-- Mathlib/Algebra/Polynomial/SumIteratedDerivative.lean
theorem aeval_sumIDeriv_of_pos [Nontrivial A] [NoZeroDivisors A] (p : R[X]) {q : ℕ} (hq : 0 < q)
    (inj_amap : Function.Injective (algebraMap R A)) :
    ∃ gp : R[X], gp.natDegree ≤ p.natDegree - q ∧
      ∀ (r : A) {p' : A[X]},
        p.map (algebraMap R A) = (X - C r) ^ (q - 1) * p' →
        aeval r (sumIDeriv p) = (q - 1)! • p'.eval r + q ! • aeval r gp

Proof state:
case inr.refine_2
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this✝ : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
this : range q = range (q - 1 + 1)
⊢ ∑ x ∈ range (q - 1), (aeval r) ((⇑derivative)^[x] p) + (aeval r) ((⇑derivative)^[q - 1] p) +
      ∑ x ∈ Ico q (p.natDegree + 1), (aeval r) ((⇑derivative)^[x] p) =
    0 + (q - 1)! • eval r p' + q ! • ∑ x ∈ Ico q (p.natDegree + 1), (aeval r) (c x)

Tactic:
congr 2

Result:
case inr.refine_2.e_a.e_a
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this✝ : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
this : range q = range (q - 1 + 1)
⊢ ∑ x ∈ range (q - 1), (aeval r) ((⇑derivative)^[x] p) = 0

case inr.refine_2.e_a.e_a
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this✝ : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
this : range q = range (q - 1 + 1)
⊢ (aeval r) ((⇑derivative)^[q - 1] p) = (q - 1)! • eval r p'

case inr.refine_2.e_a
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this✝ : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
this : range q = range (q - 1 + 1)
⊢ ∑ x ∈ Ico q (p.natDegree + 1), (aeval r) ((⇑derivative)^[x] p) = q ! • ∑ x ∈ Ico q (p.natDegree + 1), (aeval r) (c x)

Proof state:
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
⊢ range q = range (q - 1 + 1)

Tactic:
rw [tsub_add_cancel_of_le (Nat.one_le_of_lt hq)]

Result:
no goals

Proof state:
case inl
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
⊢ ∃ gp,
    gp.natDegree ≤ natDegree 0 - q ∧
      ∀ (r : A) {p' : A[X]},
        map (algebraMap R A) 0 = (X - C r) ^ (q - 1) * p' →
          (aeval r) (sumIDeriv 0) = (q - 1)! • eval r p' + q ! • (aeval r) gp

Tactic:
use 0

Result:
case h
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
⊢ natDegree 0 ≤ natDegree 0 - q ∧
    ∀ (r : A) {p' : A[X]},
      map (algebraMap R A) 0 = (X - C r) ^ (q - 1) * p' →
        (aeval r) (sumIDeriv 0) = (q - 1)! • eval r p' + q ! • (aeval r) 0

Proof state:
case inr.refine_2
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
⊢ (aeval r) (sumIDeriv p) = (q - 1)! • eval r p' + q ! • (aeval r) (∑ x ∈ Ico q (p.natDegree + 1), c x)

Tactic:
rw [← zero_add ((q - 1)! • p'.eval r)]

Result:
case inr.refine_2
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
this : range (p.natDegree + 1) = range q ∪ Ico q (p.natDegree + 1)
⊢ (aeval r) (sumIDeriv p) = 0 + (q - 1)! • eval r p' + q ! • (aeval r) (∑ x ∈ Ico q (p.natDegree + 1), c x)

Proof state:
case hq
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
p' : A[X]
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * p'
⊢ p' ≠ 0

Tactic:
rintro rfl

Result:
case hq
R : Type u_1
inst✝⁴ : CommSemiring R
A : Type u_3
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : Nontrivial A
inst✝ : NoZeroDivisors A
p : R[X]
q : ℕ
hq : 0 < q
inj_amap : Function.Injective ⇑(algebraMap R A)
p0 : p ≠ 0
c : ℕ → R[X] := fun k => if hk : q ≤ k then ⋯.choose else 0
c_le : ∀ (k : ℕ), (c k).natDegree ≤ p.natDegree - k
hc : ∀ (k : ℕ), q ≤ k → ∀ (r : A), (aeval r) ((⇑derivative)^[k] p) = q ! • (aeval r) (c k)
r : A
hp : map (algebraMap R A) p = (X - C r) ^ (q - 1) * 0
⊢ False