/-- **Cauchy integral formula** for the value at the center of a disc. If `f` is continuous on a
punctured closed disc of radius `R`, is differentiable at all but countably many points of the
interior of this disc, and has a limit `y` at the center of the disc, then the integral
$\oint_{‖z-c‖=R} \frac{f(z)}{z-c}\,dz$ is equal to `2πiy`. -/
theorem circleIntegral_sub_center_inv_smul_of_differentiable_on_off_countable_of_tendsto {c : ℂ}
    {R : ℝ} (h0 : 0 < R) {f : ℂ → E} {y : E} {s : Set ℂ} (hs : s.Countable)
    (hc : ContinuousOn f (closedBall c R \ {c}))
    (hd : ∀ z ∈ (ball c R \ {c}) \ s, DifferentiableAt ℂ f z) (hy : Tendsto f (𝓝[{c}ᶜ] c) (𝓝 y)) :
    (∮ z in C(c, R), (z - c)⁻¹ • f z) = (2 * π * I : ℂ) • y := by
  rw [← sub_eq_zero, ← norm_le_zero_iff]
  refine' le_of_forall_le_of_dense fun ε ε0 => _
  obtain ⟨δ, δ0, hδ⟩ : ∃ δ > (0 : ℝ), ∀ z ∈ closedBall c δ \ {c}, dist (f z) y < ε / (2 * π)
  exact ((nhdsWithin_hasBasis nhds_basis_closedBall _).tendsto_iff nhds_basis_ball).1 hy _
    (div_pos ε0 Real.two_pi_pos)
  obtain ⟨r, hr0, hrδ, hrR⟩ : ∃ r, 0 < r ∧ r ≤ δ ∧ r ≤ R :=
    ⟨min δ R, lt_min δ0 h0, min_le_left _ _, min_le_right _ _⟩
  have hsub : closedBall c R \ ball c r ⊆ closedBall c R \ {c} :=
    diff_subset_diff_right (singleton_subset_iff.2 <| mem_ball_self hr0)
  have hsub' : ball c R \ closedBall c r ⊆ ball c R \ {c} :=
    diff_subset_diff_right (singleton_subset_iff.2 <| mem_closedBall_self hr0.le)
  have hzne : ∀ z ∈ sphere c r, z ≠ c := fun z hz =>
    ne_of_mem_of_not_mem hz fun h => hr0.ne' <| dist_self c ▸ Eq.symm h
  /- The integral `∮ z in C(c, r), f z / (z - c)` does not depend on `0 < r ≤ R` and tends to
    `2πIy` as `r → 0`. -/
  calc
    ‖(∮ z in C(c, R), (z - c)⁻¹ • f z) - (2 * ↑π * I) • y‖ =
        ‖(∮ z in C(c, r), (z - c)⁻¹ • f z) - ∮ z in C(c, r), (z - c)⁻¹ • y‖ := by
      congr 2
      · exact circleIntegral_sub_center_inv_smul_eq_of_differentiable_on_annulus_off_countable hr0
          hrR hs (hc.mono hsub) fun z hz => hd z ⟨hsub' hz.1, hz.2⟩
      · simp [hr0.ne']
    _ = ‖∮ z in C(c, r), (z - c)⁻¹ • (f z - y)‖ := by
      simp only [smul_sub]
      have hc' : ContinuousOn (fun z => (z - c)⁻¹) (sphere c r) :=
        (continuousOn_id.sub continuousOn_const).inv₀ fun z hz => sub_ne_zero.2 <| hzne _ hz
      rw [circleIntegral.integral_sub] <;> refine' (hc'.smul _).circleIntegrable hr0.le
      · exact hc.mono <| subset_inter
          (sphere_subset_closedBall.trans <| closedBall_subset_closedBall hrR) hzne
      · exact continuousOn_const
    _ ≤ 2 * π * r * (r⁻¹ * (ε / (2 * π))) := by
      refine' circleIntegral.norm_integral_le_of_norm_le_const hr0.le fun z hz => _
      specialize hzne z hz
      rw [mem_sphere, dist_eq_norm] at hz
      

Proof state:
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℂ E
inst✝ : CompleteSpace E
c : ℂ
R : ℝ
h0 : 0 < R
f : ℂ → E
y : E
s : Set ℂ
hs : Set.Countable s
hc : ContinuousOn f (closedBall c R \ {c})
hd : ∀ z ∈ (ball c R \ {c}) \ s, DifferentiableAt ℂ f z
hy : Tendsto f (𝓝[≠] c) (𝓝 y)
ε : ℝ
ε0 : 0 < ε
δ : ℝ
δ0 : δ > 0
hδ : ∀ z ∈ closedBall c δ \ {c}, dist (f z) y < ε / (2 * π)
r : ℝ
hr0 : 0 < r
hrδ : r ≤ δ
hrR : r ≤ R
hsub : closedBall c R \ ball c r ⊆ closedBall c R \ {c}
hsub' : ball c R \ closedBall c r ⊆ ball c R \ {c}
z : ℂ
hz : ‖z - c‖ = r
hzne : z ≠ c
⊢ ‖(z - c)⁻¹ • (f z - y)‖ ≤ r⁻¹ * (ε / (2 * π))

Next tactic:
rw [norm_smul, norm_inv, hz, ← dist_eq_norm]