Analysis I, Section 10.3: Monotone functions and derivatives

I have attempted to make the translation as faithful a paraphrasing as possible of the original text. When there is a choice between a more idiomatic Lean solution and a more faithful translation, I have generally chosen the latter. In particular, there will be places where the Lean code could be "golfed" to be more elegant and idiomatic, but I have consciously avoided doing so.

Main constructions and results of this section:

Relations between monotonicity and differentiability.

namespace Chapter10

Proposition 10.3.1 / Exercise 10.3.1

theorem declaration uses `sorry`derivative_of_monotone (X:Set ℝ) {x₀:ℝ} (hx₀: ClusterPt x₀ (.principal (X \ {x₀})))
  {f:ℝ → ℝ} (hmono: Monotone f) (hderiv: DifferentiableWithinAt ℝ f X x₀) :
    derivWithin f X x₀ ≥ 0 := byX:Set ℝx₀:ℝhx₀:ClusterPt x₀ (Filter.principal (X \ {x₀}))f:ℝ → ℝhmono:Monotone fhderiv:DifferentiableWithinAt ℝ f X x₀⊢ derivWithin f X x₀ ≥ 0
  sorryAll goals completed! 🐙

theorem declaration uses `sorry`derivative_of_antitone (X:Set ℝ) {x₀:ℝ} (hx₀: ClusterPt x₀ (.principal (X \ {x₀})))
  {f:ℝ → ℝ} (hmono: Antitone f) (hderiv: DifferentiableWithinAt ℝ f X x₀) :
    derivWithin f X x₀ ≤ 0 := byX:Set ℝx₀:ℝhx₀:ClusterPt x₀ (Filter.principal (X \ {x₀}))f:ℝ → ℝhmono:Antitone fhderiv:DifferentiableWithinAt ℝ f X x₀⊢ derivWithin f X x₀ ≤ 0
  sorryAll goals completed! 🐙

Proposition 10.3.3 / Exercise 10.3.4

theorem declaration uses `sorry`strictMono_of_positive_derivative {a b:ℝ} {f:ℝ → ℝ}
  (hderiv: DifferentiableOn ℝ f (.Icc a b)) (hpos: ∀ x ∈ Set.Ioo a b, derivWithin f (.Icc a b) x > 0) :
    StrictMonoOn f (.Icc a b) := bya:ℝb:ℝf:ℝ → ℝhderiv:DifferentiableOn ℝ f (Set.Icc a b)hpos:∀ x ∈ Set.Ioo a b, derivWithin f (Set.Icc a b) x > 0⊢ StrictMonoOn f (Set.Icc a b)
  sorryAll goals completed! 🐙

              theorem declaration uses `sorry`strictAnti_of_negative_derivative {a b:ℝ} {f:ℝ → ℝ}
  (hderiv: DifferentiableOn ℝ f (.Icc a b)) (hneg: ∀ x ∈ Set.Ioo a b, derivWithin f (.Icc a b) x < 0) :
    StrictAntiOn f (.Icc a b) := bya:ℝb:ℝf:ℝ → ℝhderiv:DifferentiableOn ℝ f (Set.Icc a b)hneg:∀ x ∈ Set.Ioo a b, derivWithin f (Set.Icc a b) x < 0⊢ StrictAntiOn f (Set.Icc a b)
  sorryAll goals completed! 🐙/-- Example 10.3.2 -/
declaration uses `sorry`example : ∃ f : ℝ → ℝ, Continuous f ∧ StrictMono f ∧ ¬ DifferentiableAt ℝ f 0 := by⊢ ∃ f, Continuous f ∧ StrictMono f ∧ ¬DifferentiableAt ℝ f 0 sorryAll goals completed! 🐙/-- Exercise 10.3.3 -/
declaration uses `sorry`example : ∃ f: ℝ → ℝ, StrictMono f ∧ Differentiable ℝ f ∧ deriv f 0 = 0 := by⊢ ∃ f, StrictMono f ∧ Differentiable ℝ f ∧ deriv f 0 = 0 sorryAll goals completed! 🐙/-- Exercise 10.3.5 -/
declaration uses `sorry`example : ∃ (X : Set ℝ) (f : ℝ → ℝ), DifferentiableOn ℝ f X ∧
  (∀ x ∈ X, derivWithin f X x > 0) ∧ ¬ StrictMonoOn f X  := by⊢ ∃ X f, DifferentiableOn ℝ f X ∧ (∀ x ∈ X, derivWithin f X x > 0) ∧ ¬StrictMonoOn f X
  sorryAll goals completed! 🐙end Chapter10