-- Mathlib/AlgebraicGeometry/EllipticCurve/DivisionPolynomial/Degree.lean
private lemma natDegree_coeff_ΨSq_ofNat (n : ℕ) :
    (W.ΨSq n).natDegree ≤ n ^ 2 - 1 ∧ (W.ΨSq n).coeff (n ^ 2 - 1) = (n ^ 2 : ℤ)

Proof state:
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
n : ℕ
⊢ (W.ΨSq ↑n).natDegree ≤ n ^ 2 - 1 ∧ (W.ΨSq ↑n).coeff (n ^ 2 - 1) = ↑(↑n ^ 2)

Tactic:
let dp {m n p} : _ → (p ^ n : R[X]).natDegree ≤ n * m := natDegree_pow_le_of_le n

Result:
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
n : ℕ
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
⊢ (W.ΨSq ↑n).natDegree ≤ n ^ 2 - 1 ∧ (W.ΨSq ↑n).coeff (n ^ 2 - 1) = ↑(↑n ^ 2)

Proof state:
case succ.left
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc : ↑(n + 1) ^ 2 = WeierstrassCurve.expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1
⊢ (if Even (n + 1) then W.Ψ₂Sq else 1).natDegree ≤ if Even (n + 1) then 3 else 0

Tactic:
split_ifs <;> simp only [apply_ite natDegree, natDegree_one.le, W.natDegree_Ψ₂Sq_le]

Result:
no goals

Proof state:
case succ
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
⊢ (W.ΨSq ↑(n + 1)).natDegree ≤ (n + 1) ^ 2 - 1 ∧ (W.ΨSq ↑(n + 1)).coeff ((n + 1) ^ 2 - 1) = ↑(↑(n + 1) ^ 2)

Tactic:
have hc : (n + 1 : ℕ) ^ 2 = expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1 := by
  push_cast [← @Int.cast_inj ℚ, expCoeff_cast]
  split_ifs <;> ring1

Result:
case succ
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc : ↑(n + 1) ^ 2 = WeierstrassCurve.expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1
⊢ (W.ΨSq ↑(n + 1)).natDegree ≤ (n + 1) ^ 2 - 1 ∧ (W.ΨSq ↑(n + 1)).coeff ((n + 1) ^ 2 - 1) = ↑(↑(n + 1) ^ 2)

Proof state:
case succ
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc : ↑(n + 1) ^ 2 = WeierstrassCurve.expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1
⊢ ((W.preΨ' (n + 1) ^ 2 * if Even (n + 1) then W.Ψ₂Sq else 1).natDegree ≤
      2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0) ∧
    (W.preΨ' (n + 1) ^ 2 * if Even (n + 1) then W.Ψ₂Sq else 1).coeff
        (2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0) =
      ↑(↑(n + 1) ^ 2)

Tactic:
constructor

Result:
case succ.left
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc : ↑(n + 1) ^ 2 = WeierstrassCurve.expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1
⊢ (W.preΨ' (n + 1) ^ 2 * if Even (n + 1) then W.Ψ₂Sq else 1).natDegree ≤
    2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0

case succ.right
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dp : ∀ {m n : ℕ} {p : R[X]}, p.natDegree ≤ m → (p ^ n).natDegree ≤ n * m := fun {m n} {p} => natDegree_pow_le_of_le n
h : ∀ {n : ℕ},
  (W.preΨ' n).natDegree ≤ WeierstrassCurve.expDegree n ∧
    (W.preΨ' n).coeff (WeierstrassCurve.expDegree n) = ↑(WeierstrassCurve.expCoeff n) :=
  fun {n} => WeierstrassCurve.natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1) ^ 2 - 1 = 2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc : ↑(n + 1) ^ 2 = WeierstrassCurve.expCoeff (n + 1) ^ 2 * if Even (n + 1) then 4 else 1
⊢ (W.preΨ' (n + 1) ^ 2 * if Even (n + 1) then W.Ψ₂Sq else 1).coeff
      (2 * WeierstrassCurve.expDegree (n + 1) + if Even (n + 1) then 3 else 0) =
    ↑(↑(n + 1) ^ 2)