-- Mathlib/Computability/AkraBazzi/GrowsPolynomially.lean
lemma eventually_atTop_nonneg_or_nonpos (hf : GrowsPolynomially f) :
    (∀ᶠ x in atTop, 0 ≤ f x) ∨ (∀ᶠ x in atTop, f x ≤ 0)

Proof state:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
x : ℝ
hx : ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hx_nonneg : 0 ≤ x
h' : 3 / 4 * x ∈ Set.Icc (1 / 2 * x) x
hu✝ : f (3 / 4 * x) ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hu : (Set.Icc (c₁ * f x) (c₂ * f x)).Nonempty
⊢ f x ≤ 0

Tactic:
rw [Set.nonempty_Icc] at hu

Result:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
x : ℝ
hx : ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hx_nonneg : 0 ≤ x
h' : 3 / 4 * x ∈ Set.Icc (1 / 2 * x) x
hu✝ : f (3 / 4 * x) ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hu : c₁ * f x ≤ c₂ * f x
⊢ f x ≤ 0

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hlt : c₁ < c₂
⊢ (∀ᶠ (x : ℝ) in atTop, 0 ≤ f x) ∨ ∀ᶠ (x : ℝ) in atTop, f x ≤ 0

Tactic:
left

Result:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hlt : c₁ < c₂
⊢ ∀ᶠ (x : ℝ) in atTop, 0 ≤ f x

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
n : ℕ
hn : n ≥ 1
hyp_ind : ∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)
z : ℝ
hz : z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2))
z_nonneg : 0 ≤ z
le_2n : n₀ ⊔ 2 ≤ 2 ^ n * (n₀ ⊔ 2)
n₀_le_z : n₀ ≤ z
fz_eq_c₂fz : f z = c₂ * f z
z_to_half_z' : f (1 / 2 * z) = c₂ * f z
z_to_half_z : f (1 / 2 * z) = f z
h₁ : 2 ^ n = 1 / 2 * 2 ^ (n + 1)
⊢ 1 / 2 * z < 1 / 2 * (2 ^ (n + 1) * (n₀ ⊔ 2))

Tactic:
gcongr

Result:
case bc
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
n : ℕ
hn : n ≥ 1
hyp_ind : ∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)
z : ℝ
hz : z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2))
z_nonneg : 0 ≤ z
le_2n : n₀ ⊔ 2 ≤ 2 ^ n * (n₀ ⊔ 2)
n₀_le_z : n₀ ≤ z
fz_eq_c₂fz : f z = c₂ * f z
z_to_half_z' : f (1 / 2 * z) = c₂ * f z
z_to_half_z : f (1 / 2 * z) = f z
h₁ : 2 ^ n = 1 / 2 * 2 ^ (n + 1)
⊢ z < 2 ^ (n + 1) * (n₀ ⊔ 2)

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
⊢ (∀ᶠ (x : ℝ) in atTop, 0 ≤ f x) ∨ ∀ᶠ (x : ℝ) in atTop, f x ≤ 0

Tactic:
right

Result:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
⊢ ∀ᶠ (x : ℝ) in atTop, f x ≤ 0

Proof state:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
x : ℝ
hx : ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hx_nonneg : 0 ≤ x
⊢ f x ≤ 0

Tactic:
have h' : 3 / 4 * x ∈ Set.Icc (1 / 2 * x) x := by
  rw [Set.mem_Icc]
  exact ⟨by gcongr ?_ * x; norm_num, by linarith⟩

Result:
case h
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hgt : c₂ < c₁
x : ℝ
hx : ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
hx_nonneg : 0 ≤ x
h' : 3 / 4 * x ∈ Set.Icc (1 / 2 * x) x
⊢ f x ≤ 0

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
n : ℕ
hn : n ≥ 1
hyp_ind : ∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)
z : ℝ
hz : z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2))
z_nonneg : 0 ≤ z
le_2n : n₀ ⊔ 2 ≤ 2 ^ n * (n₀ ⊔ 2)
⊢ n₀ ≤ n₀ ⊔ 2

Tactic:
simp

Result:
no goals

Proof state:
case h.h.ha
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
n : ℕ
hn : n ≥ 1
hyp_ind : ∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)
z : ℝ
hz : z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2))
z_nonneg : 0 ≤ z
le_2n : n₀ ⊔ 2 ≤ 2 ^ n * (n₀ ⊔ 2)
n₀_le_z : n₀ ≤ z
fz_eq_c₂fz : f z = c₂ * f z
z_to_half_z' : f (1 / 2 * z) = c₂ * f z
z_to_half_z : f (1 / 2 * z) = f z
⊢ 1 ≤ 2

Tactic:
norm_num

Result:
no goals

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
⊢ ∀ n ≥ 1,
    (∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)) →
      ∀ z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)

Tactic:
intro n hn hyp_ind z hz

Result:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
n : ℕ
hn : n ≥ 1
hyp_ind : ∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)
z : ℝ
hz : z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2))
⊢ f z = f (n₀ ⊔ 2)

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
x : ℝ
hxlb : n₀ ⊔ 2 ≤ x
hxub : x < 2 * (n₀ ⊔ 2)
⊢ f x = f (n₀ ⊔ 2)

Tactic:
have h₁ := calc n₀ ≤ 1 * max n₀ 2 := by simp
                _ ≤ 2 * max n₀ 2 := by gcongr; norm_num

Result:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
x : ℝ
hxlb : n₀ ⊔ 2 ≤ x
hxub : x < 2 * (n₀ ⊔ 2)
h₁ : n₀ ≤ 2 * (n₀ ⊔ 2)
⊢ f x = f (n₀ ⊔ 2)

Proof state:
case base
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
⊢ ∀ x ∈ Set.Ico (n₀ ⊔ 2) (2 * (n₀ ⊔ 2)), f x = f (n₀ ⊔ 2)

case step
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
⊢ ∀ n ≥ 1,
    (∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)) →
      ∀ z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)

Tactic:
case base =>
  intro x ⟨hxlb, hxub⟩
  have h₁ := calc n₀ ≤ 1 * max n₀ 2 := by simp
                  _ ≤ 2 * max n₀ 2 := by gcongr; norm_num
  have h₂ := hn₀ (2 * max n₀ 2) h₁ (max n₀ 2) ⟨by simp [hxlb], by linarith⟩
  rw [h₂]
  exact hn₀ (2 * max n₀ 2) h₁ x ⟨by simp [hxlb], le_of_lt hxub⟩

Result:
case step
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
heq : c₁ = c₂
n₀ : ℝ
hn₀ : ∀ b ≥ n₀, ∀ u ∈ Set.Icc (1 / 2 * b) b, f u = c₂ * f b
⊢ ∀ n ≥ 1,
    (∀ z ∈ Set.Ico (n₀ ⊔ 2) (2 ^ n * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)) →
      ∀ z ∈ Set.Ico (2 ^ n * (n₀ ⊔ 2)) (2 ^ (n + 1) * (n₀ ⊔ 2)), f z = f (n₀ ⊔ 2)

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
c₁ : ℝ
left✝¹ : c₁ > 0
c₂ : ℝ
left✝ : c₂ > 0
h : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
heq : c₁ = c₂
c : ℝ
hc✝ : ∀ᶠ (x : ℝ) in atTop, f x = c
hneg : c < 0
x : ℝ
hc : f x = c
⊢ f x < 0

Tactic:
simpa only [hc]

Result:
no goals