Hecke operators (Lemma 12.19, Theorem 12.20)

Stream: FLT

[Bryan Wang]

Hi all! I've just opened two PRs corresponding to Lemma 12.19, Theorem 12.20 respectively - both are prerequisites for issue #584 (commutativity of Hecke operators) which I'm currently working on. I'm relatively new to this project, so let me know if there are things to fix!

[Yaël Dillies]

FLT#584

[Kevin Buzzard]

Thanks a lot! I'm away this weekend but I will have a lot of time on Monday and Tuesday to get on top of the recent FLT PRs.