-- Mathlib/Computability/AkraBazzi/AkraBazzi.lean
/-- The main proof of the upper bound part of the Akra-Bazzi theorem. The factor
`1 - ε n` does not change the asymptotic order, but is needed for the induction step to go
through. -/
lemma T_isBigO_smoothingFn_mul_asympBound :
    T =O[atTop] (fun n => (1 - ε n) * asympBound g a b n)

Proof state:
case e_a.e_f
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
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
c₁ : ℝ
hc₁ : c₁ > 0
h_sumTransform_aux : ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), c₁ * g ↑n ≤ sumTransform (p a b) g (r i n) n
n₀ : ℕ
n₀_ge_Rn₀ : R.n₀ ≤ n₀
h_smoothing_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < 1 - ε ↑y
h_smoothing_gt_half : ∀ (y : ℕ), n₀ ≤ y → 1 / 2 < 1 - ε ↑y
h_asympBound_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < asympBound g a b y
h_asympBound_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < asympBound g a b (r i y)
h_asympBound_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < asympBound g a b y
n₀_pos : 0 < n₀
h_smoothing_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < 1 - ε ↑(r i y)
bound1 : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), ↑(r i y) ^ p a b * (1 - ε ↑(r i y)) ≤ b i ^ p a b * ↑y ^ p a b * (1 - ε ↑y)
h_smoothingFn_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < 1 - ε ↑y
h_sumTransform : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), c₁ * g ↑y ≤ sumTransform (p a b) g (r i y) y
h_bi_le_r : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), b (min_bi b) / 2 * ↑y ≤ ↑(r i y)
h_base_nonempty : (Ico ⌊b (min_bi b) / 2 * ↑n₀⌋₊ n₀).Nonempty
base_max : ℝ := (Ico ⌊b' * ↑n₀⌋₊ n₀).sup' h_base_nonempty fun n => T n / ((1 - ε ↑n) * asympBound g a b n)
C : ℝ := 2 * c₁⁻¹ ⊔ base_max
hC : C = 2 * c₁⁻¹ ⊔ base_max
h_base : ∀ n ∈ Ico ⌊b' * ↑n₀⌋₊ n₀, T n ≤ C * ((1 - ε ↑n) * asympBound g a b n)
n : ℕ
h_ind : ∀ m < n, m ≥ n₀ → 0 < asympBound g a b m → 0 < 1 - ε ↑m → T m ≤ C * ((1 - ε ↑m) * asympBound g a b m)
hn : n ≥ n₀
h_asympBound_pos' : 0 < asympBound g a b n
h_one_sub_smoothingFn_pos' : 0 < 1 - ε ↑n
b_mul_n₀_le_ri : ∀ (i : α), ⌊b' * ↑n₀⌋₊ ≤ r i n
g_pos : 0 ≤ g ↑n
⊢ (fun i => a i * (C * ((1 - ε ↑(r i n)) * (↑(r i n) ^ p a b * (1 + ∑ u ∈ range (r i n), g ↑u / ↑u ^ (p a b + 1)))))) =
    fun i => C * a i * (↑(r i n) ^ p a b * (1 - ε ↑(r i n)) * (1 + ∑ u ∈ range (r i n), g ↑u / ↑u ^ (p a b + 1)))

Tactic:
ext

Result:
case e_a.e_f.h
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
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
c₁ : ℝ
hc₁ : c₁ > 0
h_sumTransform_aux : ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), c₁ * g ↑n ≤ sumTransform (p a b) g (r i n) n
n₀ : ℕ
n₀_ge_Rn₀ : R.n₀ ≤ n₀
h_smoothing_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < 1 - ε ↑y
h_smoothing_gt_half : ∀ (y : ℕ), n₀ ≤ y → 1 / 2 < 1 - ε ↑y
h_asympBound_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < asympBound g a b y
h_asympBound_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < asympBound g a b (r i y)
h_asympBound_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < asympBound g a b y
n₀_pos : 0 < n₀
h_smoothing_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < 1 - ε ↑(r i y)
bound1 : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), ↑(r i y) ^ p a b * (1 - ε ↑(r i y)) ≤ b i ^ p a b * ↑y ^ p a b * (1 - ε ↑y)
h_smoothingFn_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < 1 - ε ↑y
h_sumTransform : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), c₁ * g ↑y ≤ sumTransform (p a b) g (r i y) y
h_bi_le_r : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), b (min_bi b) / 2 * ↑y ≤ ↑(r i y)
h_base_nonempty : (Ico ⌊b (min_bi b) / 2 * ↑n₀⌋₊ n₀).Nonempty
base_max : ℝ := (Ico ⌊b' * ↑n₀⌋₊ n₀).sup' h_base_nonempty fun n => T n / ((1 - ε ↑n) * asympBound g a b n)
C : ℝ := 2 * c₁⁻¹ ⊔ base_max
hC : C = 2 * c₁⁻¹ ⊔ base_max
h_base : ∀ n ∈ Ico ⌊b' * ↑n₀⌋₊ n₀, T n ≤ C * ((1 - ε ↑n) * asympBound g a b n)
n : ℕ
h_ind : ∀ m < n, m ≥ n₀ → 0 < asympBound g a b m → 0 < 1 - ε ↑m → T m ≤ C * ((1 - ε ↑m) * asympBound g a b m)
hn : n ≥ n₀
h_asympBound_pos' : 0 < asympBound g a b n
h_one_sub_smoothingFn_pos' : 0 < 1 - ε ↑n
b_mul_n₀_le_ri : ∀ (i : α), ⌊b' * ↑n₀⌋₊ ≤ r i n
g_pos : 0 ≤ g ↑n
x✝ : α
⊢ a x✝ * (C * ((1 - ε ↑(r x✝ n)) * (↑(r x✝ n) ^ p a b * (1 + ∑ u ∈ range (r x✝ n), g ↑u / ↑u ^ (p a b + 1))))) =
    C * a x✝ * (↑(r x✝ n) ^ p a b * (1 - ε ↑(r x✝ n)) * (1 + ∑ u ∈ range (r x✝ n), g ↑u / ↑u ^ (p a b + 1)))

Proof state:
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
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
c₁ : ℝ
hc₁ : c₁ > 0
h_sumTransform_aux : ∀ᶠ (n : ℕ) in atTop, ∀ (i : α), c₁ * g ↑n ≤ sumTransform (p a b) g (r i n) n
n₀ : ℕ
n₀_ge_Rn₀ : R.n₀ ≤ n₀
h_smoothing_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < 1 - ε ↑y
h_smoothing_gt_half : ∀ (y : ℕ), n₀ ≤ y → 1 / 2 < 1 - ε ↑y
h_asympBound_pos : ∀ (y : ℕ), n₀ ≤ y → 0 < asympBound g a b y
h_asympBound_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < asympBound g a b (r i y)
h_asympBound_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < asympBound g a b y
n₀_pos : 0 < n₀
h_smoothing_r_pos : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), 0 < 1 - ε ↑(r i y)
bound1 : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), ↑(r i y) ^ p a b * (1 - ε ↑(r i y)) ≤ b i ^ p a b * ↑y ^ p a b * (1 - ε ↑y)
h_smoothingFn_floor : ∀ (y : ℕ), ⌊b' * ↑n₀⌋₊ ≤ y → 0 < 1 - ε ↑y
h_sumTransform : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), c₁ * g ↑y ≤ sumTransform (p a b) g (r i y) y
h_bi_le_r : ∀ (y : ℕ), n₀ ≤ y → ∀ (i : α), b (min_bi b) / 2 * ↑y ≤ ↑(r i y)
h_base_nonempty : (Ico ⌊b (min_bi b) / 2 * ↑n₀⌋₊ n₀).Nonempty
base_max : ℝ := (Ico ⌊b' * ↑n₀⌋₊ n₀).sup' h_base_nonempty fun n => T n / ((1 - ε ↑n) * asympBound g a b n)
C : ℝ := 2 * c₁⁻¹ ⊔ base_max
hC : C = 2 * c₁⁻¹ ⊔ base_max
h_base : ∀ n ∈ Ico ⌊b' * ↑n₀⌋₊ n₀, T n ≤ C * ((1 - ε ↑n) * asympBound g a b n)
n : ℕ
h_ind : ∀ m < n, m ≥ n₀ → 0 < asympBound g a b m → 0 < 1 - ε ↑m → T m ≤ C * ((1 - ε ↑m) * asympBound g a b m)
hn : n ≥ n₀
h_asympBound_pos' : 0 < asympBound g a b n
h_one_sub_smoothingFn_pos' : 0 < 1 - ε ↑n
i : α
⊢ ⌊b' * ↑n₀⌋₊ ≤ r i n

Tactic:
exact_mod_cast calc ⌊b' * (n₀ : ℝ)⌋₊ ≤ b' * n₀ := Nat.floor_le <| by positivity
                            _ ≤ b' * n         := by gcongr
                            _ ≤ r i n          := h_bi_le_r n hn i

Result:
no goals

Proof state:
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
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
⊢ T =O[atTop] fun n => (1 - ε ↑n) * asympBound g a b n

Tactic:
rw [isBigO_atTop_iff_eventually_exists]

Result:
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
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
⊢ ∀ᶠ (n₀ : ℕ) in atTop, ∃ c, ∀ n ≥ n₀, ‖T n‖ ≤ c * ‖(1 - ε ↑n) * asympBound g a b n‖