Ellinit and finite fields

[Cherry Kearton <cherry.kearton@durham.ac.uk>]

I want to use GP to look at elliptic curves over finite fields, but it
seems unable to handle this unless the finite field is F_p where p is prime. 
Here is an example.

? P=ffinit(5,2,y)
%1 = Mod(1, 5)*y^2 + Mod(3, 5)
? E=ellinit(Mod([0,0,0,-1,0],P))

%3 = [0, 0, 0, Mod(-1, Mod(1, 5)*y^2 + Mod(3, 5)), 0, 0, Mod(-2, Mod(1, 5)*y^2 + Mod(3, 5)), 0, Mod(-1, Mod(1, 5)*y^2 + Mod(3, 5)), Mod(48, Mod(1, 5)*y^2 + Mod(3, 5)), 0, Mod(64, Mod(1, 5)*y^2 + Mod(3, 5)), Mod(1728, Mod(1, 5)*y^2 + Mod(3, 5)), 0, 0, 0, 0, 0, 0]

? ellordinate(E,2)
  ***   invalid coefficients in roots.

Can anyone help, please?

Cherry Kearton

-- 
Dr Cherry Kearton, Reader in Pure Mathematics,
Department of Mathematical Sciences, University of Durham,
South Road, Durham, DH1 3LE, UK.

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