-- Mathlib/Analysis/BoxIntegral/DivergenceTheorem.lean
theorem hasIntegral_GP_pderiv (f : (Fin (n + 1) → ℝ) → E)
    (f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E) (s : Set (Fin (n + 1) → ℝ))
    (hs : s.Countable) (Hs : ∀ x ∈ s, ContinuousWithinAt f (Box.Icc I) x)
    (Hd : ∀ x ∈ (Box.Icc I) \ s, HasFDerivWithinAt f (f' x) (Box.Icc I) x) (i : Fin (n + 1)) :
    HasIntegral.{0, u, u} I GP (fun x => f' x (Pi.single i 1)) BoxAdditiveMap.volume
      (integral.{0, u, u} (I.face i) GP (fun x => f (i.insertNth (I.upper i) x))
          BoxAdditiveMap.volume -
        integral.{0, u, u} (I.face i) GP (fun x => f (i.insertNth (I.lower i) x))
          BoxAdditiveMap.volume)

Proof state:
case refine_2
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
n : ℕ
inst✝ : CompleteSpace E
I : Box (Fin (n + 1))
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E
s : Set (Fin (n + 1) → ℝ)
hs : s.Countable
Hs : ∀ x ∈ s, ContinuousWithinAt f (Box.Icc I) x
Hd : ∀ x ∈ Box.Icc I \ s, HasFDerivWithinAt f (f' x) (Box.Icc I) x
i : Fin (n + 1)
Hc : ContinuousOn f (Box.Icc I)
fI : ℝ → Box (Fin n) → E := fun y J => integral J GP (fun x => f (i.insertNth y x)) BoxAdditiveMap.volume
fb : ↑(Set.Icc (I.lower i) (I.upper i)) → Fin n →ᵇᵃ[↑(I.face i)] E := fun x => ⋯.toBoxAdditive
F : Fin (n + 1) →ᵇᵃ[↑I] E := BoxAdditiveMap.upperSubLower I i fI fb ⋯
c : ℝ≥0
x : Fin (n + 1) → ℝ
hx : x ∈ Box.Icc I ∩ s
ε : ℝ
ε0 : ε > 0
this : ContinuousWithinAt (fun δ => (2 * δ) ^ (n + 1) * ‖(f' x) (Pi.single i 1)‖) (Set.Ioi 0) 0
⊢ (fun δ => (2 * δ) ^ (n + 1) * ‖(f' x) (Pi.single i 1)‖) 0 < ε / 2

Tactic:
simpa using half_pos ε0

Result:
no goals

Proof state:
case refine_3
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
n : ℕ
inst✝ : CompleteSpace E
I : Box (Fin (n + 1))
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E
s : Set (Fin (n + 1) → ℝ)
hs : s.Countable
Hs : ∀ x ∈ s, ContinuousWithinAt f (Box.Icc I) x
Hd : ∀ x ∈ Box.Icc I \ s, HasFDerivWithinAt f (f' x) (Box.Icc I) x
i : Fin (n + 1)
Hc : ContinuousOn f (Box.Icc I)
fI : ℝ → Box (Fin n) → E := fun y J => integral J GP (fun x => f (i.insertNth y x)) BoxAdditiveMap.volume
fb : ↑(Set.Icc (I.lower i) (I.upper i)) → Fin n →ᵇᵃ[↑(I.face i)] E := fun x => ⋯.toBoxAdditive
F : Fin (n + 1) →ᵇᵃ[↑I] E := BoxAdditiveMap.upperSubLower I i fI fb ⋯
⊢ ∀ (c : ℝ≥0),
    ∀ x ∈ Box.Icc I ∩ s,
      ∀ ε > 0,
        ∃ δ > 0,
          ∀ J ≤ I,
            Box.Icc J ⊆ closedBall x δ →
              x ∈ Box.Icc J →
                (GP.bDistortion = true → J.distortion ≤ c) →
                  dist ((BoxAdditiveMap.volume J) ((f' x) (Pi.single i 1))) (F J) ≤ ε

Tactic:
intro c x hx ε ε0

Result:
case refine_3
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
n : ℕ
inst✝ : CompleteSpace E
I : Box (Fin (n + 1))
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E
s : Set (Fin (n + 1) → ℝ)
hs : s.Countable
Hs : ∀ x ∈ s, ContinuousWithinAt f (Box.Icc I) x
Hd : ∀ x ∈ Box.Icc I \ s, HasFDerivWithinAt f (f' x) (Box.Icc I) x
i : Fin (n + 1)
Hc : ContinuousOn f (Box.Icc I)
fI : ℝ → Box (Fin n) → E := fun y J => integral J GP (fun x => f (i.insertNth y x)) BoxAdditiveMap.volume
fb : ↑(Set.Icc (I.lower i) (I.upper i)) → Fin n →ᵇᵃ[↑(I.face i)] E := fun x => ⋯.toBoxAdditive
F : Fin (n + 1) →ᵇᵃ[↑I] E := BoxAdditiveMap.upperSubLower I i fI fb ⋯
c : ℝ≥0
x : Fin (n + 1) → ℝ
hx : x ∈ Box.Icc I ∩ s
ε : ℝ
ε0 : ε > 0
⊢ ∃ δ > 0,
    ∀ J ≤ I,
      Box.Icc J ⊆ closedBall x δ →
        x ∈ Box.Icc J →
          (GP.bDistortion = true → J.distortion ≤ c) →
            dist ((BoxAdditiveMap.volume J) ((f' x) (Pi.single i 1))) (F J) ≤ ε

Proof state:
case refine_4.intro.intro.intro.intro.refine_2
E : Type u
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
n : ℕ
inst✝ : CompleteSpace E
I : Box (Fin (n + 1))
f : (Fin (n + 1) → ℝ) → E
f' : (Fin (n + 1) → ℝ) → (Fin (n + 1) → ℝ) →L[ℝ] E
s : Set (Fin (n + 1) → ℝ)
hs : s.Countable
Hs : ∀ x ∈ s, ContinuousWithinAt f (Box.Icc I) x
Hd : ∀ x ∈ Box.Icc I \ s, HasFDerivWithinAt f (f' x) (Box.Icc I) x
i : Fin (n + 1)
Hc : ContinuousOn f (Box.Icc I)
fI : ℝ → Box (Fin n) → E := fun y J => integral J GP (fun x => f (i.insertNth y x)) BoxAdditiveMap.volume
fb : ↑(Set.Icc (I.lower i) (I.upper i)) → Fin n →ᵇᵃ[↑(I.face i)] E := fun x => ⋯.toBoxAdditive
F : Fin (n + 1) →ᵇᵃ[↑I] E := BoxAdditiveMap.upperSubLower I i fI fb ⋯
c : ℝ≥0
x : Fin (n + 1) → ℝ
hx : x ∈ Box.Icc I \ s
ε : ℝ
ε0 : ε > 0
ε' : ℝ
ε'0 : 0 < ε'
hlt : 2 * ↑c * ε' < ε
δ : ℝ
δ0 : 0 < δ
Hδ : closedBall x δ ∩ Box.Icc I ⊆ {x_1 | (fun x_2 => ‖f x_2 - f x - (f' x) (x_2 - x)‖ ≤ ε' * ‖x_2 - x‖) x_1}
J : Box (Fin (n + 1))
hle : J ≤ I
hJδ : Box.Icc J ⊆ closedBall x δ
hxJ : x ∈ Box.Icc J
hJc : GP.bDistortion = true → J.distortion ≤ c
⊢ 2 * ↑c * ε' * volume.toBoxAdditive J ≤ ε * (volume.toBoxAdditive.restrict ↑I ⋯) J

Tactic:
exact mul_le_mul_of_nonneg_right hlt.le ENNReal.toReal_nonneg

Result:
no goals