-- Mathlib/RingTheory/Polynomial/Cyclotomic/Basic.lean
theorem int_coeff_of_cyclotomic' {K : Type*} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ}
    (h : IsPrimitiveRoot ζ n) : ∃ P : ℤ[X], map (Int.castRingHom K) P =
      cyclotomic' n K ∧ P.degree = (cyclotomic' n K).degree ∧ P.Monic

Proof state:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
⊢ B.Monic

Tactic:
apply monic_prod_of_monic

Result:
case hs
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
⊢ ∀ i ∈ k.properDivisors, (cyclotomic' i K).Monic

Proof state:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Tactic:
have Bint : B ∈ lifts (Int.castRingHom K) := by
  refine Subsemiring.prod_mem (lifts (Int.castRingHom K)) ?_
  intro x hx
  have xsmall := (Nat.mem_properDivisors.1 hx).2
  obtain ⟨d, hd⟩ := (Nat.mem_properDivisors.1 hx).1
  rw [mul_comm] at hd
  exact ihk x xsmall (h.pow hpos hd)

Result:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
Bint : B ∈ lifts (Int.castRingHom K)
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Proof state:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
⊢ B ∈ lifts (Int.castRingHom K)

Tactic:
refine Subsemiring.prod_mem (lifts (Int.castRingHom K)) ?_

Result:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
⊢ ∀ c ∈ k.properDivisors, cyclotomic' c K ∈ lifts (Int.castRingHom K)

Proof state:
case h
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
huniq : (X ^ k - 1) /ₘ B = cyclotomic' k K ∧ (X ^ k - 1) %ₘ B = 0
⊢ (mapRingHom (Int.castRingHom K)) Q₁ = cyclotomic' k K

Tactic:
rw [coe_mapRingHom, map_divByMonic (Int.castRingHom K) hB₁mo, hB₁, ← huniq.1]

Result:
case h
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
huniq : (X ^ k - 1) /ₘ B = cyclotomic' k K ∧ (X ^ k - 1) %ₘ B = 0
⊢ map (Int.castRingHom K) (X ^ k - 1) /ₘ B = (X ^ k - 1) /ₘ B

Proof state:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Tactic:
have Bmo : B.Monic := by
  apply monic_prod_of_monic
  intro i _
  exact cyclotomic'.monic i K

Result:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Proof state:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Tactic:
let B : K[X] := ∏ i ∈ Nat.properDivisors k, cyclotomic' i K

Result:
case h.inr
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Proof state:
case h.inr.intro.intro.intro
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Tactic:
let Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁

Result:
case h.inr.intro.intro.intro
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Proof state:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
⊢ 0 + B * cyclotomic' k K = X ^ k - 1 ∧ degree 0 < B.degree

Tactic:
constructor

Result:
case left
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
⊢ 0 + B * cyclotomic' k K = X ^ k - 1

case right
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
k : ℕ
ihk : ∀ m < k, ∀ {ζ : K}, IsPrimitiveRoot ζ m → cyclotomic' m K ∈ lifts (Int.castRingHom K)
ζ : K
h : IsPrimitiveRoot ζ k
hpos : k > 0
B : K[X] := ∏ i ∈ k.properDivisors, cyclotomic' i K
Bmo : B.Monic
B₁ : ℤ[X]
hB₁ : map (Int.castRingHom K) B₁ = B
left✝ : B₁.degree = B.degree
hB₁mo : B₁.Monic
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
⊢ degree 0 < B.degree