.a6, 63.a6, defined over $$\Q$$, so it is also a $$\Q$$-curve.