-- Mathlib/Computability/AkraBazzi/AkraBazzi.lean
lemma isBigO_apply_r_sub_b (q : ℝ → ℝ) (hq_diff : DifferentiableOn ℝ q (Set.Ioi 1))
    (hq_poly : GrowsPolynomially fun x => ‖deriv q x‖) (i : α) :
    (fun n => q (r i n) - q (b i * n)) =O[atTop] fun n => (deriv q n) * (r i n - b i * n)

Proof state:
case 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
q : ℝ → ℝ
hq_diff : DifferentiableOn ℝ q (Set.Ioi 1)
hq_poly✝ : GrowsPolynomially fun x => ‖deriv q x‖
i : α
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
hb_lt_one : b' < 1
hb : b' ∈ Set.Ioo 0 1
hb' : ∀ (i : α), b' ≤ b i
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
hq_poly :
  ∀ᶠ (x : ℝ) in atTop,
    ∀ u ∈ Set.Icc (b' * x) x,
      (fun x => ‖deriv q x‖) u ∈ Set.Icc (c₁ * (fun x => ‖deriv q x‖) x) (c₂ * (fun x => ‖deriv q x‖) x)
h_tendsto : Tendsto (fun x => b' * x) atTop atTop
n : ℕ
hn : ∀ u ∈ Set.Icc (b' * ↑n) ↑n, ‖deriv q u‖ ∈ Set.Icc (c₁ * ‖deriv q ↑n‖) (c₂ * ‖deriv q ↑n‖)
h_bi_le_r : ∀ (i : α), b (min_bi b) / 2 * ↑n ≤ ↑(r i n)
h_ge_n₀ : R.n₀ ≤ n
h_n_pos : 0 < n
h_bn : 1 < b' * ↑n
⊢ ‖q ↑(r i n) - q (b i * ↑n)‖ ≤ c₂ * ‖deriv q ↑n‖ * ‖↑(r i n) - b i * ↑n‖

Tactic:
refine Convex.norm_image_sub_le_of_norm_deriv_le
  (s := Set.Icc (b'*n) n) (fun z hz => ?diff) (fun z hz => (hn z hz).2)
  (convex_Icc _ _) ?mem_Icc <| ⟨h_bi_le_r i, by exact_mod_cast (le_of_lt (R.r_lt_n i n h_ge_n₀))⟩

Result:
case diff
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
q : ℝ → ℝ
hq_diff : DifferentiableOn ℝ q (Set.Ioi 1)
hq_poly✝ : GrowsPolynomially fun x => ‖deriv q x‖
i : α
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
hb_lt_one : b' < 1
hb : b' ∈ Set.Ioo 0 1
hb' : ∀ (i : α), b' ≤ b i
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
hq_poly :
  ∀ᶠ (x : ℝ) in atTop,
    ∀ u ∈ Set.Icc (b' * x) x,
      (fun x => ‖deriv q x‖) u ∈ Set.Icc (c₁ * (fun x => ‖deriv q x‖) x) (c₂ * (fun x => ‖deriv q x‖) x)
h_tendsto : Tendsto (fun x => b' * x) atTop atTop
n : ℕ
hn : ∀ u ∈ Set.Icc (b' * ↑n) ↑n, ‖deriv q u‖ ∈ Set.Icc (c₁ * ‖deriv q ↑n‖) (c₂ * ‖deriv q ↑n‖)
h_bi_le_r : ∀ (i : α), b (min_bi b) / 2 * ↑n ≤ ↑(r i n)
h_ge_n₀ : R.n₀ ≤ n
h_n_pos : 0 < n
h_bn : 1 < b' * ↑n
z : ℝ
hz : z ∈ Set.Icc (b' * ↑n) ↑n
⊢ DifferentiableAt ℝ q z

case mem_Icc
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
q : ℝ → ℝ
hq_diff : DifferentiableOn ℝ q (Set.Ioi 1)
hq_poly✝ : GrowsPolynomially fun x => ‖deriv q x‖
i : α
b' : ℝ := b (min_bi b) / 2
hb_pos : 0 < b'
hb_lt_one : b' < 1
hb : b' ∈ Set.Ioo 0 1
hb' : ∀ (i : α), b' ≤ b i
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
hq_poly :
  ∀ᶠ (x : ℝ) in atTop,
    ∀ u ∈ Set.Icc (b' * x) x,
      (fun x => ‖deriv q x‖) u ∈ Set.Icc (c₁ * (fun x => ‖deriv q x‖) x) (c₂ * (fun x => ‖deriv q x‖) x)
h_tendsto : Tendsto (fun x => b' * x) atTop atTop
n : ℕ
hn : ∀ u ∈ Set.Icc (b' * ↑n) ↑n, ‖deriv q u‖ ∈ Set.Icc (c₁ * ‖deriv q ↑n‖) (c₂ * ‖deriv q ↑n‖)
h_bi_le_r : ∀ (i : α), b (min_bi b) / 2 * ↑n ≤ ↑(r i n)
h_ge_n₀ : R.n₀ ≤ n
h_n_pos : 0 < n
h_bn : 1 < b' * ↑n
⊢ b i * ↑n ∈ Set.Icc (b' * ↑n) ↑n