Thanks for the report, you are correct and there is a bug in the code. When reducing the cusp 6/7 it first reduced 6 and 7 mod the level 7 to get 6 and 0, then calls an internal function _lift_pair() which assumes that the new pair of integers is coprime.
I will raise an issue and (probably) fix it right away.JohnOn Thu, 31 Aug 2023 at 08:06, Ralf Hemmecke <hemm...@gmail.com> wrote:The following contradicts the specification of reduce_cusp for Gamma(7).
https://doc.sagemath.org/html/en/reference/arithgroup/sage/modular/arithgroup/congroup_gamma.html
sage: Gamma(7).are_equivalent(Cusp(6,7),Cusp(1,0))
True
sage: Gamma(7).reduce_cusp(Cusp(1,0))
Infinity
sage: Gamma(7).reduce_cusp(Cusp(6,7))
6/7
Ralf
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