deriv_add_lin

formal source

  57  secondDeriv f ⟨3, by decide⟩ ⟨3, by decide⟩ x
  58
  59/-- Linearity of the directional derivative. -/
  60lemma deriv_add_lin (f g : (Fin 4 → ℝ) → ℝ) (μ : Fin 4)
  61    (x : Fin 4 → ℝ) (hf : DifferentiableAt ℝ (fun t => f (coordRay x μ t)) 0)
  62    (hg : DifferentiableAt ℝ (fun t => g (coordRay x μ t)) 0) :
  63  partialDeriv_v2 (fun y => f y + g y) μ x =
  64    partialDeriv_v2 f μ x + partialDeriv_v2 g μ x := by
  65  unfold partialDeriv_v2
  66  exact deriv_add hf hg
  67
  68/-- Linearity of directional derivative (scalar multiplication). -/
  69lemma partialDeriv_v2_smul (f : (Fin 4 → ℝ) → ℝ) (c : ℝ) (μ : Fin 4)
  70    (x : Fin 4 → ℝ) (hf : DifferentiableAt ℝ (fun t => f (coordRay x μ t)) 0) :
  71  partialDeriv_v2 (fun y => c * f y) μ x = c * partialDeriv_v2 f μ x := by
  72  unfold partialDeriv_v2
  73  exact deriv_const_mul c hf
  74
  75/-- Localized version of second derivative linearity (scalar multiplication).
  76    This only requires differentiability in a neighborhood of the point x. -/
  77lemma secondDeriv_smul_local (f : (Fin 4 → ℝ) → ℝ) (c : ℝ) (μ ν : Fin 4)
  78    (x : Fin 4 → ℝ)
  79    (h1 : ∀ᶠ s in 𝓝 0, DifferentiableAt ℝ (fun t => f (coordRay (coordRay x ν s) μ t)) 0)
  80    (h2 : DifferentiableAt ℝ (fun s => partialDeriv_v2 f μ (coordRay x ν s)) 0) :
  81  secondDeriv (fun y => c * f y) μ ν x = c * secondDeriv f μ ν x := by
  82  unfold secondDeriv
  83  have h_ev : ∀ᶠ s in 𝓝 0, partialDeriv_v2 (fun z => c * f z) μ (coordRay x ν s) =
  84                          c * partialDeriv_v2 f μ (coordRay x ν s) := by
  85    apply h1.mono
  86    intro s hs
  87    exact partialDeriv_v2_smul f c μ (coordRay x ν s) hs
  88  rw [Filter.EventuallyEq.deriv_eq h_ev]
  89  exact deriv_const_mul c h2
  90