-- Lean 4 proof step from Mathlib.Computability.AkraBazzi.GrowsPolynomially (hypothesis)
Goal:
f g : ℝ → ℝ
hf : GrowsPolynomially f
hfg : ∀ ⦃c : ℝ⦄, 0 < c → ∀ᶠ (x : ℝ) in atTop, ‖g x‖ ≤ c * ‖f x‖
b : ℝ
hb : Membership.mem (γ := Set ℝ) (Set.Ioo 0 1) b
hb_ub : b < 1
⊢ ∃ c₁ > 0,
    ∃ c₂ > 0,
      ∀ᶠ (x : ℝ) in atTop,
        ∀ u ∈ Set.Icc (b * x) x,
          (fun (x : ℝ) ↦ f x + g x) u ∈ Set.Icc (c₁ * (fun (x : ℝ) ↦ f x + g x) x) (c₂ * (fun (x : ℝ) ↦ f x + g x) x)
have x : (∀ᶠ (x : ℝ) in atTop, 0 ≤ f x) ∨ ∀ᶠ (x : ℝ) in atTop, f x ≤ 0
Tactic: cases hf.eventually_atTop_nonneg_or_nonpos with
  | inl hf' => -- f is eventually non-negative
    have hf := hf b hb
    obtain ⟨c₁, hc₁_mem : 0 < c₁, c₂, hc₂_mem : 0 < c₂, hf⟩ := hf
    specialize hfg (c := 1 / 2) (by norm_num)
    refine ⟨c₁ / 3, by positivity, 3*c₂, by positivity, ?_⟩
    filter_upwards [hf,
                    (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hfg,
                    (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hf',
                    eventually_ge_atTop 0] with x hf₁ hfg' hf₂ hx_nonneg
    have hbx : b * x ≤ x := by nth_rewrite 2 [← one_mul x]; gcongr
    have hfg₂ : ‖g x‖ ≤ 1 / 2 * f x := by
      calc ‖g x‖ ≤ 1 / 2 * ‖f x‖ := hfg' x hbx
           _ = 1 / 2 * f x := by congr; exact norm_of_nonneg (hf₂ _ hbx)
    have hx_ub : f x + g x ≤ 3 / 2 * f x := by
      calc _ ≤ f x + ‖g x‖ := by gcongr; exact le_norm_self (g x)
           _ ≤ f x + 1 / 2 * f x := by gcongr
           _ = 3 / 2 * f x := by ring
    have hx_lb : 1 / 2 * f x ≤ f x + g x := by
      calc f x + g x ≥ f x - ‖g x‖ := by
                rw [sub_eq_add_neg, norm_eq_abs]; gcongr; exact neg_abs_le (g x)
           _ ≥ f x - 1 / 2 * f x := by gcongr
           _ = 1 / 2 * f x := by ring
    intro u ⟨hu_lb, hu_ub⟩
    have hfu_nonneg : 0 ≤ f u := hf₂ _ hu_lb
    have hfg₃ : ‖g u‖ ≤ 1 / 2 * f u := by
      calc ‖g u‖ ≤ 1 / 2 * ‖f u‖ := hfg' _ hu_lb
           _ = 1 / 2 * f u := by congr; simp only [norm_eq_abs, abs_eq_self, hfu_nonneg]
    refine ⟨?lb, ?ub⟩
    case lb =>
      calc f u + g u ≥ f u - ‖g u‖ := by
                  rw [sub_eq_add_neg, norm_eq_abs]; gcongr; exact neg_abs_le _
           _ ≥ f u - 1 / 2 * f u := by gcongr
           _ = 1 / 2 * f u := by ring
           _ ≥ 1 / 2 * (c₁ * f x) := by gcongr; exact (hf₁ u ⟨hu_lb, hu_ub⟩).1
           _ = c₁ / 3 * (3 / 2 * f x) := by ring
           _ ≥ c₁ / 3 * (f x + g x) := by gcongr
    case ub =>
      calc _ ≤ f u + ‖g u‖ := by gcongr; exact le_norm_self (g u)
           _ ≤ f u + 1 / 2 * f u := by gcongr
           _ = 3 / 2 * f u := by ring
           _ ≤ 3 / 2 * (c₂ * f x) := by gcongr; exact (hf₁ u ⟨hu_lb, hu_ub⟩).2
           _ = 3 * c₂ * (1 / 2 * f x) := by ring
           _ ≤ 3 * c₂ * (f x + g x) := by gcongr
  | inr hf' => -- f is eventually nonpos
    have hf := hf b hb
    obtain ⟨c₁, hc₁_mem : 0 < c₁, c₂, hc₂_mem : 0 < c₂, hf⟩ := hf
    specialize hfg (c := 1 / 2) (by norm_num)
    refine ⟨3*c₁, by positivity, c₂/3, by positivity, ?_⟩
    filter_upwards [hf,
                    (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hfg,
                    (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hf',
                    eventually_ge_atTop 0] with x hf₁ hfg' hf₂ hx_nonneg
    have hbx : b * x ≤ x := by nth_rewrite 2 [← one_mul x]; gcongr
    have hfg₂ : ‖g x‖ ≤ -1 / 2 * f x := by
      calc ‖g x‖ ≤ 1 / 2 * ‖f x‖ := hfg' x hbx
           _ = 1 / 2 * (-f x) := by congr; exact norm_of_nonpos (hf₂ x hbx)
           _ = _ := by ring
    have hx_ub : f x + g x ≤ 1 / 2 * f x := by
      calc _ ≤ f x + ‖g x‖ := by gcongr; exact le_norm_self (g x)
           _ ≤ f x + (-1 / 2 * f x) := by gcongr
           _ = 1 / 2 * f x := by ring
    have hx_lb : 3 / 2 * f x ≤ f x + g x := by
      calc f x + g x ≥ f x - ‖g x‖ := by
                rw [sub_eq_add_neg, norm_eq_abs]; gcongr; exact neg_abs_le (g x)
           _ ≥ f x + 1 / 2 * f x := by
                  rw [sub_eq_add_neg]
                  gcongr
                  refine le_of_neg_le_neg ?bc.a
                  rwa [neg_neg, ← neg_mul, ← neg_div]
           _ = 3 / 2 * f x := by ring
    intro u ⟨hu_lb, hu_ub⟩
    have hfu_nonpos : f u ≤ 0 := hf₂ _ hu_lb
    have hfg₃ : ‖g u‖ ≤ -1 / 2 * f u := by
      calc ‖g u‖ ≤ 1 / 2 * ‖f u‖ := hfg' _ hu_lb
           _ = 1 / 2 * (-f u) := by congr; exact norm_of_nonpos hfu_nonpos
           _ = -1 / 2 * f u := by ring
    refine ⟨?lb, ?ub⟩
    case lb =>
      calc f u + g u ≥ f u - ‖g u‖ := by
                  rw [sub_eq_add_neg, norm_eq_abs]; gcongr; exact neg_abs_le _
           _ ≥ f u + 1 / 2 * f u := by
                  rw [sub_eq_add_neg]
                  gcongr
                  refine le_of_neg_le_neg ?_
                  rwa [neg_neg, ← neg_mul, ← neg_div]
           _ = 3 / 2 * f u := by ring
           _ ≥ 3 / 2 * (c₁ * f x) := by gcongr; exact (hf₁ u ⟨hu_lb, hu_ub⟩).1
           _ = 3 * c₁ * (1 / 2 * f x) := by ring
           _ ≥ 3 * c₁ * (f x + g x) := by gcongr
    case ub =>
      calc _ ≤ f u + ‖g u‖ := by gcongr; exact le_norm_self (g u)
           _ ≤ f u - 1 / 2 * f u := by
                rw [sub_eq_add_neg]
                gcongr
                rwa [← neg_mul, ← neg_div]
           _ = 1 / 2 * f u := by ring
           _ ≤ 1 / 2 * (c₂ * f x) := by gcongr; exact (hf₁ u ⟨hu_lb, hu_ub⟩).2
           _ = c₂ / 3 * (3 / 2 * f x) := by ring
           _ ≤ c₂ / 3 * (f x + g x) := by gcongr