-- Mathlib/Combinatorics/Enumerative/Catalan.lean
theorem gosper_trick {n i : ℕ} (h : i ≤ n) :
    gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
      Nat.centralBinom i / (i + 1) * Nat.centralBinom (n - i) / (n - i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
have h₄ :
  ((n : ℚ) - i + 1) * (n - i + 1).centralBinom = 2 * (2 * (n - i) + 1) * (n - i).centralBinom :=
    mod_cast Nat.succ_mul_centralBinom_succ (n - i)

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
h₄ : (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom = 2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
have l₂ : (n : ℚ) - i + 1 ≠ 0 := by norm_cast

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
have l₁ : (i : ℚ) + 1 ≠ 0 := by norm_cast

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
h₄ : (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom = 2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
simp only [gosperCatalan]

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
h₄ : (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom = 2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom
⊢ ↑(i + 1).centralBinom * ↑(n + 1 - (i + 1)).centralBinom * (2 * ↑(i + 1) - ↑(n + 1)) /
        (2 * ↑(n + 1) * (↑(n + 1) + 1)) -
      ↑i.centralBinom * ↑(n + 1 - i).centralBinom * (2 * ↑i - ↑(n + 1)) / (2 * ↑(n + 1) * (↑(n + 1) + 1)) =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
have h₃ : ((i : ℚ) + 1) * (i + 1).centralBinom = 2 * (2 * i + 1) * i.centralBinom :=
  mod_cast Nat.succ_mul_centralBinom_succ i

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
h₄ : (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom = 2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom
⊢ ↑(i + 1).centralBinom * ↑(n - i).centralBinom * (2 * (↑i + 1) - (↑n + 1)) / (2 * (↑n + 1) * (↑n + 1 + 1)) -
      ↑i.centralBinom * ↑(n - i + 1).centralBinom * (2 * ↑i - (↑n + 1)) / (2 * (↑n + 1) * (↑n + 1 + 1)) =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
rw [h₁, h₂, h₃, h₄]

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
h₃ : (↑i + 1) * ↑(i + 1).centralBinom = 2 * (2 * ↑i + 1) * ↑i.centralBinom
h₄ : (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom = 2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom
⊢ 2 * (2 * ↑i + 1) * ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom * (2 * (↑i + 1) - (↑n + 1)) /
        (2 * (↑n + 1) * (↑n + 1 + 1)) -
      ↑i.centralBinom * (2 * (2 * (↑n - ↑i) + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)) * (2 * ↑i - (↑n + 1)) /
        (2 * (↑n + 1) * (↑n + 1 + 1)) =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Proof state:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)

Tactic:
have h₂ := (mul_div_cancel_left₀ (↑(Nat.centralBinom (n - i + 1))) l₂).symm

Result:
n i : ℕ
h : i ≤ n
l₁ : ↑i + 1 ≠ 0
l₂ : ↑n - ↑i + 1 ≠ 0
h₁ : ↑(i + 1).centralBinom = (↑i + 1) * ↑(i + 1).centralBinom / (↑i + 1)
h₂ : ↑(n - i + 1).centralBinom = (↑n - ↑i + 1) * ↑(n - i + 1).centralBinom / (↑n - ↑i + 1)
⊢ gosperCatalan (n + 1) (i + 1) - gosperCatalan (n + 1) i =
    ↑i.centralBinom / (↑i + 1) * ↑(n - i).centralBinom / (↑n - ↑i + 1)