## Experimental Mathematics

- Experiment. Math.
- Volume 18, Issue 1 (2009), 65-70.

### Jacobians of Genus-2 Curves with a Rational Point of Order 11

Nicolas Bernard,, Franck Leprévost, and Michael Pohst

#### Abstract

On the one hand, it is well known that Jacobians of (hyper)elliptic curves defined over $\Q$ having a rational point of order l can be used in many applications, for instance in the construction of class groups of quadratic fields with a nontrivial l-rank. On the other hand, it is also well known that 11 is the least prime number that is not the order of a rational point of an elliptic curve defined over $\Q$. It is therefore interesting to look for curves of higher genus whose Jacobians have a rational point of order 11. This problem has already been addressed, and Flynn found such a family $\Fl_t$ of genus-2 curves. Now it turns out that the Jacobian $J_0(23)$ of the modular genus-2 curve $X_0(23)$ has the required property, but does not belong to $\Fl_t$. The study of $X_0(23)$ leads to a method giving a partial solution of the considered problem. Our approach allows us to recover $X_0(23)$ and to construct another 18 distinct explicit curves of genus 2 defined over $\Q$ whose Jacobians have a rational point of order 11. Of these 19 curves, 10 do not have any rational Weierstrass point, and 9 have a rational Weierstrass point. None of these curves are $\Qb$-isomorphic to each other, nor $\Qb$-isomorphic to an element of Flynn's family $\Fl_t$. Finally, the Jacobians of these new curves are absolutely simple.

#### Article information

**Source**

Experiment. Math., Volume 18, Issue 1 (2009), 65-70.

**Dates**

First available in Project Euclid: 27 May 2009

**Permanent link to this document**

https://projecteuclid.org/euclid.em/1243430530

**Mathematical Reviews number (MathSciNet)**

MR2548987

**Zentralblatt MATH identifier**

1244.11064

**Subjects**

Primary: 11Y40: Algebraic number theory computations 11G30: Curves of arbitrary genus or genus = 1 over global fields [See also 14H25] 14H40: Jacobians, Prym varieties [See also 32G20] 14Q05: Curves

**Keywords**

Genus-2 curves torsion modular curves Jacobians rational point of order 11

#### Citation

Bernard,, Nicolas; Leprévost, Franck; Pohst, Michael. Jacobians of Genus-2 Curves with a Rational Point of Order 11. Experiment. Math. 18 (2009), no. 1, 65--70. https://projecteuclid.org/euclid.em/1243430530