-- Mathlib/Geometry/Manifold/Instances/Sphere.lean
theorem ContMDiff.codRestrict_sphere {n : ℕ} [Fact (finrank ℝ E = n + 1)] {f : M → E}
    (hf : ContMDiff I 𝓘(ℝ, E) m f) (hf' : ∀ x, f x ∈ sphere (0 : E) 1) :
    ContMDiff I (𝓡 n) m (Set.codRestrict _ _ hf' : M → sphere (0 : E) 1)

Proof state:
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
h : ContDiffOn ℝ ω (⇑U) Set.univ
H₁ :
  ContMDiffOn 𝓘(ℝ, E) (𝓡 n) ω (⇑U ∘ stereoToFun ↑(-v))
    ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)
H₂ : ContMDiffOn I 𝓘(ℝ, E) m f Set.univ
x : M
hfx : ‖f x‖ = 1
⊢ f x = -↑v ↔ ⟪f x, -↑v⟫_ℝ = 1

Tactic:
rw [inner_eq_one_iff_of_norm_one hfx]

Result:
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
h : ContDiffOn ℝ ω (⇑U) Set.univ
H₁ :
  ContMDiffOn 𝓘(ℝ, E) (𝓡 n) ω (⇑U ∘ stereoToFun ↑(-v))
    ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)
H₂ : ContMDiffOn I 𝓘(ℝ, E) m f Set.univ
x : M
hfx : ‖f x‖ = 1
⊢ ‖-↑v‖ = 1

Proof state:
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
⊢ ContMDiffOn I (𝓡 n) m (↑(extChartAt (𝓡 n) v) ∘ Set.codRestrict f (sphere 0 1) hf')
    (Set.codRestrict f (sphere 0 1) hf' ⁻¹' (extChartAt (𝓡 n) v).source)

Tactic:
let U : _ ≃ₗᵢ[ℝ] _ :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton
      n (ne_zero_of_mem_unit_sphere (-v))).repr

Result:
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
⊢ ContMDiffOn I (𝓡 n) m (↑(extChartAt (𝓡 n) v) ∘ Set.codRestrict f (sphere 0 1) hf')
    (Set.codRestrict f (sphere 0 1) hf' ⁻¹' (extChartAt (𝓡 n) v).source)

Proof state:
case h.e'_23
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
h : ContDiffOn ℝ ω (⇑U) Set.univ
H₁ :
  ContMDiffOn 𝓘(ℝ, E) (𝓡 n) ω (⇑U ∘ stereoToFun ↑(-v))
    ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)
H₂ : ContMDiffOn I 𝓘(ℝ, E) m f Set.univ
⊢ Set.codRestrict f (sphere 0 1) hf' ⁻¹' (extChartAt (𝓡 n) v).source =
    Set.univ ∩ f ⁻¹' ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)

Tactic:
ext x

Result:
case h.e'_23.h
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
h : ContDiffOn ℝ ω (⇑U) Set.univ
H₁ :
  ContMDiffOn 𝓘(ℝ, E) (𝓡 n) ω (⇑U ∘ stereoToFun ↑(-v))
    ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)
H₂ : ContMDiffOn I 𝓘(ℝ, E) m f Set.univ
x : M
⊢ x ∈ Set.codRestrict f (sphere 0 1) hf' ⁻¹' (extChartAt (𝓡 n) v).source ↔
    x ∈ Set.univ ∩ f ⁻¹' ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)

Proof state:
case h.e'_23.h
E : Type u_1
inst✝⁸ : NormedAddCommGroup E
inst✝⁷ : InnerProductSpace ℝ E
m : WithTop ℕ∞
F : Type u_2
inst✝⁶ : NormedAddCommGroup F
inst✝⁵ : NormedSpace ℝ F
H : Type u_3
inst✝⁴ : TopologicalSpace H
I : ModelWithCorners ℝ F H
M : Type u_4
inst✝³ : TopologicalSpace M
inst✝² : ChartedSpace H M
inst✝¹ : IsManifold I m M
n : ℕ
inst✝ : Fact (finrank ℝ E = n + 1)
f : M → E
hf : ContMDiff I 𝓘(ℝ, E) m f
hf' : ∀ (x : M), f x ∈ sphere 0 1
v : ↑(sphere 0 1)
U : ↥(Submodule.span ℝ {↑(-v)})ᗮ ≃ₗᵢ[ℝ] EuclideanSpace ℝ (Fin n) :=
  (OrthonormalBasis.fromOrthogonalSpanSingleton n ⋯).repr
h : ContDiffOn ℝ ω (⇑U) Set.univ
H₁ :
  ContMDiffOn 𝓘(ℝ, E) (𝓡 n) ω (⇑U ∘ stereoToFun ↑(-v))
    ({x | ((innerSL ℝ) ↑(-v)) x ≠ 1} ∩ stereoToFun ↑(-v) ⁻¹' Set.univ)
H₂ : ContMDiffOn I 𝓘(ℝ, E) m f Set.univ
x : M
hfxv : f x = -↑v ↔ ⟪f x, -↑v⟫_ℝ = 1
⊢ x ∈ Set.codRestrict f (sphere 0 1) hf' ⁻¹' (stereographic' n (-v)).source ∩ Set.univ ↔
    x ∈ Set.univ ∩ ({a | ¬⟪-↑v, f a⟫_ℝ = 1} ∩ Set.univ)

Tactic:
simp [not_iff_not, Subtype.ext_iff, hfxv, real_inner_comm]

Result:
no goals