/-- Recurrence formula for the Beta function. -/
theorem betaIntegral_recurrence {u v : ℂ} (hu : 0 < re u) (hv : 0 < re v) :
    u * betaIntegral u (v + 1) = v * betaIntegral (u + 1) v := by
  -- NB: If we knew `Gamma (u + v + 1) ≠ 0` this would be an easy consequence of
  -- `Gamma_mul_Gamma_eq_betaIntegral`; but we don't know that yet. We will prove it later, but
  -- this lemma is needed in the proof. So we give a (somewhat laborious) direct argument.
  let F : ℝ → ℂ := fun x => (x : ℂ) ^ u * (1 - (x : ℂ)) ^ v
  have hu' : 0 < re (u + 1) := by rw [add_re, one_re]; positivity
  have hv' : 0 < re (v + 1) := by rw [add_re, one_re]; positivity
  have hc : ContinuousOn F (Icc 0 1) := by
    refine' (ContinuousAt.continuousOn fun x hx => _).mul (ContinuousAt.continuousOn fun x hx => _)
    · refine' (continuousAt_cpow_const_of_re_pos (Or.inl _) hu).comp continuous_ofReal.continuousAt
      rw [ofReal_re]; exact hx.1
    · refine' (continuousAt_cpow_const_of_re_pos (Or.inl _) hv).comp
        (continuous_const.sub continuous_ofReal).continuousAt
      rw [sub_re, one_re, ofReal_re, sub_nonneg]
      exact hx.2
  have hder : ∀ x : ℝ, x ∈ Ioo (0 : ℝ) 1 →
      HasDerivAt F (u * ((x : ℂ) ^ (u - 1) * (1 - (x : ℂ)) ^ v) -
        v * ((x : ℂ) ^ u * (1 - (x : ℂ)) ^ (v - 1))) x := by
    intro x hx
    have U : HasDerivAt (fun y : ℂ => y ^ u) (u * (x : ℂ) ^ (u - 1)) ↑x := by
      have := @HasDerivAt.cpow_const _ _ _ u (hasDerivAt_id (x : ℂ)) (Or.inl ?_)
      simp only [id_eq, mul_one] at this
      · exact this
      · rw [id_eq, ofReal_re]; exact hx.1
    have V : HasDerivAt (fun y : ℂ => (1 - y) ^ v) (-v * (1 - (x : ℂ)) ^ (v - 1)) ↑x := by
      have A := @HasDerivAt.cpow_const _ _ _ v (hasDerivAt_id (1 - (x : ℂ))) (Or.inl ?_)
      swap; · rw [id.def, sub_re, one_re, ofReal_re, sub_pos]; exact hx.2
      simp_rw [id.def] at A
      have B : HasDerivAt (fun y : ℂ => 1 - y) (-1) ↑x := by
        apply HasDerivAt.const_sub; apply hasDerivAt_id
      convert HasDerivAt.comp (↑x) A B using 1
      ring
    convert (U.mul V).comp_ofReal using 1
    ring
  have h_int := ((betaIntegral_convergent hu hv').const_mul u).sub
    ((betaIntegral_convergent hu' hv).const_mul v)
  rw [add_sub_cancel, add_sub_cancel] at h_int
  have int_ev := intervalIntegral.integral_eq_sub_of_hasDerivAt_of_le zero_le_one hc hder h_int
  have hF0 : F 0 = 0 := by
    simp only [mul_eq_zero, ofReal_zero, cpow_eq_zero_iff, eq_self_iff_true, Ne.def, true_and_iff,
      sub_zero, one_cpow, one_ne_zero, or_false_iff]
    contrapose! hu; rw [hu, zero_re]
  have hF1 : F 1 = 0 := by
    simp only [mul_eq_zero, ofReal_one, one_cpow, one_ne_zero, sub_self, cpow_eq_zero_iff,
      eq_self_iff_true, Ne.def, true_and_iff, false_or_iff]
    contrapose! hv; rw [hv, zero_re]
  rw [hF0, hF1, sub_zero, intervalIntegral.integral_sub, intervalIntegral.integral_const_mul,
    intervalIntegral.integral_const_mul] at int_ev
  · rw [betaIntegral, betaIntegral, ← sub_eq_zero]
    convert int_ev <;> ring
  · apply IntervalIntegrable.const_mul
    convert betaIntegral_convergent hu hv'; ring
  · 

Proof state:
case hg
u v : ℂ
hu : 0 < u.re
hv : 0 < v.re
F : ℝ → ℂ := fun x => ↑x ^ u * (1 - ↑x) ^ v
hu' : 0 < (u + 1).re
hv' : 0 < (v + 1).re
hc : ContinuousOn F (Icc 0 1)
hder : ∀ x ∈ Ioo 0 1, HasDerivAt F (u * (↑x ^ (u - 1) * (1 - ↑x) ^ v) - v * (↑x ^ u * (1 - ↑x) ^ (v - 1))) x
h_int : IntervalIntegrable (fun x => u * (↑x ^ (u - 1) * (1 - ↑x) ^ v) - v * (↑x ^ u * (1 - ↑x) ^ (v - 1))) volume 0 1
int_ev : ∫ (y : ℝ) in 0 ..1, u * (↑y ^ (u - 1) * (1 - ↑y) ^ v) - v * (↑y ^ u * (1 - ↑y) ^ (v - 1)) = 0
hF0 : F 0 = 0
hF1 : F 1 = 0
⊢ IntervalIntegrable (fun y => v * (↑y ^ u * (1 - ↑y) ^ (v - 1))) volume 0 1

Next tactic:
apply IntervalIntegrable.const_mul