-- Mathlib/Analysis/Calculus/BumpFunction/FiniteDimension.lean
theorem u_exists :
    ∃ u : E → ℝ,
      ContDiff ℝ ∞ u ∧ (∀ x, u x ∈ Icc (0 : ℝ) 1) ∧ support u = ball 0 1 ∧ ∀ x, u (-x) = u x

Proof state:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
⊢ ∃ u, ContDiff ℝ ∞ u ∧ (∀ (x : E), u x ∈ Icc 0 1) ∧ support u = ball 0 1 ∧ ∀ (x : E), u (-x) = u x

Tactic:
obtain ⟨f, f_support, f_smooth, f_range⟩ :
    ∃ f : E → ℝ, f.support = ball (0 : E) 1 ∧ ContDiff ℝ ∞ f ∧ Set.range f ⊆ Set.Icc 0 1 :=
  A.exists_smooth_support_eq

Result:
case intro.intro.intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
⊢ ∃ u, ContDiff ℝ ∞ u ∧ (∀ (x : E), u x ∈ Icc 0 1) ∧ support u = ball 0 1 ∧ ∀ (x : E), u (-x) = u x

Proof state:
case intro.intro.intro.refine_3.refine_1
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
hx : x ∈ ball 0 1
⊢ (f x + f (-x)) / 2 ≠ 0

Tactic:
apply ne_of_gt

Result:
case intro.intro.intro.refine_3.refine_1.h
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
hx : x ∈ ball 0 1
⊢ 0 < (f x + f (-x)) / 2

Proof state:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
hx : x ∉ ball 0 1
I1 : x ∉ support f
⊢ -x ∉ ball 0 1

Tactic:
simpa using hx

Result:
no goals

Proof state:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
⊢ ∃ u, ContDiff ℝ ∞ u ∧ (∀ (x : E), u x ∈ Icc 0 1) ∧ support u = ball 0 1 ∧ ∀ (x : E), u (-x) = u x

Tactic:
have A : IsOpen (ball (0 : E) 1) := isOpen_ball

Result:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
⊢ ∃ u, ContDiff ℝ ∞ u ∧ (∀ (x : E), u x ∈ Icc 0 1) ∧ support u = ball 0 1 ∧ ∀ (x : E), u (-x) = u x

Proof state:
case intro.intro.intro.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ 0 ≤ (f x + f (-x)) / 2 ∧ (f x + f (-x)) / 2 ≤ 1

Tactic:
constructor

Result:
case intro.intro.intro.refine_2.left
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ 0 ≤ (f x + f (-x)) / 2

case intro.intro.intro.refine_2.right
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ (f x + f (-x)) / 2 ≤ 1

Proof state:
case intro.intro.intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
⊢ ∃ u, ContDiff ℝ ∞ u ∧ (∀ (x : E), u x ∈ Icc 0 1) ∧ support u = ball 0 1 ∧ ∀ (x : E), u (-x) = u x

Tactic:
refine ⟨fun x => (f x + f (-x)) / 2, ?_, ?_, ?_, ?_⟩

Result:
case intro.intro.intro.refine_1
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
⊢ ContDiff ℝ ∞ fun x => (f x + f (-x)) / 2

case intro.intro.intro.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
⊢ ∀ (x : E), (fun x => (f x + f (-x)) / 2) x ∈ Icc 0 1

case intro.intro.intro.refine_3
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
⊢ (support fun x => (f x + f (-x)) / 2) = ball 0 1

case intro.intro.intro.refine_4
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
⊢ ∀ (x : E), (fun x => (f x + f (-x)) / 2) (-x) = (fun x => (f x + f (-x)) / 2) x

Proof state:
case intro.intro.intro.refine_3.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
hx : x ∉ ball 0 1
I1 : x ∉ support f
⊢ (f x + f (-x)) / 2 = 0

Tactic:
have I2 : -x ∉ support f := by
  rw [f_support]
  simpa using hx

Result:
case intro.intro.intro.refine_3.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
hx : x ∉ ball 0 1
I1 : x ∉ support f
I2 : -x ∉ support f
⊢ (f x + f (-x)) / 2 = 0

Proof state:
case intro.intro.intro.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ (fun x => (f x + f (-x)) / 2) x ∈ Icc 0 1

Tactic:
simp only [mem_Icc]

Result:
case intro.intro.intro.refine_2
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ 0 ≤ (f x + f (-x)) / 2 ∧ (f x + f (-x)) / 2 ≤ 1

Proof state:
case intro.intro.intro.refine_2.right
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ (f x + f (-x)) / 2 ≤ 1

Tactic:
linarith [(B x).2, (B (-x)).2]

Result:
no goals

Proof state:
case intro.intro.intro.refine_2.left
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
A : IsOpen (ball 0 1)
f : E → ℝ
f_support : support f = ball 0 1
f_smooth : ContDiff ℝ ∞ f
f_range : range f ⊆ Icc 0 1
B : ∀ (x : E), f x ∈ Icc 0 1
x : E
⊢ 0 ≤ (f x + f (-x)) / 2

Tactic:
linarith [(B x).1, (B (-x)).1]

Result:
no goals