-- Lean 4 proof step from Mathlib.NumberTheory.LSeries.SumCoeff (goal)
Goal:
f : ℕ → ℂ
l : ℂ
hlim : Tendsto (fun (n : ℕ) ↦ HDiv.hDiv (α := ℂ) (∑ k ∈ Icc 1 n, f k) (↑n : ℂ)) atTop (𝓝 l)
hfS : ∀ (s : ℝ), 1 < s → LSeriesSummable f (↑s : ℂ)
ε : ℝ
hε : ε > 0
T : ℝ
hT₁ : 1 ≤ T
hT : ∀ (y : ℝ), T ≤ y → ‖∑ k ∈ Icc 1 ⌊y⌋₊, f k - l * (↑y : ℂ)‖ < ε * y
S : ℝ → ℂ := fun (t : ℝ) ↦ ∑ k ∈ Icc 1 ⌊t⌋₊, f k
C : ℝ :=
  @integral _ _ _ _ MeasureSpace.toMeasurableSpace (Measure.restrict volume (Set.Ioc 1 T)) fun (t : ℝ) ↦
    ‖S t - l * (↑t : ℂ)‖ * t ^ HSub.hSub (α := ℝ) (-1) 1
hC : 0 ≤ C
s : ℝ
hs : 1 < s
hs' : 0 ≤ (s - 1) * s
h₀ : LocallyIntegrableOn (ε := ℂ) (fun (t : ℝ) ↦ S t - l * (↑t : ℂ)) (Set.Ici 1) volume
h₁ :
  IntegrableOn (ε := ℝ) (mα := MeasureSpace.toMeasurableSpace) (fun (t : ℝ) ↦ ‖S t - l * (↑t : ℂ)‖ * t ^ (-s - 1))
    (Set.Ici 1) volume
h₂ :
  IntegrableOn (ε := ℝ) (mα := MeasureSpace.toMeasurableSpace)
    (fun (t : ℝ) ↦ ‖S t - l * (↑t : ℂ)‖ * t ^ HSub.hSub (α := ℝ) (-1) 1) (Set.Ioc 1 T) volume
h₃ :
  (((↑s : ℂ) - 1) *
      @integral _ _ _ _ MeasureSpace.toMeasurableSpace (Measure.restrict volume (Set.Ioi 1)) fun (t : ℝ) ↦
        (↑t : ℂ) ^ (-(↑s : ℂ))) =
    1
Cs : ℝ :=
  @integral _ _ _ _ MeasureSpace.toMeasurableSpace (Measure.restrict volume (Set.Ioc 1 T)) fun (t : ℝ) ↦
    ‖S t - l * (↑t : ℂ)‖ * t ^ (-s - 1)
h₄ : Cs ≤ C
⊢ ‖((↑s : ℂ) - 1) * LSeries f (↑s : ℂ) - (↑s : ℂ) * l‖ ≤ (s - 1) * s * C + s * ε
have we : ‖((↑s : ℂ) - 1) * LSeries f (↑s : ℂ) - (↑s : ℂ) * l‖ =
  ‖(((↑s : ℂ) - 1) * (↑s : ℂ) *
        @integral _ _ _ _ MeasureSpace.toMeasurableSpace (Measure.restrict volume (Set.Ioi 1)) fun (t : ℝ) ↦
          S t * (↑t : ℂ) ^ (-(↑s : ℂ) - 1)) -
      l * (↑s : ℂ) *
        (((↑s : ℂ) - 1) *
          @integral _ _ _ _ MeasureSpace.toMeasurableSpace (Measure.restrict volume (Set.Ioi 1)) fun (t : ℝ) ↦
            (↑t : ℂ) ^ (-(↑s : ℂ)))‖
Tactic: calc
    -- First, we replace `s * l` by `(s - 1) * s` times the integral of `l * t ^ (-s)` using `h₃`
    -- and replace `LSeries f s` by its integral representation.
    _ = ‖((s - 1) * s * ∫ t in Set.Ioi 1, S t * ↑t ^ (-(s : ℂ) - 1)) -
          l * s * ((s - 1) * ∫ (t : ℝ) in Set.Ioi 1, ↑t ^ (-(s : ℂ)))‖ := by
      rw [h₃, mul_one, mul_comm l, LSeries_eq_mul_integral _ zero_le_one (by rwa [ofReal_re])
        (hfS _ hs), neg_add', mul_assoc]
      exact isBigO_atTop_natCast_rpow_of_tendsto_div_rpow (a := l) (by simpa using hlim)
    _ = ‖(s - 1) * s * ∫ t in Set.Ioi 1, (S t * (t : ℂ) ^ (-s - 1 : ℂ) - l * t ^ (-s : ℂ))‖ := by
      rw [integral_sub, integral_const_mul]
      · congr; ring
      · exact (lemma₁ hlim hs).mono_set Set.Ioi_subset_Ici_self
      · exact (integrableOn_Ioi_cpow_of_lt
          (by rwa [neg_re, ofReal_re, neg_lt_neg_iff]) zero_lt_one).const_mul _
    _ = ‖(s - 1) * s * ∫ t in Set.Ioi 1, (S t - l * t) * (t : ℂ) ^ (-s - 1 : ℂ)‖ := by
      congr 2
      refine setIntegral_congr_fun measurableSet_Ioi fun t ht ↦ ?_
      replace ht : (t : ℂ) ≠ 0 := ne_zero_of_one_lt_re ht
      rw [sub_mul, cpow_sub _ _ ht, cpow_one, mul_assoc, mul_div_cancel₀ _ ht]
    _ ≤ (s - 1) * s * ∫ t in Set.Ioi 1, ‖(S t - l * ↑t) * ↑t ^ (-s - 1 : ℂ)‖ := by
      rw [norm_mul, show ((s : ℂ) - 1) * s = ((s - 1) * s : ℝ) by simp, norm_real,
        Real.norm_of_nonneg hs']
      exact mul_le_mul_of_nonneg_left (norm_integral_le_integral_norm _) hs'
    -- Next, step is to bound the integral of `‖S t - l * t‖ * t ^ (-s - 1)`.
    _ = (s - 1) * s * ∫ t in Set.Ioi 1, ‖S t - l * t‖ * t ^ (-s - 1) := by
      congr 1
      refine setIntegral_congr_fun measurableSet_Ioi fun t ht ↦ ?_
      replace ht : 0 ≤ t := zero_le_one.trans ht.le
      rw [norm_mul, show (-(s : ℂ) - 1) = (-s - 1 : ℝ) by simp, ← ofReal_cpow ht, norm_real,
        Real.norm_of_nonneg (Real.rpow_nonneg ht _)]
    -- For that, we cut the integral in two parts using `T` as the cutting point.
    _ = (s - 1) * s * (Cs + ∫ t in Set.Ioi T, ‖S t - l * t‖ * t ^ (-s - 1)) := by
      rw [← Set.Ioc_union_Ioi_eq_Ioi hT₁, setIntegral_union Set.Ioc_disjoint_Ioi_same
        measurableSet_Ioi]
      · exact h₁.mono_set <| Set.Ioc_subset_Ioi_self.trans Set.Ioi_subset_Ici_self
      · exact h₁.mono_set <| Set.Ioi_subset_Ici_self.trans <| Set.Ici_subset_Ici.mpr hT₁
    -- The first part can be bounded by `C` using `h₄`.
    _ ≤ (s - 1) * s * C + s * ((s - 1) * ∫ t in Set.Ioi T, ‖S t - l * t‖ * t ^ (-s - 1)) := by
      rw [mul_add, ← mul_assoc, mul_comm s]
      gcongr
    -- The second part is bounded using `LSeries_tendsto_sub_mul_nhds_one_of_tendsto_sum_div_aux₂`
    -- since `‖S t - l t‖ ≤ ε * t` for all `t ≥ T`.
    _ ≤ (s - 1) * s * C + s * ε := by
      gcongr
      exact LSeries_tendsto_sub_mul_nhds_one_of_tendsto_sum_div_aux₂ h₀ hε hs hT₁
        fun t ht ↦ (hT t ht.le).le