-- Mathlib/NumberTheory/ZetaValues.lean
theorem hasSum_one_div_pow_mul_fourier_mul_bernoulliFun {k : ℕ} (hk : 2 ≤ k) {x : ℝ}
    (hx : x ∈ Icc (0 : ℝ) 1) :
    HasSum (fun n : ℤ => 1 / (n : ℂ) ^ k * fourier n (x : 𝕌))
      (-(2 * π * I) ^ k / k ! * bernoulliFun k x)

Proof state:
case h.e'_5.h
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
n : ℤ
⊢ 1 / ↑n ^ k * (fourier n) ↑y = -(2 * ↑π * I) ^ k / ↑k ! * (-↑k ! / (2 * ↑π * I * ↑n) ^ k) • (fourier n) ↑y

Tactic:
rw [smul_eq_mul, ← mul_assoc, mul_div, mul_neg, div_mul_cancel₀, neg_neg, mul_pow _ (n : ℂ),
  ← div_div, div_self]

Result:
case h.e'_5.h
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
n : ℤ
⊢ (2 * ↑π * I) ^ k ≠ 0

case h.e'_5.h.h
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
n : ℤ
⊢ ↑k ! ≠ 0

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))

Tactic:
convert step2.mul_left (-(2 * ↑π * I) ^ k / (k ! : ℂ)) using 2 with n

Result:
case h.e'_5.h
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
n : ℤ
⊢ 1 / ↑n ^ k * (fourier n) ↑y = -(2 * ↑π * I) ^ k / ↑k ! * (-↑k ! / (2 * ↑π * I * ↑n) ^ k) • (fourier n) ↑y

case h.e'_6.h.e'_6
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
⊢ ↑(bernoulliFun k y) = B ↑y

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
⊢ k ≠ 1

Tactic:
omega

Result:
no goals

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx✝ : x ∈ Icc 0 1
hx : x ∈ Ico 0 1 ∨ x = 1
this :
  ∀ {y : ℝ},
    y ∈ Ico 0 1 → HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑x) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k x))

Tactic:
rcases hx with (hx | rfl)

Result:
case inl
k : ℕ
hk : 2 ≤ k
x : ℝ
hx✝ : x ∈ Icc 0 1
this :
  ∀ {y : ℝ},
    y ∈ Ico 0 1 → HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))
hx : x ∈ Ico 0 1
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑x) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k x))

case inr
k : ℕ
hk : 2 ≤ k
hx : 1 ∈ Icc 0 1
this :
  ∀ {y : ℝ},
    y ∈ Ico 0 1 → HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑1) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k 1))

Proof state:
case inl
k : ℕ
hk : 2 ≤ k
x : ℝ
hx✝ : x ∈ Icc 0 1
this :
  ∀ {y : ℝ},
    y ∈ Ico 0 1 → HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))
hx : x ∈ Ico 0 1
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑x) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k x))

Tactic:
exact this hx

Result:
no goals

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
⊢ ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k

Tactic:
rw [ContinuousMap.coe_mk]

Result:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
⊢ ∀ (n : ℤ), fourierCoeff (ofReal ∘ periodizedBernoulli k) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))

Tactic:
have step2 :=
  has_pointwise_sum_fourier_series_of_summable
    ((summable_bernoulli_fourier hk).congr fun n => (step1 n).symm) y

Result:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => fourierCoeff (⇑B) i • (fourier i) ↑y) (B ↑y)
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))

Proof state:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => fourierCoeff (⇑B) i • (fourier i) ↑y) (B ↑y)
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))

Tactic:
simp_rw [step1] at step2

Result:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
step1 : ∀ (n : ℤ), fourierCoeff (⇑B) n = -↑k ! / (2 * ↑π * I * ↑n) ^ k
step2 : HasSum (fun i => (-↑k ! / (2 * ↑π * I * ↑i) ^ k) • (fourier i) ↑y) (B ↑y)
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))

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

Tactic:
let B : C(𝕌, ℂ) :=
  ContinuousMap.mk ((↑) ∘ periodizedBernoulli k)
    (continuous_ofReal.comp (periodizedBernoulli.continuous (by omega)))

Result:
k : ℕ
hk : 2 ≤ k
x : ℝ
hx : x ∈ Icc 0 1
y : ℝ
hy : y ∈ Ico 0 1
B : C(𝕌, ℂ) := { toFun := ofReal ∘ periodizedBernoulli k, continuous_toFun := ⋯ }
⊢ HasSum (fun n => 1 / ↑n ^ k * (fourier n) ↑y) (-(2 * ↑π * I) ^ k / ↑k ! * ↑(bernoulliFun k y))