Isogenies prime degree fails on some CM curves

[gh-sachihashimoto]

The code:

K.<a> = QuadraticField(-11)
E = EllipticCurve(K, [0,1,0,-117,-541])
E.isogenies_prime_degree(53)

returns an empty list. But, since E has CM by the ring of integers of K, it has to have an isogeny of degree p for p splitting in K, so this doesn't make sense.

If you import isogenies_prime_degree_general from isogeny_small_degree.py then

isogenies_prime_degree_general(E,53) 

correctly returns

[Isogeny of degree 53 from Elliptic Curve defined by y^2 = x^3 + x^2 + (-117)*x + (-541) over Number Field in a with defining polynomial x^2 + 11 with a = 3.316624790355400?*I to Elliptic Curve defined by y^2 = x^3 + x^2 + (98560*a+42123)*x + (-12561472*a-61946205) over Number Field in a with defining polynomial x^2 + 11 with a = 3.316624790355400?*I,
 Isogeny of degree 53 from Elliptic Curve defined by y^2 = x^3 + x^2 + (-117)*x + (-541) over Number Field in a with defining polynomial x^2 + 11 with a = 3.316624790355400?*I to Elliptic Curve defined by y^2 = x^3 + x^2 + (-98560*a+42123)*x + (12561472*a-61946205) over Number Field in a with defining polynomial x^2 + 11 with a = 3.316624790355400?*I]

[alexjbest]

New commits:

​96561b3

attempt to fix 32269

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​251c829

output

[edgarcosta]

LGTM, but that test with ell = 53 is not going to fly.

Why not ell = 37? it is equally broken and takes 1/6 of the time.

[alexjbest]

@edgarcosta, right the example in code uses l=37, it might still need marking as # long time though, I'm not sure.

[edgarcosta]

Upsidaisy! Ignore my comment then.

I was hoping that the patch bot would tell us what to do regarding the necessity of the marking as # long

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​6b6dc19

fix doctest