# Properties

 Label 42.a3 Conductor $42$ Discriminant $63011844$ j-invariant $\frac{124475734657}{63011844}$ CM no Rank $0$ Torsion Structure $\Z/{2}\Z \times \Z/{2}\Z$

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Show commands for: Magma / SageMath / Pari/GP

## Minimal Weierstrass equation

magma: E := EllipticCurve([1, 1, 1, -104, 101]); // or
magma: E := EllipticCurve("42a3");
sage: E = EllipticCurve([1, 1, 1, -104, 101]) # or
sage: E = EllipticCurve("42a3")
gp: E = ellinit([1, 1, 1, -104, 101]) \\ or
gp: E = ellinit("42a3")

$y^2 + x y + y = x^{3} + x^{2} - 104 x + 101$

## Mordell-Weil group structure

$\Z/{2}\Z \times \Z/{2}\Z$

## Torsion generators

magma: TorsionSubgroup(E);
sage: E.torsion_subgroup().gens()
gp: elltors(E)

$\left(1, -1\right)$, $\left(9, -5\right)$

## Integral points

magma: IntegralPoints(E);
sage: E.integral_points()

$\left(1, -1\right)$, $\left(9, -5\right)$

Note: only one of each pair $\pm P$ is listed.

## Invariants

 magma: Conductor(E); sage: E.conductor().factor() gp: ellglobalred(E)[1] $N$ = $42$ = $2 \cdot 3 \cdot 7$ magma: Discriminant(E); sage: E.discriminant().factor() gp: E.disc $\Delta$ = $63011844$ = $2^{2} \cdot 3^{8} \cdot 7^{4}$ magma: jInvariant(E); sage: E.j_invariant().factor() gp: E.j $j$ = $\frac{124475734657}{63011844}$ = $2^{-2} \cdot 3^{-8} \cdot 7^{-4} \cdot 4993^{3}$ $\text{End} (E)$ = $\Z$ (no Complex Multiplication) $\text{ST} (E)$ = $\mathrm{SU}(2)$

## BSD invariants

 magma: Rank(E); sage: E.rank() $r$ = $0$ magma: Regulator(E); sage: E.regulator() $\text{Reg}$ = $1$ magma: RealPeriod(E); sage: E.period_lattice().omega() gp: E.omega[1] $\Omega$ ≈ $1.73772372875$ magma: TamagawaNumbers(E); sage: E.tamagawa_numbers() gp: gr=ellglobalred(E); [[gr[4][i,1],gr[5][i][4]] | i<-[1..#gr[4][,1]]] $\prod_p c_p$ = $8$  = $2\cdot2\cdot2$ magma: Order(TorsionSubgroup(E)); sage: E.torsion_order() gp: elltors(E)[1] $\#E_{\text{tor}}$ = $4$ magma: MordellWeilShaInformation(E); sage: E.sha().an_numerical() Ш$_{\text{an}}$ = $1$ (exact)

## Modular invariants

### Modular form42.2.1.a

magma: ModularForm(E);
sage: E.q_eigenform(20)
gp: xy = elltaniyama(E);
gp: deriv(xy[1])/(2*xy[2]+E.a1*xy[1]+E.a3)

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

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

### Modular degree and optimality

magma: ModularDegree(E);
sage: E.modular_degree()
16 : curve is not $\Gamma_0(N)$-optimal

### Special L-value attached to the curve

magma: Lr1 where r,Lr1 := AnalyticRank(E: Precision:=12);
sage: r = E.rank();
sage: E.lseries().dokchitser().derivative(1,r)/r.factorial()
gp: ar = ellanalyticrank(E);
gp: ar[2]/factorial(ar[1])

$L(E,1)$ ≈ $0.868861864374$

## Local data

magma: [LocalInformation(E,p) : p in BadPrimes(E)];
sage: E.local_data()
gp: ellglobalred(E)[5]
prime Tamagawa number Kodaira symbol Reduction type Root number ord($N$) ord($\Delta$) ord$(j)_{-}$
$2$ $2$ $I_{2}$ Split multiplicative -1 1 2 2
$3$ $2$ $I_{8}$ Non-split multiplicative 1 1 8 8
$7$ $2$ $I_{4}$ Non-split multiplicative 1 1 4 4

## Galois representations

The image of the 2-adic representation attached to this elliptic curve is the subgroup of $\GL(2,\Z_2)$ with Rouse label X200b.

This subgroup is the pull-back of the subgroup of $\GL(2,\Z_2/2^3\Z_2)$ generated by $\left(\begin{array}{rr} 5 & 2 \\ 4 & 1 \end{array}\right),\left(\begin{array}{rr} 1 & 0 \\ 0 & 7 \end{array}\right),\left(\begin{array}{rr} 1 & 0 \\ 4 & 1 \end{array}\right)$ and has index 96.

magma: [GaloisRepresentation(E,p): p in PrimesUpTo(20)];
sage: rho = E.galois_representation();
sage: [rho.image_type(p) for p in rho.non_surjective()]

The mod $p$ Galois representation has maximal image $\GL(2,\F_p)$ for all primes $p$ except those listed.

prime Image of Galois representation
$2$ Cs

## $p$-adic data

### $p$-adic regulators

sage: [E.padic_regulator(p) for p in primes(3,20) if E.conductor().valuation(p)<2]

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

## Iwasawa invariants

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

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

## Isogenies

This curve has non-trivial cyclic isogenies of degree $d$ for $d=$ 2 and 4.
Its isogeny class 42.a consists of 6 curves linked by isogenies of degrees dividing 8.

## Growth of torsion in number fields

The number fields $K$ of degree up to 7 such that $E(K)_{\rm tors}$ is strictly larger than $E(\Q)_{\rm tors}$ $\cong \Z/{2}\Z \times \Z/{2}\Z$ are as follows:

$[K:\Q]$ $K$ $E(K)_{\rm tors}$ Base-change curve
2 $\Q(\sqrt{2})$ $\Z/2\Z \times \Z/4\Z$ 2.2.8.1-882.1-a4
$\Q(\sqrt{-1})$ $\Z/2\Z \times \Z/4\Z$ 2.0.4.1-882.1-a3
$\Q(\sqrt{-2})$ $\Z/2\Z \times \Z/4\Z$ 2.0.8.1-882.2-a3
4 $\Q(\zeta_{8})$ $\Z/4\Z \times \Z/4\Z$ Not in database
4.2.2048.1 $\Z/2\Z \times \Z/8\Z$ Not in database
$\Q(i, \sqrt{7})$ $\Z/2\Z \times \Z/8\Z$ Not in database

We only show fields where the torsion growth is primitive. For each field $K$ we either show its label, or a defining polynomial when $K$ is not in the database.