-- Mathlib/Analysis/Calculus/BumpFunction/FiniteDimension.lean
theorem IsOpen.exists_smooth_support_eq {s : Set E} (hs : IsOpen s) :
    ∃ f : E → ℝ, f.support = s ∧ ContDiff ℝ ∞ f ∧ Set.range f ⊆ Set.Icc 0 1

Proof state:
case inl
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
hs : IsOpen ∅
⊢ ∃ f, support f = ∅ ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1

Tactic:
exact
  ⟨fun _ => 0, Function.support_zero, contDiff_const, by
    simp only [range_const, singleton_subset_iff, left_mem_Icc, zero_le_one]⟩

Result:
no goals

Proof state:
case inr.intro.intro.intro.intro.intro.intro.intro.refine_1.h₁
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
g_comp_supp : ∀ (n : ℕ), HasCompactSupport (g n)
g_nonneg : ∀ (n : ℕ) (x : E), 0 ≤ g n x
δ : ℕ → ℝ≥0
δpos : ∀ (i : ℕ), 0 < δ i
c : ℝ≥0
δc : HasSum δ c
c_lt : c < 1
r : ℕ → ℝ
rpos : ∀ (n : ℕ), 0 < r n
hr : ∀ (n i : ℕ), i ≤ n → ∀ (x : E), ‖iteratedFDeriv ℝ i (r n • g n) x‖ ≤ ↑(δ n)
S : ∀ (x : E), Summable fun n => (r n • g n) x
x : E
hx : ¬∑' (n : ℕ), r n * g n x = 0
⊢ x ∈ s

Tactic:
contrapose! hx

Result:
case inr.intro.intro.intro.intro.intro.intro.intro.refine_1.h₁
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
g_comp_supp : ∀ (n : ℕ), HasCompactSupport (g n)
g_nonneg : ∀ (n : ℕ) (x : E), 0 ≤ g n x
δ : ℕ → ℝ≥0
δpos : ∀ (i : ℕ), 0 < δ i
c : ℝ≥0
δc : HasSum δ c
c_lt : c < 1
r : ℕ → ℝ
rpos : ∀ (n : ℕ), 0 < r n
hr : ∀ (n i : ℕ), i ≤ n → ∀ (x : E), ‖iteratedFDeriv ℝ i (r n • g n) x‖ ≤ ↑(δ n)
S : ∀ (x : E), Summable fun n => (r n • g n) x
x : E
hx : x ∉ s
⊢ ∑' (n : ℕ), r n * g n x = 0

Proof state:
case inr.intro.intro.intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
⊢ ∃ f, support f = s ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1

Tactic:
have g_comp_supp : ∀ n, HasCompactSupport (g n) := fun n => (g0 n).2.2.1

Result:
case inr.intro.intro.intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
g_comp_supp : ∀ (n : ℕ), HasCompactSupport (g n)
⊢ ∃ f, support f = s ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1

Proof state:
case inr
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
⊢ ∃ f, support f = s ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1

Tactic:
obtain ⟨T, T_count, hT⟩ : ∃ T : Set ι, T.Countable ∧ ⋃ f ∈ T, support (f : E → ℝ) = s := by
  have : ⋃ f : ι, (f : E → ℝ).support = s := by
    refine Subset.antisymm (iUnion_subset fun f => f.2.1) ?_
    intro x hx
    rcases exists_smooth_tsupport_subset (hs.mem_nhds hx) with ⟨f, hf⟩
    let g : ι := ⟨f, (subset_tsupport f).trans hf.1, hf.2.1, hf.2.2.1, hf.2.2.2.1⟩
    have : x ∈ support (g : E → ℝ) := by
      simp only [g, hf.2.2.2.2, Subtype.coe_mk, mem_support, Ne, one_ne_zero,
        not_false_iff]
    exact mem_iUnion_of_mem _ this
  simp_rw [← this]
  apply isOpen_iUnion_countable
  rintro ⟨f, hf⟩
  exact hf.2.2.1.continuous.isOpen_support

Result:
case inr.intro.intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
⊢ ∃ f, support f = s ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1

Proof state:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
x : E
hx : x ∈ s
⊢ x ∈ ⋃ f, support ↑f

Tactic:
rcases exists_smooth_tsupport_subset (hs.mem_nhds hx) with ⟨f, hf⟩

Result:
case intro
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
x : E
hx : x ∈ s
f : E → ℝ
hf : tsupport f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 ∧ f x = 1
⊢ x ∈ ⋃ f, support ↑f

Proof state:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
g_comp_supp : ∀ (n : ℕ), HasCompactSupport (g n)
g_nonneg : ∀ (n : ℕ) (x : E), 0 ≤ g n x
δ : ℕ → ℝ≥0
δpos : ∀ (i : ℕ), 0 < δ i
c : ℝ≥0
δc : HasSum δ c
c_lt : c < 1
r : ℕ → ℝ
rpos : ∀ (n : ℕ), 0 < r n
hr : ∀ (n i : ℕ), i ≤ n → ∀ (x : E), ‖iteratedFDeriv ℝ i (r n • g n) x‖ ≤ ↑(δ n)
S : ∀ (x : E), Summable fun n => (r n • g n) x
x : E
hx : x ∉ s
⊢ ∀ (n : ℕ), g n x = 0

Tactic:
intro n

Result:
E : Type u_1
inst✝² : NormedAddCommGroup E
inst✝¹ : NormedSpace ℝ E
inst✝ : FiniteDimensional ℝ E
s : Set E
hs : IsOpen s
h's : s.Nonempty
ι : Type (max 0 u_1) := { f // support f ⊆ s ∧ HasCompactSupport f ∧ ContDiff ℝ ∞ f ∧ range f ⊆ Icc 0 1 }
T : Set ι
T_count : T.Countable
hT : ⋃ f ∈ T, support ↑f = s
g0 : ℕ → ι
hg : T = range g0
g : ℕ → E → ℝ := fun n => ↑(g0 n)
g_s : ∀ (n : ℕ), support (g n) ⊆ s
s_g : ∀ x ∈ s, ∃ n, x ∈ support (g n)
g_smooth : ∀ (n : ℕ), ContDiff ℝ ∞ (g n)
g_comp_supp : ∀ (n : ℕ), HasCompactSupport (g n)
g_nonneg : ∀ (n : ℕ) (x : E), 0 ≤ g n x
δ : ℕ → ℝ≥0
δpos : ∀ (i : ℕ), 0 < δ i
c : ℝ≥0
δc : HasSum δ c
c_lt : c < 1
r : ℕ → ℝ
rpos : ∀ (n : ℕ), 0 < r n
hr : ∀ (n i : ℕ), i ≤ n → ∀ (x : E), ‖iteratedFDeriv ℝ i (r n • g n) x‖ ≤ ↑(δ n)
S : ∀ (x : E), Summable fun n => (r n • g n) x
x : E
hx : x ∉ s
n : ℕ
⊢ g n x = 0