-- 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:
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)
hχ : χ (-1) = 1
hq : χ.IsQuadratic
h₁ : (fun a => ↑(χ₈ a) * τ ^ ↑a) = fun a => χ a * ψ₈char a
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
have hg₁ : gaussSum χ ψ₈char = 2 * (τ - τ ^ 3) := by
  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]
  simp only [Int.cast_zero, zero_mul, Int.cast_one, Fin.val_one, pow_one, one_mul, zero_add,
    Fin.val_two, add_zero, Int.reduceNeg, Int.cast_neg, neg_mul]
  linear_combination (τ ^ 3 - τ) * τ_spec

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)
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)
⊢ 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
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 : (χ (-1) * ↑(Fintype.card (ZMod 8))) ^ (ringChar FF ^ ↑n / 2) = χ (↑(ringChar FF) ^ ↑n)
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
rw [ZMod.card, ← hchar, hχ, one_mul, ← hc, ← Nat.cast_pow (ringChar F), ← hc] at h

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)
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)
⊢ 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
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)
⊢ 2 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))

Tactic:
convert_to (8 : F) ^ (Fintype.card F / 2) = _

Result:
case h.e'_2
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)
⊢ 2 ^ (Fintype.card F / 2) = 8 ^ (Fintype.card F / 2)

case intro.intro.convert_2
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 ^ (Fintype.card F / 2) = ↑(χ₈ ↑(Fintype.card F))