-- Lean 4 proof step from Mathlib.Computability.AkraBazzi.SumTransform (goal)
Goal:
α : Type u_1
inst : Fintype α
T : ℕ → ℝ
g : ℝ → ℝ
a b : α → ℝ
r : α → ℕ → ℕ
inst_1 : Nonempty α
R : AkraBazziRecurrence T g a b r
c₁ : ℝ
hc₁_mem : Membership.mem (γ := Set ℝ) (Set.Ioo 0 1) c₁
hc₁ : ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), c₁ * (↑n : ℝ) ≤ (↑(r i n) : ℝ)
c₂ : ℝ
hc₂_mem : c₂ > 0
hc₂ : ∀ᶠ (n : ℕ) in atTop, ∀ u ∈ Set.Icc (c₁ * (↑n : ℝ)) (↑n : ℝ), c₂ * g (↑n : ℝ) ≤ g u
c₃ : ℝ
hc₃_mem : Membership.mem (γ := Set ℝ) (Set.Ioo 0 1) c₃
hc₃ : ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), (↑(r i n) : ℝ) ≤ c₃ * (↑n : ℝ)
hc₁_pos : 0 < c₁
hc₃' : 0 < 1 - c₃
n : ℕ
hn₁ : ∀ (i : α), c₁ * (↑n : ℝ) ≤ (↑(r i n) : ℝ)
hn₂ : ∀ u ∈ Set.Icc (c₁ * (↑n : ℝ)) (↑n : ℝ), c₂ * g (↑n : ℝ) ≤ g u
hn₃ : ∀ (i : α), (↑(r i n) : ℝ) ≤ c₃ * (↑n : ℝ)
hrpos : ∀ (i : α), 0 < r i n
hr_lt_n : ∀ (i : α), r i n < n
hn_pos : 0 < n
i : α
hrpos_i : 0 < r i n
g_nonneg : 0 ≤ g (↑n : ℝ)
hp : p a b + 1 < 0
⊢ min (c₂ * (1 - c₃)) ((1 - c₃) * c₂ / c₁ ^ (p a b + 1)) * g (↑n : ℝ) ≤ sumTransform (p a b) g (r i n) n
have calc : sumTransform (p a b) g (r i n) n =
  HPow.hPow (α := ℝ) (↑n : ℝ) (p a b) * ∑ u ∈ Ico (r i n) n, g (↑u : ℝ) / HPow.hPow (α := ℝ) (↑u : ℝ) (p a b + 1)
Tactic: calc sumTransform (p a b) g (r i n) n
        = n ^ (p a b) * (∑ u ∈ Finset.Ico (r i n) n, g u / u ^ ((p a b) + 1)) := by rfl
      _ ≥ n ^ (p a b) * (∑ u ∈ Finset.Ico (r i n) n, c₂ * g n / u ^ ((p a b) + 1)) := by
        gcongr with u hu
        rw [Finset.mem_Ico] at hu
        have hu' : u ∈ Set.Icc (r i n) n := ⟨hu.1, by lia⟩
        refine hn₂ u ?_
        rw [Set.mem_Icc]
        refine ⟨?_, by norm_cast; lia⟩
        calc c₁ * n ≤ r i n := by exact hn₁ i
                  _ ≤ u := by exact_mod_cast hu'.1
      _ ≥ n ^ (p a b) * (∑ _u ∈ Finset.Ico (r i n) n, c₂ * g n / (r i n) ^ ((p a b) + 1)) := by
        gcongr n ^ (p a b) * (Finset.Ico (r i n) n).sum (fun _ => c₂ * g n / ?_) with u hu
        · rw [Finset.mem_Ico] at hu
          have := calc 0 < r i n := hrpos_i
                      _ ≤ u := hu.1
          positivity
        · rw [Finset.mem_Ico] at hu
          exact rpow_le_rpow_of_nonpos (by positivity)
            (by exact_mod_cast hu.1) (le_of_lt hp)
      _ ≥ n ^ p a b * #(Ico (r i n) n) • (c₂ * g n / r i n ^ (p a b + 1)) := by
          gcongr; exact Finset.card_nsmul_le_sum _ _ _ (fun x _ => by rfl)
      _ = n ^ p a b * #(Ico (r i n) n) * (c₂ * g n / r i n ^ (p a b + 1)) := by
          rw [nsmul_eq_mul, mul_assoc]
      _ ≥ n ^ p a b * #(Ico (r i n) n) * (c₂ * g n / (c₁ * n) ^ (p a b + 1)) := by
          gcongr n ^ p a b * #(Ico (r i n) n) * (c₂ * g n / ?_)
          exact rpow_le_rpow_of_nonpos (by positivity) (hn₁ i) (le_of_lt hp)
      _ = n ^ (p a b) * (n - r i n) * (c₂ * g n / (c₁ * n) ^ ((p a b) + 1)) := by
          congr; rw [Nat.card_Ico, Nat.cast_sub (le_of_lt <| hr_lt_n i)]
      _ ≥ n ^ (p a b) * (n - c₃ * n) * (c₂ * g n / (c₁ * n) ^ ((p a b) + 1)) := by
          gcongr; exact hn₃ i
      _ = n ^ (p a b) * n * (1 - c₃) * (c₂ * g n / (c₁ * n) ^ ((p a b) + 1)) := by ring
      _ = n ^ (p a b) * n * (1 - c₃) * (c₂ * g n / (c₁ ^ ((p a b) + 1) * n ^ ((p a b) + 1))) := by
          rw [Real.mul_rpow (by positivity) (by positivity)]
      _ = (n ^ ((p a b) + 1) / n ^ ((p a b) + 1)) * (1 - c₃) * c₂ * g n / c₁ ^ ((p a b) + 1) := by
          rw [← Real.rpow_add_one (by positivity) (p a b)]; ring
      _ = (1 - c₃) * c₂ / c₁ ^ ((p a b) + 1) * g n := by
          rw [div_self (by positivity), one_mul]; ring
      _ ≥ min (c₂ * (1 - c₃)) ((1 - c₃) * c₂ / c₁ ^ ((p a b) + 1)) * g n := by
          gcongr; exact min_le_right _ _