Re: ell from j

[John Cremona <john.cremona@gmail.com>]

On 8 June 2012 23:05, Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr> wrote:
> On Fri, Jun 08, 2012 at 09:04:57PM +0100, John Cremona wrote:
>> I wrote a similar function for Sage,  called EllipticCurve_from_j, but
>> it is not normally called directly, rather one does EllipticCurve
>> (j=...).  Can gp use named parameters, so you could do ellinit(j=...)?
>>
>> The Sage function has special cases for characteristics 2 and 3, and
>> also over Q it gives a minimal quadratic twist.
>
> The computation of the minimal quadratic twist requires the factorisation
> of j/(1728-j) which can be large, so this would change the complexity of
> the function.

That's a good point, so this should be made optional.  Something for
sage-devel rather than pari-dev!

John

>
> Cheers,
> Bill.
>