-- Mathlib/FieldTheory/SeparableDegree.lean
theorem natSepDegree_eq_one_iff_eq_X_sub_C_pow : (minpoly F x).natSepDegree = 1 ↔
    ∃ n : ℕ, (minpoly F x).map (algebraMap F E) = (X - C x) ^ q ^ n

Proof state:
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this : ExpChar E q
⊢ (minpoly F x).natSepDegree = 1 ↔ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
haveI := expChar_of_injective_ringHom (C_injective (R := E)) q

Result:
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
⊢ (minpoly F x).natSepDegree = 1 ↔ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Proof state:
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : (algebraMap F E) ((minpoly F x).coeff 0) = -1 * x ^ q ^ n
⊢ (algebraMap F E) (-(minpoly F x).coeff 0) = x ^ q ^ n

Tactic:
rw [map_neg, h, neg_mul, one_mul, neg_neg]

Result:
no goals

Proof state:
case refine_1.intro.intro
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
hx : (Polynomial.aeval x) (X ^ q ^ n - C y) = (Polynomial.aeval x) (minpoly F x)
⊢ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
rw [minpoly.aeval, map_sub, map_pow, aeval_X, aeval_C, sub_eq_zero, eq_comm] at hx

Result:
case refine_1.intro.intro
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
hx : (algebraMap F E) y = x ^ q ^ n
⊢ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Proof state:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : (algebraMap F E) ((minpoly F x).coeff 0) = (0 - x) ^ q ^ n
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Tactic:
rw [zero_sub, neg_pow, neg_one_pow_expChar_pow] at h

Result:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : (algebraMap F E) ((minpoly F x).coeff 0) = -1 * x ^ q ^ n
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Proof state:
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
⊢ (minpoly F x).natSepDegree = 1 ↔ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
haveI := expChar_of_injective_algebraMap (algebraMap F E).injective q

Result:
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this : ExpChar E q
⊢ (minpoly F x).natSepDegree = 1 ↔ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Proof state:
case h
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
hx : (algebraMap F E) y = x ^ q ^ n
⊢ Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
rw [h, Polynomial.map_sub, Polynomial.map_pow, map_X, map_C, hx, map_pow,
  ← sub_pow_expChar_pow_of_commute _ _ (commute_X _)]

Result:
no goals

Proof state:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : constantCoeff (Polynomial.map (algebraMap F E) (minpoly F x)) = constantCoeff ((X - C x) ^ q ^ n)
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Tactic:
simp_rw [map_pow, map_sub, constantCoeff_apply, coeff_map, coeff_X_zero, coeff_C_zero] at h

Result:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : (algebraMap F E) ((minpoly F x).coeff 0) = (0 - x) ^ q ^ n
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Proof state:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Tactic:
apply_fun constantCoeff at h

Result:
case refine_2
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
x✝ : ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n
n : ℕ
h : constantCoeff (Polynomial.map (algebraMap F E) (minpoly F x)) = constantCoeff ((X - C x) ^ q ^ n)
⊢ ∃ n, x ^ q ^ n ∈ (algebraMap F E).range

Proof state:
case refine_1.intro.intro
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
hx : (algebraMap F E) y = x ^ q ^ n
⊢ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
use n

Result:
case h
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
hx : (algebraMap F E) y = x ^ q ^ n
⊢ Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Proof state:
case refine_1
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h : (minpoly F x).natSepDegree = 1
⊢ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n

Tactic:
obtain ⟨n, y, h⟩ := (natSepDegree_eq_one_iff_eq_X_pow_sub_C q).1 h

Result:
case refine_1.intro.intro
F : Type u
E : Type v
inst✝³ : Field F
inst✝² : Ring E
inst✝¹ : IsDomain E
inst✝ : Algebra F E
q : ℕ
hF : ExpChar F q
x : E
this✝ : ExpChar E q
this : ExpChar E[X] q
h✝ : (minpoly F x).natSepDegree = 1
n : ℕ
y : F
h : minpoly F x = X ^ q ^ n - C y
⊢ ∃ n, Polynomial.map (algebraMap F E) (minpoly F x) = (X - C x) ^ q ^ n