-- Mathlib/NumberTheory/ZetaValues.lean
theorem hasSum_one_div_nat_pow_mul_sin {k : ℕ} (hk : k ≠ 0) {x : ℝ} (hx : x ∈ Icc (0 : ℝ) 1) :
    HasSum (fun n : ℕ => 1 / (n : ℝ) ^ (2 * k + 1) * Real.sin (2 * π * n * x))
      ((-1 : ℝ) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / (2 * k + 1)! *
        (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))).eval x)

Proof state:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ (-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) / (2 * I) =
    ↑((-1) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
        Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Tactic:
rw [ofReal_mul, ofReal_div, ofReal_div, ofReal_mul, ofReal_pow, ofReal_pow, ofReal_neg,
  ofReal_natCast, ofReal_mul, ofReal_two, ofReal_one, ← div_div, div_I,
  div_mul_eq_mul_div₀]

Result:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ -((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) * I / 2) =
    (-1) ^ (k + 1) * (2 * ↑π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      ↑(Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Proof state:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
⊢ HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * Real.sin (2 * π * ↑n * x))
    ((-1) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Tactic:
have ofReal_two : ((2 : ℝ) : ℂ) = 2 := by norm_cast

Result:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * Real.sin (2 * π * ↑n * x))
    ((-1) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Proof state:
case h.e'_6
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ (-1) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))) =
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) / (2 * I)).re

Tactic:
convert (ofReal_re _).symm

Result:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ (-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) / (2 * I) =
    ↑((-1) ^ (k + 1) * (2 * π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
        Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Proof state:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this✝ :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
this : ∀ (α β γ δ : ℂ), α * I * β / γ * δ * I = I ^ 2 * α * β / γ * δ
⊢ -((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) * I / 2) =
    (-1) ^ (k + 1) * (2 * ↑π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      ↑(Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Tactic:
rw [this, I_sq]

Result:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this✝ :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
this : ∀ (α β γ δ : ℂ), α * I * β / γ * δ * I = I ^ 2 * α * β / γ * δ
⊢ -(-1 * (-1) ^ (k + 1) * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x) / 2) =
    (-1) ^ (k + 1) * (2 * ↑π) ^ (2 * k + 1) / 2 / ↑(2 * k + 1)! *
      ↑(Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1))))

Proof state:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
x✝ : ℕ
⊢ 1 / ↑x✝ ^ (2 * k + 1) * ((fourier ↑x✝) ↑x - (fourier (-↑x✝)) ↑x) / (2 * I) =
    ↑(1 / ↑x✝ ^ (2 * k + 1) * Real.sin (2 * π * ↑x✝ * x))

Tactic:
rw [ofReal_mul]

Result:
case h.e'_3.h.e'_1
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
x✝ : ℕ
⊢ 1 / ↑x✝ ^ (2 * k + 1) * ((fourier ↑x✝) ↑x - (fourier (-↑x✝)) ↑x) / (2 * I) =
    ↑(1 / ↑x✝ ^ (2 * k + 1)) * ↑(Real.sin (2 * π * ↑x✝ * x))

Proof state:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
⊢ ∀ (α β γ δ : ℂ), α * I * β / γ * δ * I = I ^ 2 * α * β / γ * δ

Tactic:
intros

Result:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
α✝ β✝ γ✝ δ✝ : ℂ
⊢ α✝ * I * β✝ / γ✝ * δ✝ * I = I ^ 2 * α✝ * β✝ / γ✝ * δ✝

Proof state:
case h.e'_6.e_a.e_a
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
⊢ (-1) ^ k * -1 * I * (2 * ↑π) ^ (2 * k + 1) = -((2 * ↑π) ^ (2 * k + 1) * ((-1) ^ k * I))

Tactic:
ring

Result:
no goals

Proof state:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
⊢ ↑2 = 2

Tactic:
norm_cast

Result:
no goals

Proof state:
case h.e'_3.h.e'_1.e_a
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
this :
  HasSum (fun n => 1 / ↑n ^ (2 * k + 1) * ((fourier ↑n) ↑x - (fourier (-↑n)) ↑x))
    ((-1) ^ (k + 1) * I * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! * ↑(bernoulliFun (2 * k + 1) x))
ofReal_two : ↑2 = 2
x✝ : ℕ
⊢ 1 / ↑x✝ ^ (2 * k + 1) = 1 / ↑↑x✝ ^ (2 * k + 1)

Tactic:
rfl

Result:
no goals