-- Mathlib/RingTheory/MvPolynomial/Groebner.lean
theorem degree_reduce_lt {f b : MvPolynomial σ R} (hb : IsUnit (m.leadingCoeff b))
    (hbf : m.degree b ≤ m.degree f) (hf : m.degree f ≠ 0) :
    m.degree (m.reduce hb f) ≺[m] m.degree f

Proof state:
case right
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
K : m.degree (m.reduce hb f) = m.degree f
⊢ False

Tactic:
nth_rewrite 1 [← K] at H'

Result:
case right
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree (m.reduce hb f)) (m.reduce hb f) = 0
K : m.degree (m.reduce hb f) = m.degree f
⊢ False

Proof state:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree (m.reduce hb f)) ≤ m.toSyn (m.degree f) ∧ m.toSyn (m.degree (m.reduce hb f)) ≠ m.toSyn (m.degree f)

Tactic:
constructor

Result:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree (m.reduce hb f)) ≤ m.toSyn (m.degree f)

case right
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree (m.reduce hb f)) ≠ m.toSyn (m.degree f)

Proof state:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f) * b)) ≤ m.toSyn (m.degree f)

Tactic:
apply le_of_le_of_eq degree_mul_le

Result:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b) =
    m.toSyn (m.degree f)

Proof state:
case hnc
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
⊢ ¬f = 0

Tactic:
intro hf0

Result:
case hnc
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
hf0 : f = 0
⊢ False

Proof state:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree (m.reduce hb f)) ≤ m.toSyn (m.degree f)

Tactic:
classical
apply le_trans degree_sub_le
simp only [sup_le_iff, le_refl, true_and]
apply le_of_le_of_eq degree_mul_le
rw [m.toSyn.injective.eq_iff]
exact H.symm

Result:
no goals

Proof state:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree (m.reduce hb f)) ≤ m.toSyn (m.degree f)

Tactic:
apply le_trans degree_sub_le

Result:
case left
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
H' : coeff (m.degree f) (m.reduce hb f) = 0
⊢ m.toSyn (m.degree f) ⊔ m.toSyn (m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f) * b)) ≤
    m.toSyn (m.degree f)

Proof state:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
⊢ coeff (m.degree f) f = coeff (m.degree f) ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f) * b)

Tactic:
nth_rewrite 2 [H]

Result:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
⊢ coeff (m.degree f) f =
    coeff (m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b)
      ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f) * b)

Proof state:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
⊢ coeff (m.degree f) (m.reduce hb f) = 0

Tactic:
simp only [reduce, coeff_sub, sub_eq_zero]

Result:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
H : m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b
⊢ coeff (m.degree f) f = coeff (m.degree f) ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f) * b)

Proof state:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
⊢ m.degree f = m.degree ((monomial (m.degree f - m.degree b)) (↑hb.unit⁻¹ * m.leadingCoeff f)) + m.degree b

Tactic:
rw [degree_monomial, if_neg]

Result:
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
⊢ m.degree f = m.degree f - m.degree b + m.degree b

case hnc
σ : Type u_1
m : MonomialOrder σ
R : Type u_2
inst✝ : CommRing R
f b : MvPolynomial σ R
hb : IsUnit (m.leadingCoeff b)
hbf : m.degree b ≤ m.degree f
hf : m.degree f ≠ 0
⊢ ¬↑hb.unit⁻¹ * m.leadingCoeff f = 0