Elliptic curve with Cremona label 21a2 (LMFDB label 21.a2)

• Label: 21a2
• Conductor: $21$
• Discriminant: $21609$
• j-invariant: $\frac{13027640977}{21609}$
• CM: no
• Rank: $0$
• Torsion structure: $\Z/{2}\Z \oplus \Z/{2}\Z$

Minimal Weierstrass equation

$y^2+xy=x^3-49x-136$

(, )

Mordell-Weil group structure

$\Z/{2}\Z \oplus \Z/{2}\Z$

Mordell-Weil generators

$P$	$\hat{h}(P)$	Order
$(-4, 2)$	$0$	$2$
$(8, -4)$	$0$	$2$

Integral points

$\left(-4, 2\right)$, $\left(8, -4\right)$

Invariants

Conductor:	$N$	=	$21$	=	$3 \cdot 7$
Discriminant:	$\Delta$	=	$21609$	=	$3^{2} \cdot 7^{4}$
j-invariant:	$j$	=	$\frac{13027640977}{21609}$	=	$3^{-2} \cdot 7^{-4} \cdot 13^{3} \cdot 181^{3}$
Endomorphism ring:	$\mathrm{End}(E)$	=	$\Z$
Geometric endomorphism ring:	$\mathrm{End}(E_{\overline{\Q}})$	=	$\Z$    (no potential complex multiplication)
Sato-Tate group:	$\mathrm{ST}(E)$	=	$\mathrm{SU}(2)$
Faltings height:	$h_{\mathrm{Faltings}}$	≈	$-0.27236820688903288395035020606$
Stable Faltings height:	$h_{\mathrm{stable}}$	≈	$-0.27236820688903288395035020606$
$abc$ quality:	$Q$	≈	$1.0814902740568197$
Szpiro ratio:	$\sigma_{m}$	≈	$7.649915427654956$

BSD invariants

Analytic rank:	$r_{\mathrm{an}}$	=	$0$
Mordell-Weil rank:	$r$	=	$0$
Regulator:	$\mathrm{Reg}(E/\Q)$	=	$1$
Real period:	$\Omega$	≈	$1.8044616215539682223736006256$
Tamagawa product:	$\prod_{p}c_p$	=	$4$  = $2\cdot2$
Torsion order:	$\#E(\Q)_{\mathrm{tor}}$	=	$4$
Special value:	$L(E,1)$	≈	$0.45111540538849205559340015640$
Analytic order of Ш:	Ш${}_{\mathrm{an}}$	=	$1$    (exact)

BSD formula

$\begin{aligned} 0.451115405 \approx L(E,1) & = \frac{\# Ш(E/\Q)\cdot \Omega_E \cdot \mathrm{Reg}(E/\Q) \cdot \prod_p c_p}{\#E(\Q)_{\rm tor}^2} \\ & \approx \frac{1 \cdot 1.804462 \cdot 1.000000 \cdot 4}{4^2} \\ & \approx 0.451115405\end{aligned}$

Modular invariants

Modular form   21.2.a.a

$q - q^{2} + q^{3} - q^{4} - 2 q^{5} - q^{6} - q^{7} + 3 q^{8} + q^{9} + 2 q^{10} + 4 q^{11} - q^{12} - 2 q^{13} + q^{14} - 2 q^{15} - q^{16} - 6 q^{17} - q^{18} + 4 q^{19} + O(q^{20})$

For more coefficients, see the Downloads section to the right.

Modular degree:	2
$\Gamma_0(N)$-optimal:	no
Manin constant:	1

Local data at primes of bad reduction

This elliptic curve is semistable. There are 2 primes $p$ of bad reduction:

$p$	Tamagawa number	Kodaira symbol	Reduction type	Root number	$\mathrm{ord}_p(N)$	$\mathrm{ord}_p(\Delta)$	$\mathrm{ord}_p(\mathrm{den}(j))$
$3$	$2$	$I_{2}$	split multiplicative	-1	1	2	2
$7$	$2$	$I_{4}$	nonsplit multiplicative	1	1	4	4

Galois representations

The $\ell$-adic Galois representation has maximal image for all primes $\ell$ except those listed in the table below.

prime $\ell$	mod-$\ell$ image	$\ell$-adic image
$2$	2Cs	8.48.0.34

The image $H:=\rho_E(\Gal(\overline{\Q}/\Q))$ of the adelic Galois representation has level $168 = 2^{3} \cdot 3 \cdot 7$, index $192$, genus $1$, and generators

$\left(\begin{array}{rr} 89 & 44 \\ 64 & 93 \end{array}\right),\left(\begin{array}{rr} 119 & 2 \\ 150 & 163 \end{array}\right),\left(\begin{array}{rr} 73 & 8 \\ 124 & 33 \end{array}\right),\left(\begin{array}{rr} 5 & 4 \\ 164 & 165 \end{array}\right),\left(\begin{array}{rr} 7 & 48 \\ 162 & 121 \end{array}\right),\left(\begin{array}{rr} 161 & 8 \\ 160 & 9 \end{array}\right),\left(\begin{array}{rr} 1 & 0 \\ 8 & 1 \end{array}\right),\left(\begin{array}{rr} 1 & 8 \\ 0 & 1 \end{array}\right)$.

The torsion field $K:=\Q(E[168])$ is a degree-$774144$ Galois extension of $\Q$ with $\Gal(K/\Q)$ isomorphic to the projection of $H$ to $\GL_2(\Z/168\Z)$.

The table below list all primes $\ell$ for which the Serre invariants associated to the mod-$\ell$ Galois representation are exceptional.

$\ell$	Reduction type	Serre weight	Serre conductor
$2$	good	$2$	$1$
$3$	split multiplicative	$4$	$7$
$7$	nonsplit multiplicative	$8$	$3$

Isogenies

This curve has non-trivial cyclic isogenies of degree $d$ for $d=$ 2 and 4.
Its isogeny class 21a consists of 6 curves linked by isogenies of degrees dividing 8.

Twists

This elliptic curve is its own minimal quadratic twist.

Growth of torsion in number fields

The number fields $K$ of degree less than 24 such that $E(K)_{\rm tors}$ is strictly larger than $E(\Q)_{\rm tors}$ $\cong \Z/{2}\Z \oplus \Z/{2}\Z$ are as follows:

$[K:\Q]$	$K$	$E(K)_{\rm tors}$	Base change curve
$2$	$\Q(\sqrt{-1})$	$\Z/2\Z \oplus \Z/4\Z$	2.0.4.1-441.1-a5
$2$	$\Q(\sqrt{-3})$	$\Z/2\Z \oplus \Z/4\Z$	2.0.3.1-147.2-a7
$2$	$\Q(\sqrt{3})$	$\Z/2\Z \oplus \Z/4\Z$	2.2.12.1-147.1-a6
$4$	$\Q(\zeta_{12})$	$\Z/4\Z \oplus \Z/4\Z$	not in database
$4$	4.0.432.1	$\Z/2\Z \oplus \Z/8\Z$	not in database
$8$	8.0.157351936.1	$\Z/2\Z \oplus \Z/8\Z$	not in database
$8$	8.0.12745506816.7	$\Z/2\Z \oplus \Z/8\Z$	not in database
$8$	8.4.114709561344.5	$\Z/2\Z \oplus \Z/8\Z$	not in database
$8$	8.0.2985984.1	$\Z/4\Z \oplus \Z/8\Z$	not in database
$8$	8.2.425329947.2	$\Z/2\Z \oplus \Z/6\Z$	not in database
$16$	16.0.162447943996702457856.1	$\Z/4\Z \oplus \Z/8\Z$	not in database
$16$	deg 16	$\Z/4\Z \oplus \Z/8\Z$	not in database
$16$	16.0.49323011734109159424.1	$\Z/2\Z \oplus \Z/16\Z$	not in database
$16$	deg 16	$\Z/2\Z \oplus \Z/12\Z$	not in database
$16$	deg 16	$\Z/2\Z \oplus \Z/12\Z$	not in database
$16$	deg 16	$\Z/2\Z \oplus \Z/12\Z$	not in database

We only show fields where the torsion growth is primitive. For fields not in the database, click on the degree shown to reveal the defining polynomial.

Iwasawa invariants

$p$	2	3	7
Reduction type	ord	split	nonsplit
$\lambda$-invariant(s)	1	1	0
$\mu$-invariant(s)	1	0	0

All Iwasawa $\lambda$ and $\mu$-invariants for primes $p\ge 3$ of good reduction are zero.

$p$-adic regulators

All $p$-adic regulators are identically $1$ since the rank is $0$.