feat(ring_theory/polynomial/cyclotomic/*): ɸₙ(1) = 1 (#10483)
feat(ring_theory/polynomial/cyclotomic/*): ɸₙ(1) = 1 (#10483)

(for `n` not a prime power)



Co-authored-by: Eric <480064f38bcfd78eaf0148211ef61a38099706aa@users.noreply.github.com>
Co-authored-by: Yury G. Kudryashov <2c59405d0ec2e119a1c3d2318bdb4a6ce50051ae@urkud.name>
Co-authored-by: Johan Commelin <759412786bc533369b22377bf83fb9056c5b25b2@commelin.net>

leanprover-community/mathlib,leanprover-community/mathlib