feat(group_theory/specific_groups/cyclic): |G|=expG ↔ G is cyclic (#10692)
feat(group_theory/specific_groups/cyclic): |G|=expG ↔ G is cyclic (#10692)



Co-authored-by: Eric <480064f38bcfd78eaf0148211ef61a38099706aa@users.noreply.github.com>
Co-authored-by: Johan Commelin <759412786bc533369b22377bf83fb9056c5b25b2@commelin.net>

leanprover-community/mathlib,leanprover-community/mathlib