-- Mathlib/RingTheory/WittVector/FrobeniusFractionField.lean
theorem exists_frobenius_solution_fractionRing {a : FractionRing (𝕎 k)} (ha : a ≠ 0) :
    ∃ᵉ (b ≠ 0) (m : ℤ), φ b * a = (p : FractionRing (𝕎 k)) ^ m * b

Proof state:
case mk.mk.intro.intro.intro.intro.intro.intro
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
a : FractionRing (𝕎 k)
m : ℕ
r' : 𝕎 k
hr' : r'.coeff 0 ≠ 0
hr0 : ↑p ^ m * r' ≠ 0
n : ℕ
q' : 𝕎 k
hq' : q'.coeff 0 ≠ 0
hq : ↑p ^ n * q' ∈ nonZeroDivisors (𝕎 k)
hq0 : ↑p ^ n * q' ≠ 0
hrq : Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 ≠ 0
⊢ ∃ b,
    b ≠ 0 ∧
      ∃ m_1, φ b * Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 = ↑p ^ m_1 * b

Tactic:
let b := frobeniusRotation p hr' hq'

Result:
case mk.mk.intro.intro.intro.intro.intro.intro
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
a : FractionRing (𝕎 k)
m : ℕ
r' : 𝕎 k
hr' : r'.coeff 0 ≠ 0
hr0 : ↑p ^ m * r' ≠ 0
n : ℕ
q' : 𝕎 k
hq' : q'.coeff 0 ≠ 0
hq : ↑p ^ n * q' ∈ nonZeroDivisors (𝕎 k)
hq0 : ↑p ^ n * q' ≠ 0
hrq : Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 ≠ 0
b : 𝕎 k := frobeniusRotation p hr' hq'
⊢ ∃ b,
    b ≠ 0 ∧
      ∃ m_1, φ b * Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 = ↑p ^ m_1 * b

Proof state:
case mk.mk.intro.intro.intro.intro.intro.intro.refine_1
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
a : FractionRing (𝕎 k)
m : ℕ
r' : 𝕎 k
hr' : r'.coeff 0 ≠ 0
hr0 : ↑p ^ m * r' ≠ 0
n : ℕ
q' : 𝕎 k
hq' : q'.coeff 0 ≠ 0
hq : ↑p ^ n * q' ∈ nonZeroDivisors (𝕎 k)
hq0 : ↑p ^ n * q' ≠ 0
hrq : Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 ≠ 0
b : 𝕎 k := frobeniusRotation p hr' hq'
⊢ (algebraMap (𝕎 k) (FractionRing (𝕎 k))) b ≠ 0

Tactic:
simpa only [map_zero] using
  (IsFractionRing.injective (WittVector p k) (FractionRing (WittVector p k))).ne
    (frobeniusRotation_nonzero p hr' hq')

Result:
no goals

Proof state:
case mk.mk.intro.intro.intro
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
a : FractionRing (𝕎 k)
q : 𝕎 k
hq : q ∈ nonZeroDivisors (𝕎 k)
hq0 : q ≠ 0
m : ℕ
r' : 𝕎 k
hr' : r'.coeff 0 ≠ 0
hrq : Localization.mk (↑p ^ m * r', ⟨q, hq⟩).1 (↑p ^ m * r', ⟨q, hq⟩).2 ≠ 0
hr0 : ↑p ^ m * r' ≠ 0
⊢ ∃ b, b ≠ 0 ∧ ∃ m_1, φ b * Localization.mk (↑p ^ m * r', ⟨q, hq⟩).1 (↑p ^ m * r', ⟨q, hq⟩).2 = ↑p ^ m_1 * b

Tactic:
obtain ⟨n, q', hq', rfl⟩ := exists_eq_pow_p_mul q hq0

Result:
case mk.mk.intro.intro.intro.intro.intro.intro
p : ℕ
hp : Fact (Nat.Prime p)
k : Type u_1
inst✝² : Field k
inst✝¹ : CharP k p
inst✝ : IsAlgClosed k
a : FractionRing (𝕎 k)
m : ℕ
r' : 𝕎 k
hr' : r'.coeff 0 ≠ 0
hr0 : ↑p ^ m * r' ≠ 0
n : ℕ
q' : 𝕎 k
hq' : q'.coeff 0 ≠ 0
hq : ↑p ^ n * q' ∈ nonZeroDivisors (𝕎 k)
hq0 : ↑p ^ n * q' ≠ 0
hrq : Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 ≠ 0
⊢ ∃ b,
    b ≠ 0 ∧
      ∃ m_1, φ b * Localization.mk (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).1 (↑p ^ m * r', ⟨↑p ^ n * q', hq⟩).2 = ↑p ^ m_1 * b