-- Mathlib/RingTheory/Polynomial/Eisenstein/IsIntegral.lean
theorem mem_adjoin_of_smul_prime_smul_of_minpoly_isEisensteinAt {B : PowerBasis K L}
    (hp : Prime p) (hBint : IsIntegral R B.gen) {z : L} (hzint : IsIntegral R z)
    (hz : p • z ∈ adjoin R ({B.gen} : Set L)) (hei : (minpoly R B.gen).IsEisensteinAt 𝓟) :
    z ∈ adjoin R ({B.gen} : Set L)

Proof state:
case neg.hi.convert_2.intro
R : Type u
K : Type v
L : Type z
p : R
inst✝¹⁰ : CommRing R
inst✝⁹ : Field K
inst✝⁸ : Field L
inst✝⁷ : Algebra K L
inst✝⁶ : Algebra R L
inst✝⁵ : Algebra R K
inst✝⁴ : IsScalarTower R K L
inst✝³ : Algebra.IsSeparable K L
inst✝² : IsDomain R
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
B : PowerBasis K L
hp : _root_.Prime p
hBint : IsIntegral R B.gen
z : L
hzint : IsIntegral R z
this✝ : Module.Finite K L
P : R[X] := minpoly R B.gen
hei : P.IsEisensteinAt (Submodule.span R {p})
hndiv : ¬p ^ 2 ∣ P.coeff 0
hP : P = minpoly R B.gen
n : ℕ
hn : B.dim = n.succ
this : NoZeroSMulDivisors R L
x✝ : L[X] := Polynomial.map (algebraMap R L) P
Q₁ : R[X]
Q : R[X] := Q₁ %ₘ P
hQ₁ : Q = Q₁ %ₘ P
hQzero : ¬Q = 0
j : ℕ
hj : j + 1 ∈ range (Q.natDegree + 1)
H : j + 1 + 1 ≤ P.natDegree
Hj : Q.natDegree + 1 = j + 1 + (Q.natDegree - j)
g : ℕ → R
hg : ∀ k ∈ range (j + 1), Q.coeff k • B.gen ^ k = (algebraMap R L) p * g k • B.gen ^ k
f : ℕ → L
hf :
  ∀ (i : ℕ),
    (Polynomial.map (algebraMap R L) (minpoly R B.gen)).natDegree ≤ i →
      f i ∈ adjoin R {B.gen} ∧ (algebraMap R L) p * f i = B.gen ^ i
hf₁ :
  ∀ k ∈ (range (Q.natDegree - j)).erase 0,
    Q.coeff (j + 1 + k) • B.gen ^ (j + 1 + k) * B.gen ^ (P.natDegree - (j + 2)) =
      (algebraMap R L) p * Q.coeff (j + 1 + k) • f (k + P.natDegree - 1)
hQ :
  (Algebra.norm K) ((algebraMap R L) (Q.coeff (j + 1)) * B.gen ^ (j + 1 + (P.natDegree - (j + 2)))) =
    (Algebra.norm K)
      (p • z * B.gen ^ (P.natDegree - (j + 2)) -
        (algebraMap R L) p *
          (∑ i ∈ (range (Q.natDegree - j)).erase 0, Q.coeff (j + 1 + i) • f (i + P.natDegree - 1) +
            ∑ i ∈ range (j + 1), g i • B.gen ^ i * B.gen ^ (P.natDegree - (j + 2))))
hintsum :
  IsIntegral R
    (z * B.gen ^ (P.natDegree - (j + 2)) -
      (∑ x ∈ (range (Q.natDegree - j)).erase 0, Q.coeff (j + 1 + x) • f (x + P.natDegree - 1) +
        ∑ x ∈ range (j + 1), g x • B.gen ^ x * B.gen ^ (P.natDegree - (j + 2))))
r : R
hr :
  (algebraMap R K) r =
    (Algebra.norm K)
      (z * B.gen ^ (P.natDegree - (j + 2)) -
        (∑ x ∈ (range (Q.natDegree - j)).erase 0, Q.coeff (j + 1 + x) • f (x + P.natDegree - 1) +
          ∑ x ∈ range (j + 1), g x • B.gen ^ x * B.gen ^ (P.natDegree - (j + 2))))
⊢ p ^ n.succ ∣ Q.coeff j.succ ^ n.succ * (minpoly R B.gen).coeff 0 ^ (j.succ + (P.natDegree - (j + 2)))

Tactic:
rw [Algebra.smul_def, mul_assoc, ← mul_sub, _root_.map_mul, algebraMap_apply R K L, map_pow,
  Algebra.norm_algebraMap, _root_.map_mul, algebraMap_apply R K L, Algebra.norm_algebraMap,
  finrank B, ← hr, PowerBasis.norm_gen_eq_coeff_zero_minpoly,
  minpoly.isIntegrallyClosed_eq_field_fractions' K hBint, coeff_map,
  show (-1 : K) = algebraMap R K (-1) by simp, ← map_pow, ← map_pow, ← _root_.map_mul, ←
  map_pow, ← _root_.map_mul, ← map_pow, ← _root_.map_mul] at hQ

Result:
case neg.hi.convert_2.intro
R : Type u
K : Type v
L : Type z
p : R
inst✝¹⁰ : CommRing R
inst✝⁹ : Field K
inst✝⁸ : Field L
inst✝⁷ : Algebra K L
inst✝⁶ : Algebra R L
inst✝⁵ : Algebra R K
inst✝⁴ : IsScalarTower R K L
inst✝³ : Algebra.IsSeparable K L
inst✝² : IsDomain R
inst✝¹ : IsFractionRing R K
inst✝ : IsIntegrallyClosed R
B : PowerBasis K L
hp : _root_.Prime p
hBint : IsIntegral R B.gen
z : L
hzint : IsIntegral R z
this✝ : Module.Finite K L
P : R[X] := minpoly R B.gen
hei : P.IsEisensteinAt (Submodule.span R {p})
hndiv : ¬p ^ 2 ∣ P.coeff 0
hP : P = minpoly R B.gen
n : ℕ
hn : B.dim = n.succ
this : NoZeroSMulDivisors R L
x✝ : L[X] := Polynomial.map (algebraMap R L) P
Q₁ : R[X]
Q : R[X] := Q₁ %ₘ P
hQ₁ : Q = Q₁ %ₘ P
hQzero : ¬Q = 0
j : ℕ
hj : j + 1 ∈ range (Q.natDegree + 1)
H : j + 1 + 1 ≤ P.natDegree
Hj : Q.natDegree + 1 = j + 1 + (Q.natDegree - j)
g : ℕ → R
hg : ∀ k ∈ range (j + 1), Q.coeff k • B.gen ^ k = (algebraMap R L) p * g k • B.gen ^ k
f : ℕ → L
hf :
  ∀ (i : ℕ),
    (Polynomial.map (algebraMap R L) (minpoly R B.gen)).natDegree ≤ i →
      f i ∈ adjoin R {B.gen} ∧ (algebraMap R L) p * f i = B.gen ^ i
hf₁ :
  ∀ k ∈ (range (Q.natDegree - j)).erase 0,
    Q.coeff (j + 1 + k) • B.gen ^ (j + 1 + k) * B.gen ^ (P.natDegree - (j + 2)) =
      (algebraMap R L) p * Q.coeff (j + 1 + k) • f (k + P.natDegree - 1)
hintsum :
  IsIntegral R
    (z * B.gen ^ (P.natDegree - (j + 2)) -
      (∑ x ∈ (range (Q.natDegree - j)).erase 0, Q.coeff (j + 1 + x) • f (x + P.natDegree - 1) +
        ∑ x ∈ range (j + 1), g x • B.gen ^ x * B.gen ^ (P.natDegree - (j + 2))))
r : R
hQ :
  (algebraMap R K)
      (Q.coeff (j + 1) ^ B.dim * ((-1) ^ B.dim * (minpoly R B.gen).coeff 0) ^ (j + 1 + (P.natDegree - (j + 2)))) =
    (algebraMap R K) (p ^ B.dim * r)
hr :
  (algebraMap R K) r =
    (Algebra.norm K)
      (z * B.gen ^ (P.natDegree - (j + 2)) -
        (∑ x ∈ (range (Q.natDegree - j)).erase 0, Q.coeff (j + 1 + x) • f (x + P.natDegree - 1) +
          ∑ x ∈ range (j + 1), g x • B.gen ^ x * B.gen ^ (P.natDegree - (j + 2))))
⊢ p ^ n.succ ∣ Q.coeff j.succ ^ n.succ * (minpoly R B.gen).coeff 0 ^ (j.succ + (P.natDegree - (j + 2)))