-- Mathlib/LinearAlgebra/Basis/VectorSpace.lean
theorem LinearMap.exists_leftInverse_of_injective (f : V →ₗ[K] V') (hf_inj : LinearMap.ker f = ⊥) :
    ∃ g : V' →ₗ[K] V, g.comp f = LinearMap.id

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : B ⊆ ⇑f ⁻¹' this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
b : ↑(Basis.ofVectorSpaceIndex K V)
fb_eq : f ↑b = hC ⟨f ↑b, ⋯⟩
⊢ C.restrict (invFun ⇑f) ⟨f ↑b, ⋯⟩ = ↑b

Tactic:
exact leftInverse_invFun (LinearMap.ker_eq_bot.1 hf_inj) _

Result:
no goals

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
⊢ ∃ g, g ∘ₗ f = id

Tactic:
have hB₀ : _ := hB.linearIndependent.linearIndepOn_id

Result:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
⊢ ∃ g, g ∘ₗ f = id

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
⊢ ∃ g, g ∘ₗ f = id

Tactic:
have : LinearIndepOn K _root_.id (f '' B) := by
  have h₁ : LinearIndepOn K _root_.id (f '' Set.range (Basis.ofVectorSpace K V)) :=
    LinearIndepOn.image (f := f) hB₀ (show Disjoint _ _ by simp [hf_inj])
  rwa [Basis.range_ofVectorSpace K V] at h₁

Result:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
⊢ ∃ g, g ∘ₗ f = id

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
⊢ Disjoint (span K (Set.range ⇑(Basis.ofVectorSpace K V))) (ker f)

Tactic:
simp [hf_inj]

Result:
no goals

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : B ⊆ ⇑f ⁻¹' this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
b : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ((hC.constr ℕ) (C.restrict (invFun ⇑f)) ∘ₗ f) (hB b) = id (hB b)

Tactic:
have fb_eq : f b = hC ⟨f b, BC b.2⟩ := by
  change f b = Basis.extend this _
  simp_rw [Basis.extend_apply_self]

Result:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : B ⊆ ⇑f ⁻¹' this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
b : ↑(Basis.ofVectorSpaceIndex K V)
fb_eq : f ↑b = hC ⟨f ↑b, ⋯⟩
⊢ ((hC.constr ℕ) (C.restrict (invFun ⇑f)) ∘ₗ f) (hB b) = id (hB b)

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : ⇑f '' B ⊆ this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
⊢ ∃ g, g ∘ₗ f = id

Tactic:
haveI Vinh : Inhabited V := ⟨0⟩

Result:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : ⇑f '' B ⊆ this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
⊢ ∃ g, g ∘ₗ f = id

Proof state:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : B ⊆ ⇑f ⁻¹' this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
b : ↑(Basis.ofVectorSpaceIndex K V)
⊢ f ↑b = hC ⟨f ↑b, ⋯⟩

Tactic:
change f b = Basis.extend this _

Result:
K : Type u_3
V : Type u_4
V' : Type u_5
inst✝⁴ : DivisionRing K
inst✝³ : AddCommGroup V
inst✝² : AddCommGroup V'
inst✝¹ : Module K V
inst✝ : Module K V'
f : V →ₗ[K] V'
hf_inj : ker f = ⊥
B : Set V := Basis.ofVectorSpaceIndex K V
hB : Basis (↑(Basis.ofVectorSpaceIndex K V)) K V := Basis.ofVectorSpace K V
hB₀ : LinearIndepOn K _root_.id (Set.range ⇑hB)
this : LinearIndepOn K _root_.id (⇑f '' B)
C : Set V' := this.extend ⋯
BC : B ⊆ ⇑f ⁻¹' this.extend ⋯
hC : Basis (↑(this.extend ⋯)) K V' := Basis.extend this
Vinh : Inhabited V
b : ↑(Basis.ofVectorSpaceIndex K V)
⊢ f ↑b = (Basis.extend this) ⟨f ↑b, ⋯⟩