-- Mathlib/CategoryTheory/Preadditive/Schur.lean
theorem finrank_endomorphism_eq_one {X : C} (isIso_iff_nonzero : ∀ f : X ⟶ X, IsIso f ↔ f ≠ 0)
    [I : FiniteDimensional 𝕜 (X ⟶ X)] : finrank 𝕜 (X ⟶ X) = 1

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
⊢ ∃ c, c • 𝟙 X = f

Tactic:
obtain ⟨c, nu⟩ := spectrum.nonempty_of_isAlgClosed_of_finiteDimensional 𝕜 (End.of f)

Result:
case intro
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : c ∈ spectrum 𝕜 (End.of f)
⊢ ∃ c, c • 𝟙 X = f

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
⊢ finrank 𝕜 (X ⟶ X) = 1

Tactic:
have id_nonzero := (isIso_iff_nonzero (𝟙 X)).mp (by infer_instance)

Result:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
⊢ finrank 𝕜 (X ⟶ X) = 1

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
⊢ ∀ (w : X ⟶ X), ∃ c, c • 𝟙 X = w

Tactic:
intro f

Result:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
⊢ ∃ c, c • 𝟙 X = f

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
⊢ finrank 𝕜 (X ⟶ X) = 1

Tactic:
refine finrank_eq_one (𝟙 X) id_nonzero ?_

Result:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
⊢ ∀ (w : X ⟶ X), ∃ c, c • 𝟙 X = w

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this : Nontrivial (End X)
⊢ ∃ c, c • 𝟙 X = f

Tactic:
have : FiniteDimensional 𝕜 (End X) := I

Result:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
⊢ ∃ c, c • 𝟙 X = f

Proof state:
case intro
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : c ∈ spectrum 𝕜 (End.of f)
⊢ ∃ c, c • 𝟙 X = f

Tactic:
use c

Result:
case h
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : c ∈ spectrum 𝕜 (End.of f)
⊢ c • 𝟙 X = f

Proof state:
case h
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : c ∈ spectrum 𝕜 (End.of f)
⊢ c • 𝟙 X = f

Tactic:
rw [spectrum.mem_iff, IsUnit.sub_iff, isUnit_iff_isIso, isIso_iff_nonzero, Ne,
  Classical.not_not, sub_eq_zero, Algebra.algebraMap_eq_smul_one] at nu

Result:
case h
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : End.of f = c • 1
⊢ c • 𝟙 X = f

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
⊢ IsIso (𝟙 X)

Tactic:
infer_instance

Result:
no goals

Proof state:
case h
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this✝ : Nontrivial (End X)
this : FiniteDimensional 𝕜 (End X)
c : 𝕜
nu : End.of f = c • 1
⊢ c • 𝟙 X = f

Tactic:
exact nu.symm

Result:
no goals

Proof state:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
⊢ ∃ c, c • 𝟙 X = f

Tactic:
have : Nontrivial (End X) := nontrivial_of_ne _ _ id_nonzero

Result:
C : Type u_1
inst✝⁴ : Category.{u_3, u_1} C
inst✝³ : Preadditive C
𝕜 : Type u_2
inst✝² : Field 𝕜
inst✝¹ : IsAlgClosed 𝕜
inst✝ : Linear 𝕜 C
X : C
isIso_iff_nonzero : ∀ (f : X ⟶ X), IsIso f ↔ f ≠ 0
I : FiniteDimensional 𝕜 (X ⟶ X)
id_nonzero : 𝟙 X ≠ 0
f : X ⟶ X
this : Nontrivial (End X)
⊢ ∃ c, c • 𝟙 X = f