-- 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'_5.h
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) * Real.sin (2 * π * ↑x✝ * x) =
    (1 / ↑x✝ ^ (2 * k + 1) * ((fourier ↑x✝) ↑x - (fourier (-↑x✝)) ↑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
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:
case h.e'_3.h.e'_1.e_a.e_a.e_a.e_a.e_z
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
⊢ 2 * ↑π * I * -↑x✝ * ↑x = -(2 * ↑π * ↑x✝ * ↑x) * I

Tactic:
ring

Result:
no goals

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 * (-1) ^ (k + 1) * (2 * ↑π) ^ (2 * k + 1) / ↑(2 * k + 1)! *
          ↑(Polynomial.eval x (Polynomial.map (algebraMap ℚ ℝ) (Polynomial.bernoulli (2 * k + 1)))) /
        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:
ring

Result:
no goals

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:
congr

Result:
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))

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
⊢ ((fourier ↑x✝) ↑x - (fourier (-↑x✝)) ↑x) / (2 * I) = ↑(Real.sin (2 * π * ↑x✝ * x))

Proof state:
k : ℕ
hk : k ≠ 0
x : ℝ
hx : x ∈ Icc 0 1
⊢ 2 ≤ 2 * k + 1

Tactic:
omega

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))
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))))

Tactic:
convert ((hasSum_iff _ _).mp (this.div_const (2 * I))).1

Result:
case h.e'_5.h
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) * Real.sin (2 * π * ↑x✝ * x) =
    (1 / ↑x✝ ^ (2 * k + 1) * ((fourier ↑x✝) ↑x - (fourier (-↑x✝)) ↑x) / (2 * I)).re

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

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 [← mul_div]

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:
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:
rw [ofReal_div, ofReal_one, ofReal_pow]

Result:
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)

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) * 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:
have : ∀ α β γ δ : ℂ, α * I * β / γ * δ * I = I ^ 2 * α * β / γ * δ := by intros; ring

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) ^ (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))))