-- 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
Bmo : B.Monic
x : ℕ
hx : x ∈ k.properDivisors
⊢ cyclotomic' x K ∈ lifts (Int.castRingHom K)

Tactic:
have xsmall := (Nat.mem_properDivisors.1 hx).2

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
x : ℕ
hx : x ∈ k.properDivisors
xsmall : x < k
⊢ cyclotomic' x 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
⊢ map (Int.castRingHom K) (X ^ k - 1) /ₘ B = (X ^ k - 1) /ₘ B

Tactic:
simp

Result:
no goals

Proof state:
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

Tactic:
simpa only [degree_zero, bot_lt_iff_ne_bot, Ne, degree_eq_bot] using Bmo.ne_zero

Result:
no goals

Proof state:
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

Tactic:
intro i _

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 : ℕ
a✝ : i ∈ k.properDivisors
⊢ (cyclotomic' i K).Monic

Proof state:
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

Tactic:
rw [zero_add, mul_comm, ← prod_cyclotomic'_eq_X_pow_sub_one hpos h, ←
  Nat.cons_self_properDivisors hpos.ne', Finset.prod_cons]

Result:
no goals

Proof state:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
ζ : K
n : ℕ
h : IsPrimitiveRoot ζ n
⊢ ∃ P, map (Int.castRingHom K) P = cyclotomic' n K ∧ P.degree = (cyclotomic' n K).degree ∧ P.Monic

Tactic:
refine lifts_and_degree_eq_and_monic ?_ (cyclotomic'.monic n K)

Result:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
ζ : K
n : ℕ
h : IsPrimitiveRoot ζ n
⊢ cyclotomic' n K ∈ lifts (Int.castRingHom K)

Proof state:
case 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
x : ℕ
hx : x ∈ k.properDivisors
xsmall : x < k
d : ℕ
hd : k = d * x
⊢ cyclotomic' x K ∈ lifts (Int.castRingHom K)

Tactic:
exact ihk x xsmall (h.pow hpos hd)

Result:
no goals

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
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
huniq : (X ^ k - 1) /ₘ B = cyclotomic' k K ∧ (X ^ k - 1) %ₘ B = 0
⊢ ∃ x, (mapRingHom (Int.castRingHom K)) x = cyclotomic' k K

Tactic:
use Q₁

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
⊢ (mapRingHom (Int.castRingHom K)) Q₁ = cyclotomic' k K

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

Tactic:
simp only [cyclotomic'_zero, coe_mapRingHom, Polynomial.map_one]

Result:
no goals

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
Q₁ : ℤ[X] := (X ^ k - 1) /ₘ B₁
huniq : (X ^ k - 1) /ₘ B = cyclotomic' k K ∧ (X ^ k - 1) %ₘ B = 0
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)

Tactic:
simp only [lifts, RingHom.mem_rangeS]

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
huniq : (X ^ k - 1) /ₘ B = cyclotomic' k K ∧ (X ^ k - 1) %ₘ B = 0
⊢ ∃ x, (mapRingHom (Int.castRingHom K)) x = cyclotomic' k K

Proof state:
K : Type u_2
inst✝¹ : CommRing K
inst✝ : IsDomain K
ζ : K
n : ℕ
h : IsPrimitiveRoot ζ n
⊢ cyclotomic' n K ∈ lifts (Int.castRingHom K)

Tactic:
induction' n using Nat.strong_induction_on with k ihk generalizing ζ

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
⊢ cyclotomic' k K ∈ lifts (Int.castRingHom K)