Module: Mathlib.Computability.AkraBazzi.SumTransform

Goal state before:
pp: α : Type u_1
inst✝¹ : Fintype α
T : ℕ → ℝ
g : ℝ → ℝ
a b : α → ℝ
r : α → ℕ → ℕ
inst✝ : Nonempty α
R : AkraBazziRecurrence T g a b r
⊢ ∃ c > 0, ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), c * g ↑n ≤ sumTransform (p a b) g (r i n) n

Tactic:
by
  obtain ⟨c₁, hc₁_mem, hc₁⟩ := R.exists_eventually_const_mul_le_r
  obtain ⟨c₂, hc₂_mem, hc₂⟩ := R.g_grows_poly.eventually_atTop_ge_nat hc₁_mem
  obtain ⟨c₃, hc₃_mem, hc₃⟩ := R.exists_eventually_r_le_const_mul
  have hc₁_pos : 0 < c₁ := hc₁_mem.1
  have hc₃' : 0 < (1 - c₃) := by have := hc₃_mem.2; linarith
  refine ⟨min (c₂ * (1 - c₃)) ((1 - c₃) * c₂ / c₁^((p a b) + 1)), by positivity, ?_⟩
  filter_upwards [hc₁, hc₂, hc₃, R.eventually_r_pos, R.eventually_r_lt_n, eventually_gt_atTop 0]
    with n hn₁ hn₂ hn₃ hrpos hr_lt_n hn_pos
  intro i
  have hrpos_i := hrpos i
  have g_nonneg : 0 ≤ g n := R.g_nonneg n (by positivity)
  cases le_or_gt 0 (p a b + 1) with
  | inl hp => -- 0 ≤ (p a b) + 1
    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)) := 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 / n ^ ((p a b) + 1)) := by
        gcongr 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 le_of_lt hu.2
      _ ≥ n ^ p a b * #(Ico (r i n) n) • (c₂ * g n / 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 / n ^ (p a b + 1)) := by
        rw [nsmul_eq_mul, mul_assoc]
      _ = n ^ (p a b) * (n - r i n) * (c₂ * g n / 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 / n ^ ((p a b) + 1)) := by
        gcongr; exact hn₃ i
      _ = n ^ (p a b) * n * (1 - c₃) * (c₂ * g n / n ^ ((p a b) + 1)) := by ring
      _ = c₂ * (1 - c₃) * g n * (n ^ ((p a b) + 1) / n ^ ((p a b) + 1)) := by
        rw [← Real.rpow_add_one (by positivity) (p a b)]; ring
      _ = c₂ * (1 - c₃) * g n := by rw [div_self (by positivity), mul_one]
      _ ≥ min (c₂ * (1 - c₃)) ((1 - c₃) * c₂ / c₁ ^ ((p a b) + 1)) * g n := by
        gcongr; exact min_le_left _ _
  | inr hp => -- (p a b) + 1 < 0
    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 _ _

Goal state after: