-- Mathlib/Computability/AkraBazzi/GrowsPolynomially.lean
lemma congr_of_eventuallyEq {f g : ℝ → ℝ} (hfg : f =ᶠ[atTop] g) (hg : GrowsPolynomially g) :
    GrowsPolynomially f

Proof state:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
⊢ ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
intro u hu

Result:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)

Tactic:
exact hx₁ u hu

Result:
no goals

Proof state:
case intro.intro.intro.intro
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
refine ⟨c₁, hc₁_mem, c₂, hc₂_mem, ?_⟩

Result:
case intro.intro.intro.intro
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
case intro.intro.intro.intro
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
filter_upwards [hg', (tendsto_id.const_mul_atTop hb.1).eventually_forall_ge_atTop hfg, hfg]
  with x hx₁ hx₂ hx₃

Result:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
⊢ ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
have hg' := hg b hb

Result:
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
hg' : ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
hg' : ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
obtain ⟨c₁, hc₁_mem, c₂, hc₂_mem, hg'⟩ := hg'

Result:
case intro.intro.intro.intro
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
⊢ GrowsPolynomially f

Tactic:
intro b hb

Result:
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
⊢ ∃ c₁ > 0, ∃ c₂ > 0, ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Proof state:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ f u ∈ Set.Icc (c₁ * f x) (c₂ * f x)

Tactic:
rw [hx₂ u hu.1, hx₃]

Result:
case h
f g : ℝ → ℝ
hfg : f =ᶠ[atTop] g
hg : GrowsPolynomially g
b : ℝ
hb : b ∈ Set.Ioo 0 1
c₁ : ℝ
hc₁_mem : c₁ > 0
c₂ : ℝ
hc₂_mem : c₂ > 0
hg' : ∀ᶠ (x : ℝ) in atTop, ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
x : ℝ
hx₁ : ∀ u ∈ Set.Icc (b * x) x, g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)
hx₂ : ∀ (y : ℝ), b * id x ≤ y → f y = g y
hx₃ : f x = g x
u : ℝ
hu : u ∈ Set.Icc (b * x) x
⊢ g u ∈ Set.Icc (c₁ * g x) (c₂ * g x)