-- Mathlib/Analysis/SpecialFunctions/Trigonometric/EulerSineProd.lean
theorem integral_sin_mul_sin_mul_cos_pow_eq (hn : 2 ≤ n) (hz : z ≠ 0) :
    (∫ x in (0 : ℝ)..π / 2, Complex.sin (2 * z * x) * sin x * (cos x : ℂ) ^ (n - 1)) =
      (n / (2 * z) * ∫ x in (0 : ℝ)..π / 2, Complex.cos (2 * z * x) * (cos x : ℂ) ^ n) -
        (n - 1) / (2 * z) *
          ∫ x in (0 : ℝ)..π / 2, Complex.cos (2 * z * x) * (cos x : ℂ) ^ (n - 2)

Proof state:
case h.e'_9
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ Complex.cos ↑x ^ n - (↑n - 1) * Complex.sin ↑x ^ 2 * Complex.cos ↑x ^ (n - 2) =
    Complex.cos ↑x * ((fun x => x ^ (n - 1)) ∘ Complex.cos) ↑x +
      Complex.sin ↑x * (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x)

Tactic:
rw [mul_neg, mul_neg, ← sub_eq_add_neg, Function.comp_apply]

Result:
case h.e'_9
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ Complex.cos ↑x ^ n - (↑n - 1) * Complex.sin ↑x ^ 2 * Complex.cos ↑x ^ (n - 2) =
    Complex.cos ↑x * Complex.cos ↑x ^ (n - 1) -
      Complex.sin ↑x * (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * Complex.sin ↑x)

Proof state:
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x

Tactic:
convert ((Complex.hasDerivAt_sin x).mul c).comp_ofReal using 1

Result:
case h.e'_8
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) = fun y => Complex.sin ↑y * ((fun x => x ^ (n - 1)) ∘ Complex.cos) ↑y

case h.e'_9
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ ↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2) =
    Complex.cos ↑x * ((fun x => x ^ (n - 1)) ∘ Complex.cos) ↑x +
      Complex.sin ↑x * (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x)

Proof state:
case convert_2.hu
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
⊢ Continuous fun x => Complex.sin (2 * z * ↑x)

Tactic:
exact Complex.continuous_sin.comp (continuous_const.mul Complex.continuous_ofReal)

Result:
no goals

Proof state:
case h.e'_3
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
⊢ ∫ (x : ℝ) in 0 ..π / 2,
      ↑n / (2 * z) * (Complex.cos (2 * z * ↑x) * ↑(cos x) ^ n) -
        (↑n - 1) / (2 * z) * (Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (n - 2)) =
    ∫ (x : ℝ) in 0 ..π / 2,
      -((↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) * (-Complex.cos (2 * z * ↑x) / (2 * z)))

Tactic:
refine integral_congr fun x _ => ?_

Result:
case h.e'_3
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
x : ℝ
x✝ : x ∈ uIcc 0 (π / 2)
⊢ ↑n / (2 * z) * (Complex.cos (2 * z * ↑x) * ↑(cos x) ^ n) -
      (↑n - 1) / (2 * z) * (Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (n - 2)) =
    -((↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) * (-Complex.cos (2 * z * ↑x) / (2 * z)))

Proof state:
case convert_2
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
⊢ IntervalIntegrable (fun x => Complex.sin (2 * z * ↑x)) volume 0 (π / 2)

Tactic:
apply Continuous.intervalIntegrable

Result:
case convert_2.hu
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
⊢ Continuous fun x => Complex.sin (2 * z * ↑x)

Proof state:
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
x : ℝ
x✝ : x ∈ uIcc 0 (π / 2)
⊢ Complex.cos ↑x ^ n = Complex.cos ↑x ^ (n - 2) * Complex.cos ↑x ^ 2

Tactic:
conv_lhs => rw [← Nat.sub_add_cancel hn, pow_add]

Result:
no goals

Proof state:
case h.e'_3.hg.hu
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
⊢ Continuous fun x => (↑n - 1) / (2 * z) * (Complex.cos (2 * z * ↑x) * ↑(cos x) ^ (n - 2))

Tactic:
exact
  continuous_const.mul
    ((Complex.continuous_cos.comp (continuous_const.mul Complex.continuous_ofReal)).mul
      ((Complex.continuous_ofReal.comp continuous_cos).pow (n - 2)))

Result:
no goals

Proof state:
case h.e'_9.e_a
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
this : ↑(n - 1) = ↑n - 1
⊢ (↑n - 1) * Complex.sin ↑x ^ 2 * Complex.cos ↑x ^ (n - 2) =
    Complex.sin ↑x * ((↑n - 1) * Complex.cos ↑x ^ (n - (1 + 1)) * Complex.sin ↑x)

Tactic:
ring

Result:
no goals

Proof state:
case h.e'_2
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
der1 :
  ∀ x ∈ uIcc 0 (π / 2),
    HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x
x : ℝ
x✝ : x ∈ uIcc 0 (π / 2)
⊢ Complex.sin (2 * z * ↑x) * ↑(sin x) * ↑(cos x) ^ (n - 1) = ↑(sin x) * ↑(cos x) ^ (n - 1) * Complex.sin (2 * z * ↑x)

Tactic:
ring_nf

Result:
no goals

Proof state:
case h.e'_9.e_a
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
this : ↑(n - 1) = ↑n - 1
⊢ (↑n - 1) * Complex.sin ↑x ^ 2 * Complex.cos ↑x ^ (n - 2) =
    Complex.sin ↑x * (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * Complex.sin ↑x)

Tactic:
rw [Nat.sub_sub, this]

Result:
case h.e'_9.e_a
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
this : ↑(n - 1) = ↑n - 1
⊢ (↑n - 1) * Complex.sin ↑x ^ 2 * Complex.cos ↑x ^ (n - 2) =
    Complex.sin ↑x * ((↑n - 1) * Complex.cos ↑x ^ (n - (1 + 1)) * Complex.sin ↑x)

Proof state:
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
⊢ HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x

Tactic:
have c := HasDerivAt.comp (x : ℂ) (hasDerivAt_pow (n - 1) _) (Complex.hasDerivAt_cos x)

Result:
z : ℂ
n : ℕ
hn : 2 ≤ n
hz : z ≠ 0
x : ℝ
a✝ : x ∈ uIcc 0 (π / 2)
c : HasDerivAt ((fun x => x ^ (n - 1)) ∘ Complex.cos) (↑(n - 1) * Complex.cos ↑x ^ (n - 1 - 1) * -Complex.sin ↑x) ↑x
⊢ HasDerivAt (fun y => ↑(sin y) * ↑(cos y) ^ (n - 1)) (↑(cos x) ^ n - (↑n - 1) * ↑(sin x) ^ 2 * ↑(cos x) ^ (n - 2)) x