-- Mathlib/FieldTheory/JacobsonNoether.lean
theorem exists_separable_and_not_isCentral' {L D : Type*} [Field L] [DivisionRing D]
    [Algebra L D] [Algebra.IsAlgebraic L D] [Algebra.IsCentral L D]
  (hneq : (⊥ : Subalgebra L D) ≠ ⊤) :
    ∃ x : D, x ∉ (⊥ : Subalgebra L D) ∧ IsSeparable L x

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
let _ : Algebra L (center D) := equiv.toRingHom.toAlgebra

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝ : Algebra L ↥k := equiv.toRingHom.toAlgebra
⊢ ∃ x ∉ ⊥, IsSeparable L x

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
set φ := Subalgebra.equivOfEq (⊥ : Subalgebra L D) (.center L D) hcenter.symm

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
⊢ ∃ x ∉ ⊥, IsSeparable L x

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝¹ : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
have _ : IsScalarTower L (center D) D := .of_algebraMap_eq fun _ ↦ rfl

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝² : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝¹ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
x✝ : IsScalarTower L (↥k) D
⊢ ∃ x ∉ ⊥, IsSeparable L x

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝² : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝¹ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
x✝ : IsScalarTower L (↥k) D
x : ↥k
⊢ (algebraMap (↥k) D) x = (algebraMap L D) ((algebraMap (↥k) L) x)

Tactic:
rw [IsScalarTower.algebraMap_apply L (center D)]

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝² : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝¹ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
x✝ : IsScalarTower L (↥k) D
x : ↥k
⊢ (algebraMap (↥k) D) x = (algebraMap (↥k) D) ((algebraMap L ↥k) ((algebraMap (↥k) L) x))

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝⁴ : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝³ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
x✝² : IsScalarTower L (↥k) D
x✝¹ : IsScalarTower (↥k) L D
x✝ : Algebra.IsAlgebraic (↥k) D
x : D
hxd : x ∉ k
hx : IsSeparable (↥k) x
⊢ x ∉ ⊥

Tactic:
rwa [← Subalgebra.center_toSubring L, hcenter] at hxd

Result:
no goals

Proof state:
case intro.intro
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
x✝⁴ : Algebra L ↥k := equiv.toRingHom.toAlgebra
x✝³ : Algebra (↥k) L := equiv.symm.toRingHom.toAlgebra
x✝² : IsScalarTower L (↥k) D
x✝¹ : IsScalarTower (↥k) L D
x✝ : Algebra.IsAlgebraic (↥k) D
x : D
hxd : x ∉ k
hx : IsSeparable (↥k) x
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
exact ⟨x, ⟨by rwa [← Subalgebra.center_toSubring L, hcenter] at hxd, IsSeparable.tower_top _ hx⟩⟩

Result:
no goals

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
have hcenter : Subalgebra.center L D = ⊥ := le_bot_iff.mp IsCentral.out

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
⊢ ∃ x ∉ ⊥, IsSeparable L x

Proof state:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
⊢ ∃ x ∉ ⊥, IsSeparable L x

Tactic:
set equiv : L ≃+* (center D) := ((botEquiv L D).symm.trans φ).toRingEquiv

Result:
L : Type u_2
D : Type u_3
inst✝⁴ : Field L
inst✝³ : DivisionRing D
inst✝² : Algebra L D
inst✝¹ : Algebra.IsAlgebraic L D
inst✝ : IsCentral L D
hneq : ⊥ ≠ ⊤
hcenter : Subalgebra.center L D = ⊥
ntrivial : k ≠ ⊤
φ : ↥⊥ ≃ₐ[L] ↥(Subalgebra.center L D) := ⊥.equivOfEq (Subalgebra.center L D) ⋯
equiv : L ≃+* ↥k := ((botEquiv L D).symm.trans φ).toRingEquiv
⊢ ∃ x ∉ ⊥, IsSeparable L x