-- Mathlib/NumberTheory/LSeries/HurwitzZetaOdd.lean
/-- Formula for `completedSinZeta` as a Dirichlet series in the convergence range
(second version, with sum over `ℕ`). -/
lemma hasSum_nat_completedSinZeta (a : ℝ) {s : ℂ} (hs : 1 < re s) :
    HasSum (fun n : ℕ ↦ Gammaℝ (s + 1) * Real.sin (2 * π * a * n) / (n : ℂ) ^ s)
    (completedSinZeta a s)

Proof state:
case inr
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
h : n ≠ 0
⊢ (s + 1).Gammaℝ * ((cexp (-↑(2 * π * a * ↑n) * I) - cexp (↑(2 * π * a * ↑n) * I)) * I / 2) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * 1 * cexp (2 * ↑π * I * ↑a * ↑n) + (s + 1).Gammaℝ * -I * -1 * cexp (2 * ↑π * I * ↑a * -↑n)) /
        2 /
      ↑n ^ s

Tactic:
simp only [← mul_div_assoc, push_cast, mul_assoc (Gammaℝ _), ← mul_add]

Result:
case inr
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
h : n ≠ 0
⊢ (s + 1).Gammaℝ * ((cexp (-(2 * ↑π * ↑a * ↑n) * I) - cexp (2 * ↑π * ↑a * ↑n * I)) * I) / 2 / ↑n ^ s =
    (s + 1).Gammaℝ * (-I * 1 * cexp (2 * ↑π * I * ↑a * ↑n) + -I * -1 * cexp (2 * ↑π * I * ↑a * -↑n)) / 2 / ↑n ^ s

Proof state:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
⊢ HasSum (fun n => (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s) (completedSinZeta (↑a) s)

Tactic:
refine this.congr_fun fun n ↦ ?_

Result:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
          (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
        ↑n ^ s /
      2

Proof state:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      (s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑↑n) / ↑|↑n| ^ s / 2 +
        (s + 1).Gammaℝ * -I * ↑(-↑n).sign * cexp (2 * ↑π * I * ↑a * ↑(-↑n)) / ↑|(-↑n)| ^ s / 2)
    (completedSinZeta (↑a) s + (s + 1).Gammaℝ * -I * ↑(Int.sign 0) * cexp (2 * ↑π * I * ↑a * ↑0) / ↑|0| ^ s / 2)
⊢ HasSum (fun n => (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s) (completedSinZeta (↑a) s)

Tactic:
simp_rw [Int.sign_zero, Int.cast_zero, mul_zero, zero_mul, zero_div, add_zero, abs_neg,
  Int.sign_neg, Nat.abs_cast, Int.cast_neg, Int.cast_natCast, ← add_div] at this

Result:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
⊢ HasSum (fun n => (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s) (completedSinZeta (↑a) s)

Proof state:
a : ℝ
s : ℂ
hs : 1 < s.re
⊢ HasSum (fun n => (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s) (completedSinZeta (↑a) s)

Tactic:
have := (hasSum_int_completedSinZeta a hs).nat_add_neg

Result:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      (s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑↑n) / ↑|↑n| ^ s / 2 +
        (s + 1).Gammaℝ * -I * ↑(-↑n).sign * cexp (2 * ↑π * I * ↑a * ↑(-↑n)) / ↑|(-↑n)| ^ s / 2)
    (completedSinZeta (↑a) s + (s + 1).Gammaℝ * -I * ↑(Int.sign 0) * cexp (2 * ↑π * I * ↑a * ↑0) / ↑|0| ^ s / 2)
⊢ HasSum (fun n => (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s) (completedSinZeta (↑a) s)

Proof state:
case inr.e_a.e_a.e_a
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
h : n ≠ 0
⊢ (cexp (-(2 * ↑π * ↑a * ↑n) * I) - cexp (2 * ↑π * ↑a * ↑n * I)) * I =
    -I * 1 * cexp (2 * ↑π * I * ↑a * ↑n) + -I * -1 * cexp (2 * ↑π * I * ↑a * -↑n)

Tactic:
rw [mul_one, mul_neg_one, neg_neg, neg_mul I, ← sub_eq_neg_add, ← mul_sub, mul_comm,
  mul_neg, neg_mul]

Result:
case inr.e_a.e_a.e_a
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
h : n ≠ 0
⊢ I * (cexp (-(2 * ↑π * ↑a * ↑n * I)) - cexp (2 * ↑π * ↑a * ↑n * I)) =
    I * (cexp (-(2 * ↑π * I * ↑a * ↑n)) - cexp (2 * ↑π * I * ↑a * ↑n))

Proof state:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
          (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
        2 /
      ↑n ^ s

Tactic:
rcases eq_or_ne n 0 with rfl | h

Result:
case inl
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑0)) / ↑0 ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑0).sign * cexp (2 * ↑π * I * ↑a * ↑0) +
          (s + 1).Gammaℝ * -I * -↑(↑0).sign * cexp (2 * ↑π * I * ↑a * -↑0)) /
        2 /
      ↑0 ^ s

case inr
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
h : n ≠ 0
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
          (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
        2 /
      ↑n ^ s

Proof state:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
          (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
        ↑n ^ s /
      2

Tactic:
rw [div_right_comm]

Result:
a : ℝ
s : ℂ
hs : 1 < s.re
this :
  HasSum
    (fun n =>
      ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
            (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
          ↑n ^ s /
        2)
    (completedSinZeta (↑a) s)
n : ℕ
⊢ (s + 1).Gammaℝ * ↑(Real.sin (2 * π * a * ↑n)) / ↑n ^ s =
    ((s + 1).Gammaℝ * -I * ↑(↑n).sign * cexp (2 * ↑π * I * ↑a * ↑n) +
          (s + 1).Gammaℝ * -I * -↑(↑n).sign * cexp (2 * ↑π * I * ↑a * -↑n)) /
        2 /
      ↑n ^ s