-- Mathlib/RingTheory/MvPowerSeries/LexOrder.lean
theorem coeff_mul_of_add_lexOrder {φ ψ : MvPowerSeries σ R}
    {p q : σ →₀ ℕ} (hp : lexOrder φ = toLex p) (hq : lexOrder ψ = toLex q) :
    coeff R (p + q) (φ * ψ) = coeff R p φ * coeff R q ψ

Proof state:
case h₀
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
⊢ ∀ b ∈ Finset.antidiagonal (p + q), b ≠ (p, q) → (coeff R b.1) φ * (coeff R b.2) ψ = 0

Tactic:
rintro ⟨u, v⟩ h h'

Result:
case h₀.mk
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h : (u, v) ∈ Finset.antidiagonal (p + q)
h' : (u, v) ≠ (p, q)
⊢ (coeff R (u, v).1) φ * (coeff R (u, v).2) ψ = 0

Proof state:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ False

Tactic:
apply h'

Result:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ (u, v) = (p, q)

Proof state:
case neg.right.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ toLex v = toLex q

Tactic:
apply Or.resolve_right (eq_or_gt_of_le hv)

Result:
case neg.right.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ ¬toLex q < toLex v

Proof state:
case neg.right
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ v = q

Tactic:
apply toLex.injective

Result:
case neg.right.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ toLex v = toLex q

Proof state:
case h₀.mk
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
⊢ (coeff R u) φ * (coeff R v) ψ = 0

Tactic:
by_cases hu : toLex u < toLex p

Result:
case pos
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex u < toLex p
⊢ (coeff R u) φ * (coeff R v) ψ = 0

case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : ¬toLex u < toLex p
⊢ (coeff R u) φ * (coeff R v) ψ = 0

Proof state:
case neg.left.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ toLex u = toLex p

Tactic:
apply Or.resolve_right (eq_or_gt_of_le hu)

Result:
case neg.left.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
⊢ ¬toLex p < toLex u

Proof state:
case neg.right.a
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : toLex q ≤ toLex v
hv' : toLex q < toLex v
⊢ False

Tactic:
exact not_le.mpr (add_lt_add_of_le_of_lt hu hv') (le_of_eq h)

Result:
no goals

Proof state:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : ¬toLex u < toLex p
⊢ ¬↑(toLex q) ≤ ↑(toLex v)

Tactic:
simp only [not_lt] at hu

Result:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
⊢ ¬↑(toLex q) ≤ ↑(toLex v)

Proof state:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
⊢ ¬↑(toLex q) ≤ ↑(toLex v)

Tactic:
intro hv

Result:
case neg
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
hu : toLex p ≤ toLex u
hv : ↑(toLex q) ≤ ↑(toLex v)
⊢ False

Proof state:
case h₀.mk
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
⊢ (coeff R (u, v).1) φ * (coeff R (u, v).2) ψ = 0

Tactic:
simp only

Result:
case h₀.mk
σ : Type u_1
R : Type u_2
inst✝² : Semiring R
inst✝¹ : LinearOrder σ
inst✝ : WellFoundedGT σ
φ ψ : MvPowerSeries σ R
p q : σ →₀ ℕ
hp : φ.lexOrder = ↑(toLex p)
hq : ψ.lexOrder = ↑(toLex q)
u v : σ →₀ ℕ
h' : (u, v) ≠ (p, q)
h : u + v = p + q
⊢ (coeff R u) φ * (coeff R v) ψ = 0