l3lab/ntp-mathlib huggingface dataset:
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
⊢ ∃ R > 0, ∀ r ∈ Ioo 0 R, x ∈ A f (fderiv 𝕜 f x) r ε

Next tactic:
obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
⊢ 0 < δ

Next tactic:
positivity
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  

Proof state:
case intro.intro
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
⊢ ∃ R > 0, ∀ r ∈ Ioo 0 R, x ∈ A f (fderiv 𝕜 f x) r ε

Next tactic:
refine' ⟨R, R_pos, fun r hr => _⟩
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  

Proof state:
case intro.intro
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
⊢ x ∈ A f (fderiv 𝕜 f x) r ε

Next tactic:
have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  

Proof state:
case intro.intro
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
⊢ x ∈ A f (fderiv 𝕜 f x) r ε

Next tactic:
refine' ⟨r, this, fun y hy z hz => _⟩
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  

Proof state:
case intro.intro
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ < ε * r

Next tactic:
calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by rw [mem_ball_iff_norm] at hz hy; gcongr
    _ = (ε / 2) * r := by ring
    _ < ε * r := by gcongr; exacts [hr.1, half_lt_self hε]
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ = ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖

Next tactic:
simp only [map_sub]
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ ‖f z - f y - ((fderiv 𝕜 f x) z - (fderiv 𝕜 f x) y)‖ =
    ‖f z - f x - ((fderiv 𝕜 f x) z - (fderiv 𝕜 f x) x) - (f y - f x - ((fderiv 𝕜 f x) y - (fderiv 𝕜 f x) x))‖

Next tactic:
abel_nf
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ δ * ‖z - x‖ + δ * ‖y - x‖ ≤ δ * r + δ * r

Next tactic:
rw [mem_ball_iff_norm] at hz hy
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by rw [mem_ball_iff_norm] at hz hy; 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : ‖y - x‖ < r
z : E
hz : ‖z - x‖ < r
⊢ δ * ‖z - x‖ + δ * ‖y - x‖ ≤ δ * r + δ * r

Next tactic:
gcongr
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by rw [mem_ball_iff_norm] at hz hy; gcongr
    _ = (ε / 2) * r := by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ δ * r + δ * r = ε / 2 * r

Next tactic:
ring
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by rw [mem_ball_iff_norm] at hz hy; gcongr
    _ = (ε / 2) * r := by ring
    _ < ε * r := by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ ε / 2 * r < ε * r

Next tactic:
gcongr
theorem mem_A_of_differentiable {ε : ℝ} (hε : 0 < ε) {x : E} (hx : DifferentiableAt 𝕜 f x) :
    ∃ R > 0, ∀ r ∈ Ioo (0 : ℝ) R, x ∈ A f (fderiv 𝕜 f x) r ε := by
  let δ := (ε / 2) / 2
  obtain ⟨R, R_pos, hR⟩ :
      ∃ R > 0, ∀ y ∈ ball x R, ‖f y - f x - fderiv 𝕜 f x (y - x)‖ ≤ δ * ‖y - x‖ :=
    eventually_nhds_iff_ball.1 <| hx.hasFDerivAt.isLittleO.bound <| by positivity
  refine' ⟨R, R_pos, fun r hr => _⟩
  have : r ∈ Ioc (r / 2) r := right_mem_Ioc.2 <| half_lt_self hr.1
  refine' ⟨r, this, fun y hy z hz => _⟩
  calc
    ‖f z - f y - (fderiv 𝕜 f x) (z - y)‖ =
        ‖f z - f x - (fderiv 𝕜 f x) (z - x) - (f y - f x - (fderiv 𝕜 f x) (y - x))‖ :=
      by simp only [map_sub]; abel_nf
    _ ≤ ‖f z - f x - (fderiv 𝕜 f x) (z - x)‖ + ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ :=
      norm_sub_le _ _
    _ ≤ δ * ‖z - x‖ + δ * ‖y - x‖ :=
      add_le_add (hR _ (ball_subset_ball hr.2.le hz)) (hR _ (ball_subset_ball hr.2.le hy))
    _ ≤ δ * r + δ * r := by rw [mem_ball_iff_norm] at hz hy; gcongr
    _ = (ε / 2) * r := by ring
    _ < ε * r := by gcongr; 

Proof state:
case a0
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ 0 < r
case bc
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
ε : ℝ
hε : 0 < ε
x : E
hx : DifferentiableAt 𝕜 f x
δ : ℝ := ε / 2 / 2
R : ℝ
R_pos : R > 0
hR : ∀ y ∈ ball x R, ‖f y - f x - (fderiv 𝕜 f x) (y - x)‖ ≤ δ * ‖y - x‖
r : ℝ
hr : r ∈ Ioo 0 R
this : r ∈ Ioc (r / 2) r
y : E
hy : y ∈ ball x r
z : E
hz : z ∈ ball x r
⊢ ε / 2 < ε

Next tactic:
exacts [hr.1, half_lt_self hε]
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
⊢ ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε

Next tactic:
refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
⊢ 0 ≤ 4 * ‖c‖ * ε

Next tactic:
positivity
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
⊢ ∀ (x : E), r / 2 / ‖c‖ ≤ ‖x‖ → ‖x‖ < r / 2 → ‖(L₁ - L₂) x‖ ≤ 4 * ‖c‖ * ε * ‖x‖

Next tactic:
intro y ley ylt
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  intro y ley ylt
  

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
y : E
ley : r / 2 / ‖c‖ ≤ ‖y‖
ylt : ‖y‖ < r / 2
⊢ ‖(L₁ - L₂) y‖ ≤ 4 * ‖c‖ * ε * ‖y‖

Next tactic:
rw [div_div, div_le_iff' (mul_pos (by norm_num : (0 : ℝ) < 2) (zero_lt_one.trans hc))] at ley
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  intro y ley ylt
  rw [div_div, div_le_iff' (mul_pos (by 

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
y : E
ley : r / (2 * ‖c‖) ≤ ‖y‖
ylt : ‖y‖ < r / 2
⊢ 0 < 2

Next tactic:
norm_num
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  intro y ley ylt
  rw [div_div, div_le_iff' (mul_pos (by norm_num : (0 : ℝ) < 2) (zero_lt_one.trans hc))] at ley
  

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
y : E
ley : r ≤ 2 * ‖c‖ * ‖y‖
ylt : ‖y‖ < r / 2
⊢ ‖(L₁ - L₂) y‖ ≤ 4 * ‖c‖ * ε * ‖y‖

Next tactic:
calc
    ‖(L₁ - L₂) y‖ = ‖f (x + y) - f x - L₂ (x + y - x) - (f (x + y) - f x - L₁ (x + y - x))‖ := by
      simp
    _ ≤ ‖f (x + y) - f x - L₂ (x + y - x)‖ + ‖f (x + y) - f x - L₁ (x + y - x)‖ := (norm_sub_le _ _)
    _ ≤ ε * r + ε * r := by
      apply add_le_add
      · apply le_of_mem_A h₂
        · simp only [le_of_lt (half_pos hr), mem_closedBall, dist_self]
        · simp only [dist_eq_norm, add_sub_cancel', mem_closedBall, ylt.le]
      · apply le_of_mem_A h₁
        · simp only [le_of_lt (half_pos hr), mem_closedBall, dist_self]
        · simp only [dist_eq_norm, add_sub_cancel', mem_closedBall, ylt.le]
    _ = 2 * ε * r := by ring
    _ ≤ 2 * ε * (2 * ‖c‖ * ‖y‖) := (mul_le_mul_of_nonneg_left ley (mul_nonneg (by norm_num) hε.le))
    _ = 4 * ‖c‖ * ε * ‖y‖ := by ring
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  intro y ley ylt
  rw [div_div, div_le_iff' (mul_pos (by norm_num : (0 : ℝ) < 2) (zero_lt_one.trans hc))] at ley
  calc
    ‖(L₁ - L₂) y‖ = ‖f (x + y) - f x - L₂ (x + y - x) - (f (x + y) - f x - L₁ (x + y - x))‖ := by
      

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
y : E
ley : r ≤ 2 * ‖c‖ * ‖y‖
ylt : ‖y‖ < r / 2
⊢ ‖(L₁ - L₂) y‖ = ‖f (x + y) - f x - L₂ (x + y - x) - (f (x + y) - f x - L₁ (x + y - x))‖

Next tactic:
simp
theorem norm_sub_le_of_mem_A {c : 𝕜} (hc : 1 < ‖c‖) {r ε : ℝ} (hε : 0 < ε) (hr : 0 < r) {x : E}
    {L₁ L₂ : E →L[𝕜] F} (h₁ : x ∈ A f L₁ r ε) (h₂ : x ∈ A f L₂ r ε) : ‖L₁ - L₂‖ ≤ 4 * ‖c‖ * ε := by
  refine' op_norm_le_of_shell (half_pos hr) (by positivity) hc _
  intro y ley ylt
  rw [div_div, div_le_iff' (mul_pos (by norm_num : (0 : ℝ) < 2) (zero_lt_one.trans hc))] at ley
  calc
    ‖(L₁ - L₂) y‖ = ‖f (x + y) - f x - L₂ (x + y - x) - (f (x + y) - f x - L₁ (x + y - x))‖ := by
      simp
    _ ≤ ‖f (x + y) - f x - L₂ (x + y - x)‖ + ‖f (x + y) - f x - L₁ (x + y - x)‖ := (norm_sub_le _ _)
    _ ≤ ε * r + ε * r := by
      

Proof state:
𝕜 : Type u_1
inst✝⁴ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝³ : NormedAddCommGroup E
inst✝² : NormedSpace 𝕜 E
F : Type u_3
inst✝¹ : NormedAddCommGroup F
inst✝ : NormedSpace 𝕜 F
f : E → F
K : Set (E →L[𝕜] F)
c : 𝕜
hc : 1 < ‖c‖
r ε : ℝ
hε : 0 < ε
hr : 0 < r
x : E
L₁ L₂ : E →L[𝕜] F
h₁ : x ∈ A f L₁ r ε
h₂ : x ∈ A f L₂ r ε
y : E
ley : r ≤ 2 * ‖c‖ * ‖y‖
ylt : ‖y‖ < r / 2
⊢ ‖f (x + y) - f x - L₂ (x + y - x)‖ + ‖f (x + y) - f x - L₁ (x + y - x)‖ ≤ ε * r + ε * r

Next tactic:
apply add_le_add