Re: ell from j

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

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.  You are welcome to
reuse that code :)

John

On 8 June 2012 20:37, Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr> wrote:
> Dear PARI developers,
>
> It has been suggested to add a function to PARI which return an elliptic curve with
> a prescribed j-invariant.
>
> An example of such function is
>
> ellfromj(j)=if(j==0,[0,0,0,0,1],if(j==1728,[0,0,0,1,0],[0,0,0,3,2]*(j/(1728-j))));
>
> How should we call it ? What should it return ? (an ell2,an ell5, a smallell, etc ?)
>
> Cheers,
> Bill.
>