protected lemma GrowsPolynomially.rpow (p : ℝ) (hf : GrowsPolynomially f)
    (hf_nonneg : ∀ᶠ x in atTop, 0 ≤ f x) : GrowsPolynomially fun x => (f x) ^ p := by
  intro b hb
  obtain ⟨c₁, (hc₁_mem : 0 < c₁), c₂, hc₂_mem, hfnew⟩ := hf b hb
  have hc₁p : 0 < c₁ ^ p := Real.rpow_pos_of_pos hc₁_mem _
  have hc₂p : 0 < c₂ ^ p := Real.rpow_pos_of_pos hc₂_mem _
  obtain _ | hp := le_or_lt 0 p
  case inl =>    -- 0 ≤ p
    refine ⟨c₁^p, hc₁p, ?_⟩
    refine ⟨c₂^p, hc₂p, ?_⟩
    filter_upwards [eventually_gt_atTop 0, hfnew, hf_nonneg,
        (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hf_nonneg]
        with x _ hf₁ hf_nonneg hf_nonneg₂
    intro u hu
    have fu_nonneg : 0 ≤ f u := hf_nonneg₂ u hu.1
    refine ⟨?lb, ?ub⟩
    case lb => calc
      c₁^p * (f x)^p = (c₁ * f x)^p := by rw [mul_rpow (le_of_lt hc₁_mem) hf_nonneg]
        _ ≤ _ := by gcongr; exact (hf₁ u hu).1
    case ub => calc
      (f u)^p ≤ (c₂ * f x)^p := by gcongr; exact (hf₁ u hu).2
        _ = _ := by rw [← mul_rpow (le_of_lt hc₂_mem) hf_nonneg]
  case inr =>   -- p < 0
    rcases hf.eventually_atTop_zero_or_pos_or_neg with hzero|hpos|hneg
    case inl => -- eventually zero
      refine ⟨1, by norm_num, 1, by norm_num, ?_⟩
      filter_upwards [hzero, hfnew] with x hx hx'
      intro u hu
      simp only [hx, ne_eq, zero_rpow (ne_of_lt hp), mul_zero, le_refl, not_true, lt_self_iff_false,
        Set.Icc_self, Set.mem_singleton_iff]
      simp only [hx, mul_zero, Set.Icc_self, Set.mem_singleton_iff] at hx'
      rw [hx' u hu, zero_rpow (ne_of_lt hp)]
    case inr.inl => -- eventually positive
      refine ⟨c₂^p, hc₂p, ?_⟩
      refine ⟨c₁^p, hc₁p, ?_⟩
      filter_upwards [eventually_gt_atTop 0, hfnew, hpos,
          (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hpos]
          with x _ hf₁ hf_pos hf_pos₂
      intro u hu
      refine ⟨?lb, ?ub⟩
      

Proof state:
case lb
f : ℝ → ℝ
p : ℝ
hf : GrowsPolynomially f
hf_nonneg : ∀ᶠ (x : ℝ) in atTop, 0 ≤ f x
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : 0 < c₁
c₂ : ℝ
hc₂_mem : c₂ > 0
hfnew : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hc₁p : 0 < c₁ ^ p
hc₂p : 0 < c₂ ^ p
hp : p < 0
hpos : ∀ᶠ (x : ℝ) in atTop, 0 < f x
x : ℝ
a✝ : 0 < x
hf₁ : ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hf_pos : 0 < f x
hf_pos₂ : ∀ (y : ℝ), b * id x ≤ y → 0 < f y
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ c₂ ^ p * f x ^ p ≤ f u ^ p
case ub
f : ℝ → ℝ
p : ℝ
hf : GrowsPolynomially f
hf_nonneg : ∀ᶠ (x : ℝ) in atTop, 0 ≤ f x
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : 0 < c₁
c₂ : ℝ
hc₂_mem : c₂ > 0
hfnew : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hc₁p : 0 < c₁ ^ p
hc₂p : 0 < c₂ ^ p
hp : p < 0
hpos : ∀ᶠ (x : ℝ) in atTop, 0 < f x
x : ℝ
a✝ : 0 < x
hf₁ : ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hf_pos : 0 < f x
hf_pos₂ : ∀ (y : ℝ), b * id x ≤ y → 0 < f y
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ f u ^ p ≤ c₁ ^ p * f x ^ p

Next tactic:
case lb => calc
        c₂^p * (f x)^p = (c₂ * f x)^p := by rw [mul_rpow (le_of_lt hc₂_mem) (le_of_lt hf_pos)]
          _ ≤ _ := rpow_le_rpow_of_exponent_nonpos (hf_pos₂ u hu.1) (hf₁ u hu).2 (le_of_lt hp)