-- Mathlib/Computability/AkraBazzi/GrowsPolynomially.lean
lemma eventually_zero_of_frequently_zero (hf : GrowsPolynomially f) (hf' : ∃ᶠ x in atTop, f x = 0) :
    ∀ᶠ x in atTop, f x = 0

Proof state:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
k : ℕ
z : ℝ
hxz : x ≤ z
hz : z ∈ Set.Icc (2 ^ (-(↑k + 1) - 1) * x₀) (2 ^ (-(↑k + 1)) * x₀)
hx' : x ≤ 2 ^ (-↑k - 1) * x₀
ih : f (2 ^ (-↑k - 1) * x₀) = 0
hx : z ∈ Set.Icc (1 / 2 * (2 ^ (-↑k - 1) * x₀)) (2 ^ (-↑k - 1) * x₀) → f z = 0
h₁ : -1 + (-↑k - 1) = -↑k - 2
⊢ -(↑k + 1) - 1 = -↑k - 2

Tactic:
ring

Result:
no goals

Proof state:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
k : ℕ
ih : ∀ (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑k - 1) * x₀) (2 ^ (-↑k) * x₀) → f z = 0
z : ℝ
hxz : x ≤ z
hz : z ∈ Set.Icc (2 ^ (-(↑k + 1) - 1) * x₀) (2 ^ (-(↑k + 1)) * x₀)
⊢ x ≤ 2 ^ (-↑k - 1) * x₀

Tactic:
calc x ≤ z := hxz
  _ ≤ _ := by simp only [neg_add, ← sub_eq_add_neg] at hz; exact hz.2

Result:
no goals

Proof state:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ logb 2 (x / x₀) ≤ logb 2 (2 ^ (-↑⌊-logb 2 (x / x₀)⌋₊))

Tactic:
rw [← rpow_intCast, logb_rpow (by norm_num) (by norm_num), ← neg_le_neg_iff]

Result:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ -↑(-↑⌊-logb 2 (x / x₀)⌋₊) ≤ -logb 2 (x / x₀)

Proof state:
f : ℝ → ℝ
hf : GrowsPolynomially f
hf' : ∃ᶠ (x : ℝ) in atTop, f x = 0
⊢ ∀ᶠ (x : ℝ) in atTop, f x = 0

Tactic:
obtain ⟨c₁, hc₁_mem, c₂, hc₂_mem, hf⟩ := hf (1/2) (by norm_num)

Result:
case intro.intro.intro.intro
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∃ᶠ (x : ℝ) in atTop, f x = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
⊢ ∀ᶠ (x : ℝ) in atTop, f x = 0

Proof state:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ x / x₀ ≤ 1

Tactic:
rw [div_le_one x₀_pos]

Result:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ x ≤ x₀

Proof state:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ x / x₀ ≤ 2 ^ (-↑⌊-logb 2 (x / x₀)⌋₊)

Tactic:
refine (logb_le_logb (b := 2) (by norm_num) (by positivity)
  (zpow_pos (by norm_num) _)).mp ?_

Result:
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
hmain : ∀ (m : ℕ) (z : ℝ), x ≤ z → z ∈ Set.Icc (2 ^ (-↑m - 1) * x₀) (2 ^ (-↑m) * x₀) → f z = 0
⊢ logb 2 (x / x₀) ≤ logb 2 (2 ^ (-↑⌊-logb 2 (x / x₀)⌋₊))

Proof state:
case succ
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
k : ℕ
z : ℝ
hxz : x ≤ z
hz : z ∈ Set.Icc (2 ^ (-(↑k + 1) - 1) * x₀) (2 ^ (-(↑k + 1)) * x₀)
hx' : x ≤ 2 ^ (-↑k - 1) * x₀
ih : f (2 ^ (-↑k - 1) * x₀) = 0
hx : z ∈ Set.Icc (1 / 2 * (2 ^ (-↑k - 1) * x₀)) (2 ^ (-↑k - 1) * x₀) → f z = 0
⊢ f z = 0

Tactic:
refine hx ⟨?lb₁, ?ub₁⟩

Result:
case lb₁
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
k : ℕ
z : ℝ
hxz : x ≤ z
hz : z ∈ Set.Icc (2 ^ (-(↑k + 1) - 1) * x₀) (2 ^ (-(↑k + 1)) * x₀)
hx' : x ≤ 2 ^ (-↑k - 1) * x₀
ih : f (2 ^ (-↑k - 1) * x₀) = 0
hx : z ∈ Set.Icc (1 / 2 * (2 ^ (-↑k - 1) * x₀)) (2 ^ (-↑k - 1) * x₀) → f z = 0
⊢ 1 / 2 * (2 ^ (-↑k - 1) * x₀) ≤ z

case ub₁
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx_pos : 0 < x
x₀ : ℝ
hx₀_ge : x₀ ≥ x ⊔ 1
hx₀ : f x₀ = 0
x₀_pos : 0 < x₀
k : ℕ
z : ℝ
hxz : x ≤ z
hz : z ∈ Set.Icc (2 ^ (-(↑k + 1) - 1) * x₀) (2 ^ (-(↑k + 1)) * x₀)
hx' : x ≤ 2 ^ (-↑k - 1) * x₀
ih : f (2 ^ (-↑k - 1) * x₀) = 0
hx : z ∈ Set.Icc (1 / 2 * (2 ^ (-↑k - 1) * x₀)) (2 ^ (-↑k - 1) * x₀) → f z = 0
⊢ z ≤ 2 ^ (-↑k - 1) * x₀

Proof state:
case intro.intro.intro.intro
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
⊢ ∀ᶠ (x : ℝ) in atTop, f x = 0

Tactic:
filter_upwards [eventually_forall_ge_atTop.mpr hf, eventually_gt_atTop 0] with x hx hx_pos

Result:
case h
f : ℝ → ℝ
hf✝ : GrowsPolynomially f
hf' : ∀ (a : ℝ), ∃ b ≥ a, f b = 0
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hf : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (1 / 2 * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)
x : ℝ
hx : ∀ (y : ℝ), x ≤ y → ∀ u ∈ Set.Icc (1 / 2 * y) y, f u ∈ Set.Icc (c₁ * f y) (c₂ * f y)
hx_pos : 0 < x
⊢ f x = 0