-- Module: Mathlib.RingTheory.Nilpotent.Exp
Goal:
A : Type u_1
inst : Ring A
inst_1 : Module ℚ A
a b : A
h₁ : Commute a b
n₁ : ℕ
hn₁ : a ^ n₁ = 0
n₂ : ℕ
hn₂ : b ^ n₂ = 0
N : ℕ := max n₁ n₂
h₄ : a ^ (N + 1) = 0
h₅ : b ^ (N + 1) = 0
R2N : Finset ℕ := range (2 * N + 1)
hR2N : R2N = range (2 * N + 1)
RN : Finset ℕ := range (N + 1)
hRN : RN = range (N + 1)
s₁ :
  Eq.{u_1 + 1} (α := A) (∑ i ∈ R2N, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ ((a + b) ^ i))
    (∑ ij ∈ RN ×ˢ RN, HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
z₁ :
  Eq.{u_1 + 1} (α := A)
    (∑ ij ∈ R2N ×ˢ R2N with ¬ij.1 + ij.2 ≤ 2 * N,
      HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
    0
split₁ :
  Eq.{u_1 + 1} (α := A)
    (∑ x ∈ R2N ×ˢ R2N with x.1 + x.2 ≤ 2 * N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
    (∑ x ∈ R2N ×ˢ R2N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
z₂ :
  Eq.{u_1 + 1} (α := A)
    (∑ ij ∈ R2N ×ˢ R2N with ¬(ij.1 ≤ N ∧ ij.2 ≤ N),
      HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
    0
split₂ :
  Eq.{u_1 + 1} (α := A)
    (∑ x ∈ R2N ×ˢ R2N with x.1 ≤ N ∧ x.2 ≤ N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
    (∑ x ∈ R2N ×ˢ R2N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
restrict :
  Eq.{u_1 + 1} (α := A)
    (∑ ij ∈ R2N ×ˢ R2N with ij.1 ≤ N ∧ ij.2 ≤ N,
      HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
    (∑ ij ∈ RN ×ˢ RN, HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
s₂ :
  HMul.hMul (α := A) (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (a ^ i))
      (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (b ^ i)) =
    ∑ ij ∈ RN ×ˢ RN, HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2)
rwa rwa₁ rwa₂ rwa₃ :
  ∀
    (s₁ :
      ∑ i ∈ R2N, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ ((a + b) ^ i) =
        HMul.hMul (α := A) (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (a ^ i))
          (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (b ^ i)))
    (z₁ :
      Eq.{u_1 + 1} (α := A)
        (∑ ij ∈ R2N ×ˢ R2N with ¬ij.1 + ij.2 ≤ 2 * N,
          HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
        0)
    (split₁ :
      Eq.{u_1 + 1} (α := A)
        (∑ x ∈ R2N ×ˢ R2N with x.1 + x.2 ≤ 2 * N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
        (∑ x ∈ R2N ×ˢ R2N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2)))
    (z₂ :
      Eq.{u_1 + 1} (α := A)
        (∑ ij ∈ R2N ×ˢ R2N with ¬(ij.1 ≤ N ∧ ij.2 ≤ N),
          HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
        0)
    (split₂ :
      Eq.{u_1 + 1} (α := A)
        (∑ x ∈ R2N ×ˢ R2N with x.1 ≤ N ∧ x.2 ≤ N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2))
        (∑ x ∈ R2N ×ˢ R2N, HMul.hMul (α := ℚ) (↑x.1! : ℚ)⁻¹ (↑x.2! : ℚ)⁻¹ • (a ^ x.1 * b ^ x.2)))
    (restrict :
      Eq.{u_1 + 1} (α := A)
        (∑ ij ∈ R2N ×ˢ R2N with ij.1 ≤ N ∧ ij.2 ≤ N,
          HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2))
        (∑ ij ∈ RN ×ˢ RN, HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2)))
    (s₂ :
      HMul.hMul (α := A) (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (a ^ i))
          (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (b ^ i)) =
        ∑ ij ∈ RN ×ˢ RN, HMul.hMul (α := ℚ) (↑ij.1! : ℚ)⁻¹ (↑ij.2! : ℚ)⁻¹ • (a ^ ij.1 * b ^ ij.2)),
    ∑ i ∈ R2N, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ ((a + b) ^ i) =
      HMul.hMul (α := A) (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (a ^ i))
        (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (b ^ i))
⊢ ∑ i ∈ R2N, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ ((a + b) ^ i) =
    HMul.hMul (α := A) (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (a ^ i))
      (∑ i ∈ RN, HSMul.hSMul (α := ℚ) (↑i ! : ℚ)⁻¹ (b ^ i))

Tactic:
rwa [s₂.symm] at s₁

Premises used: Eq.symm