Improve handling of easy cases in lift_to_sl2z()

[pjbruin]

The function lift_to_sl2z(c, d, N) returns larger coefficients than needed in the cases c = 0 or d = 0. Also, the (trivial) case N = 1 is not treated correctly.

Before:

sage: lift_to_sl2z(0, 41, 42)
[-1, -2, 42, 83]
sage: lift_to_sl2z(0, 5, 42)
[17, 19, 42, 47]
sage: lift_to_sl2z(5, 0, 42)
[-2, -101, 5, 252]
sage: lift_to_sl2z(0, 0, 1)
Traceback (most recent call last):
...
AttributeError: Element (0, 0) not in P1.

After:

sage: lift_to_sl2z(0, 41, 42)
[-1, 0, 0, -1]
sage: lift_to_sl2z(0, 5, 42)
[17, 2, 42, 5]
sage: lift_to_sl2z(5, 0, 42)
[-2, -17, 5, 42]
sage: lift_to_sl2z(0, 0, 1)
[1, 0, 0, 1]

Component: modular forms

Author: Peter Bruin

Branch/Commit: bcc2c19

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/29246

[pjbruin]

Branch: u/pbruin/29246-lift_to_sl2z

[pjbruin]

Commit: bcc2c19

[mkoeppe]

comment:2

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[fchapoton]

comment:4

ok, let it be

[fchapoton]

Reviewer: Frédéric Chapoton

[vbraun]

Changed branch from u/pbruin/29246-lift_to_sl2z to bcc2c19