-- Mathlib/Dynamics/Newton.lean
theorem existsUnique_nilpotent_sub_and_aeval_eq_zero
    (h : IsNilpotent (aeval x P)) (h' : IsUnit (aeval x <| derivative P)) :
    ∃! r, IsNilpotent (x - r) ∧ aeval r P = 0

Proof state:
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
⊢ ∃! r, IsNilpotent (r - x) ∧ (aeval r) P = 0

Tactic:
refine existsUnique_of_exists_of_unique ?_ fun r₁ r₂ ⟨hr₁, hr₁'⟩ ⟨hr₂, hr₂'⟩ ↦ ?_

Result:
case refine_1
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
⊢ ∃ x_1, IsNilpotent (x_1 - x) ∧ (aeval x_1) P = 0

case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
⊢ r₁ = r₂

Proof state:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
this : IsUnit ((aeval r₁) (derivative P))
⊢ IsUnit ((aeval r₁) (derivative P) + u * (r₂ - x - (r₁ - x)))

Tactic:
refine IsNilpotent.isUnit_add_left_of_commute ?_ this (Commute.all _ _)

Result:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
this : IsUnit ((aeval r₁) (derivative P))
⊢ IsNilpotent (u * (r₂ - x - (r₁ - x)))

Proof state:
case refine_1.intro
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
n : ℕ
hn : (aeval x) P ^ n = 0
⊢ (aeval (P.newtonMap^[n] x)) P = 0

Tactic:
rw [← zero_dvd_iff, ← pow_eq_zero_of_le (n.lt_two_pow_self).le hn]

Result:
case refine_1.intro
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
n : ℕ
hn : (aeval x) P ^ n = 0
⊢ (aeval x) P ^ 2 ^ n ∣ (aeval (P.newtonMap^[n] x)) P

Proof state:
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
⊢ ∃! r, IsNilpotent (x - r) ∧ (aeval r) P = 0

Tactic:
simp_rw [(neg_sub _ x).symm, isNilpotent_neg_iff]

Result:
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
⊢ ∃! r, IsNilpotent (r - x) ∧ (aeval r) P = 0

Proof state:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
⊢ r₁ = r₂

Tactic:
suffices IsUnit (aeval r₁ (derivative P) + u * (r₂ - r₁)) by
  rwa [derivative_map, eval_map_algebraMap, eval_map_algebraMap, eval_map_algebraMap,
    add_sub_cancel, hr₂', hr₁', zero_add, pow_two, ← mul_assoc, ← add_mul, eq_comm,
    this.mul_right_eq_zero, sub_eq_zero, eq_comm] at hu

Result:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
⊢ IsUnit ((aeval r₁) (derivative P) + u * (r₂ - r₁))

Proof state:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
⊢ r₁ = r₂

Tactic:
have ⟨u, hu⟩ := binomExpansion (P.map (algebraMap R S)) r₁ (r₂ - r₁)

Result:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
⊢ r₁ = r₂

Proof state:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
this : IsUnit ((aeval r₁) (derivative P))
⊢ IsUnit ((aeval r₁) (derivative P) + u * (r₂ - r₁))

Tactic:
rw [← sub_sub_sub_cancel_right r₂ r₁ x]

Result:
case refine_2
R : Type u_1
S : Type u_2
inst✝² : CommRing R
inst✝¹ : CommRing S
inst✝ : Algebra R S
P : R[X]
x : S
h : IsNilpotent ((aeval x) P)
h' : IsUnit ((aeval x) (derivative P))
r₁ r₂ : S
x✝¹ : IsNilpotent (r₁ - x) ∧ (aeval r₁) P = 0
x✝ : IsNilpotent (r₂ - x) ∧ (aeval r₂) P = 0
hr₁ : IsNilpotent (r₁ - x)
hr₁' : (aeval r₁) P = 0
hr₂ : IsNilpotent (r₂ - x)
hr₂' : (aeval r₂) P = 0
u : S
hu :
  eval (r₁ + (r₂ - r₁)) (map (algebraMap R S) P) =
    eval r₁ (map (algebraMap R S) P) + eval r₁ (derivative (map (algebraMap R S) P)) * (r₂ - r₁) + u * (r₂ - r₁) ^ 2
this : IsUnit ((aeval r₁) (derivative P))
⊢ IsUnit ((aeval r₁) (derivative P) + u * (r₂ - x - (r₁ - x)))