-- Mathlib/NumberTheory/GaussSum.lean
theorem FiniteField.two_pow_card {F : Type*} [Fintype F] [Field F] (hF : ringChar F ≠ 2) :
    (2 : F) ^ (Fintype.card F / 2) = χ₈ (Fintype.card F)

Proof state:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
have hp2 (n : ℕ) : (2 ^ n : F) ≠ 0 := pow_ne_zero n (Ring.two_ne_zero hF)

Result:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Proof state:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
let χ := χ₈.ringHomComp (Int.castRingHom FF)

Result:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
χ : MulChar (ZMod 8) FF := χ₈.ringHomComp (Int.castRingHom FF)
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Proof state:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
χ : MulChar (ZMod 8) FF := χ₈.ringHomComp (Int.castRingHom FF)
hχ : χ (-1) = 1
hq : χ.IsQuadratic
h₁ : (fun a => ↑(χ₈ a) * τ ^ ↑a) = fun a => χ a * ψ₈char a
hg₁ : gaussSum χ ψ₈char = 2 * (τ - τ ^ 3)
hg : gaussSum χ ψ₈char ^ 2 = χ (-1) * ↑(Fintype.card (ZMod 8))
h : ↑8 ^ (Fintype.card F / 2) = χ ↑(Fintype.card F)
⊢ 8 = 2 ^ 2 * 2

Tactic:
norm_num

Result:
no goals

Proof state:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
have hchar := Algebra.ringChar_eq F FF

Result:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Proof state:
case intro.intro.convert_2.a
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
χ : MulChar (ZMod 8) FF := χ₈.ringHomComp (Int.castRingHom FF)
hχ : χ (-1) = 1
hq : χ.IsQuadratic
h₁ : (fun a => ↑(χ₈ a) * τ ^ ↑a) = fun a => χ a * ψ₈char a
hg₁ : gaussSum χ ψ₈char = 2 * (τ - τ ^ 3)
hg : gaussSum χ ψ₈char ^ 2 = χ (-1) * ↑(Fintype.card (ZMod 8))
h : ↑8 ^ (Fintype.card F / 2) = χ ↑(Fintype.card F)
⊢ (algebraMap F FF) (8 ^ (Fintype.card F / 2)) = (algebraMap F FF) ↑(χ₈ ↑(Fintype.card F))

Tactic:
simpa only [map_pow, map_ofNat, map_intCast, Nat.cast_ofNat] using h

Result:
no goals

Proof state:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
⊢ ¬ringChar FF ∣ 8

Tactic:
rw [Ne, ← Nat.prime_dvd_prime_iff_eq FFp Nat.prime_two] at hFF

Result:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ¬ringChar FF ∣ 2
⊢ ¬ringChar FF ∣ 8

Proof state:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
let FF := CyclotomicField 8 F

Result:
case intro.intro
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Proof state:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
χ : MulChar (ZMod 8) FF := χ₈.ringHomComp (Int.castRingHom FF)
hχ : χ (-1) = 1
hq : χ.IsQuadratic
h₁ : (fun a => ↑(χ₈ a) * τ ^ ↑a) = fun a => χ a * ψ₈char a
⊢ gaussSum χ ψ₈char = 2 * (τ - τ ^ 3)

Tactic:
rw [gaussSum, ← h₁, Fin.sum_univ_eight,
  show χ₈ 0 = 0 from rfl, show χ₈ 1 = 1 from rfl, show χ₈ 2 = 0 from rfl,
  show χ₈ 3 = -1 from rfl, show χ₈ 4 = 0 from rfl, show χ₈ 5 = -1 from rfl,
  show χ₈ 6 = 0 from rfl, show χ₈ 7 = 1 from rfl,
  show ((3 : Fin 8) : ℕ) = 3 from rfl, show ((5 : Fin 8) : ℕ) = 5 from rfl,
  show ((7 : Fin 8) : ℕ) = 7 from rfl]

Result:
F : Type u_1
inst✝¹ : Fintype F
inst✝ : Field F
hF : ringChar F ≠ 2
hp2 : ∀ (n : ℕ), 2 ^ n ≠ 0
n : ℕ+
hp : Nat.Prime (ringChar F)
hc : Fintype.card F = ringChar F ^ ↑n
FF : Type u_1 := CyclotomicField 8 F
hchar : ringChar F = ringChar FF
FFp : Nat.Prime (ringChar FF)
this : Fact (Nat.Prime (ringChar FF))
hFF : ringChar FF ≠ 2
hu : IsUnit ↑(ringChar FF)
ψ₈ : PrimitiveAddChar (ZMod ↑8) F := primitiveZModChar 8 F ⋯
ψ₈char : AddChar (ZMod 8) FF := ψ₈.char
τ : FF := ψ₈char 1
τ_spec : τ ^ 4 = -1
χ : MulChar (ZMod 8) FF := χ₈.ringHomComp (Int.castRingHom FF)
hχ : χ (-1) = 1
hq : χ.IsQuadratic
h₁ : (fun a => ↑(χ₈ a) * τ ^ ↑a) = fun a => χ a * ψ₈char a
⊢ ↑0 * τ ^ ↑0 + ↑1 * τ ^ ↑1 + ↑0 * τ ^ ↑2 + ↑(-1) * τ ^ 3 + ↑0 * τ ^ ↑4 + ↑(-1) * τ ^ 5 + ↑0 * τ ^ ↑6 + ↑1 * τ ^ 7 =
    2 * (τ - τ ^ 3)