Module: Mathlib.AlgebraicGeometry.EllipticCurve.DivisionPolynomial.Degree

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

Tactic:
refine natDegree_sub_le_of_le (dm (dm natDegree_X_le (dp h.1)) ?_) (dm (dm h.1 h.1) ?_)

Goal state after:
case succ.succ.left.refine_1
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dm : ∀ {m n : ℕ} {p q : R[X]}, p.natDegree ≤ m → q.natDegree ≤ n → (p * q).natDegree ≤ m + n :=
  fun {m n} {p q} ↦ natDegree_mul_le_of_le
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
cm : ∀ {m n : ℕ} {p q : R[X]}, p.natDegree ≤ m → q.natDegree ≤ n → (p * q).coeff (m + n) = p.coeff m * q.coeff n :=
  fun {m n} {p q} ↦ coeff_mul_add_eq_of_natDegree_le
h : ∀ {n : ℕ}, (W.preΨ' n).natDegree ≤ expDegree n ∧ (W.preΨ' n).coeff (expDegree n) = ↑(expCoeff n) :=
  fun {n} ↦ natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1 + 1) ^ 2 = 1 + 2 * expDegree (n + 2) + if Even (n + 1) then 0 else 3
hd' : (n + 1 + 1) ^ 2 = expDegree (n + 3) + expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc :
  1 =
    (1 * expCoeff (n + 2) ^ 2 * if Even (n + 1) then 1 else 4) -
      expCoeff (n + 3) * expCoeff (n + 1) * if Even (n + 1) then 4 else 1
⊢ (if Even (n + 1) then 1 else W.Ψ₂Sq).natDegree ≤ if Even (n + 1) then 0 else 3

case succ.succ.left.refine_2
R : Type u
inst✝ : CommRing R
W : WeierstrassCurve R
dm : ∀ {m n : ℕ} {p q : R[X]}, p.natDegree ≤ m → q.natDegree ≤ n → (p * q).natDegree ≤ m + n :=
  fun {m n} {p q} ↦ natDegree_mul_le_of_le
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
cm : ∀ {m n : ℕ} {p q : R[X]}, p.natDegree ≤ m → q.natDegree ≤ n → (p * q).coeff (m + n) = p.coeff m * q.coeff n :=
  fun {m n} {p q} ↦ coeff_mul_add_eq_of_natDegree_le
h : ∀ {n : ℕ}, (W.preΨ' n).natDegree ≤ expDegree n ∧ (W.preΨ' n).coeff (expDegree n) = ↑(expCoeff n) :=
  fun {n} ↦ natDegree_coeff_preΨ' W n
n : ℕ
hd : (n + 1 + 1) ^ 2 = 1 + 2 * expDegree (n + 2) + if Even (n + 1) then 0 else 3
hd' : (n + 1 + 1) ^ 2 = expDegree (n + 3) + expDegree (n + 1) + if Even (n + 1) then 3 else 0
hc :
  1 =
    (1 * expCoeff (n + 2) ^ 2 * if Even (n + 1) then 1 else 4) -
      expCoeff (n + 3) * expCoeff (n + 1) * 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