-- Mathlib/LinearAlgebra/Coevaluation.lean
theorem contractLeft_assoc_coevaluation :
    (contractLeft K V).rTensor _ ∘ₗ
        (TensorProduct.assoc K _ _ _).symm.toLinearMap ∘ₗ
          (coevaluation K V).lTensor (Module.Dual K V) =
      (TensorProduct.lid K _).symm.toLinearMap ∘ₗ (TensorProduct.rid K _).toLinearMap

Proof state:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ∑ x : ↑(Basis.ofVectorSpaceIndex K V),
      (LinearMap.rTensor (Dual K V) (contractLeft K V))
        (((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] (Basis.ofVectorSpace K V) x) ⊗ₜ[K]
          (Basis.ofVectorSpace K V).coord x) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Tactic:
simp only [LinearMap.rTensor_tmul, contractLeft_apply]

Result:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ∑ x : ↑(Basis.ofVectorSpaceIndex K V),
      ((Basis.ofVectorSpace K V).dualBasis j) ((Basis.ofVectorSpace K V) x) ⊗ₜ[K] (Basis.ofVectorSpace K V).coord x =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Proof state:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (LinearMap.rTensor (Dual K V) (contractLeft K V))
      (∑ x : ↑(Basis.ofVectorSpaceIndex K V),
        (TensorProduct.assoc K (Dual K V) V (Dual K V)).symm
          ((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K]
            (Basis.ofVectorSpace K V) x ⊗ₜ[K] (Basis.ofVectorSpace K V).coord x)) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Tactic:
simp only [assoc_symm_tmul]

Result:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (LinearMap.rTensor (Dual K V) (contractLeft K V))
      (∑ x : ↑(Basis.ofVectorSpaceIndex K V),
        ((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] (Basis.ofVectorSpace K V) x) ⊗ₜ[K]
          (Basis.ofVectorSpace K V).coord x) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Proof state:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (((TensorProduct.mk K (Dual K V) K).compr₂
          (LinearMap.rTensor (Dual K V) (contractLeft K V) ∘ₗ
            ↑(TensorProduct.assoc K (Dual K V) V (Dual K V)).symm ∘ₗ LinearMap.lTensor (Dual K V) (coevaluation K V)))
        ((Basis.ofVectorSpace K V).dualBasis j))
      1 =
    (((TensorProduct.mk K (Dual K V) K).compr₂
          (↑(TensorProduct.lid K (Dual K V)).symm ∘ₗ ↑(TensorProduct.rid K (Dual K V))))
        ((Basis.ofVectorSpace K V).dualBasis j))
      1

Tactic:
rw [LinearMap.compr₂_apply, LinearMap.compr₂_apply, TensorProduct.mk_apply]

Result:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (LinearMap.rTensor (Dual K V) (contractLeft K V) ∘ₗ
        ↑(TensorProduct.assoc K (Dual K V) V (Dual K V)).symm ∘ₗ LinearMap.lTensor (Dual K V) (coevaluation K V))
      ((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] 1) =
    (↑(TensorProduct.lid K (Dual K V)).symm ∘ₗ ↑(TensorProduct.rid K (Dual K V)))
      ((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] 1)

Proof state:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ∑ x : ↑(Basis.ofVectorSpaceIndex K V),
      ((Basis.ofVectorSpace K V).dualBasis j) ((Basis.ofVectorSpace K V) x) ⊗ₜ[K] (Basis.ofVectorSpace K V).coord x =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Tactic:
simp only [Basis.coe_dualBasis, Basis.coord_apply, Basis.repr_self_apply, TensorProduct.ite_tmul]

Result:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (∑ x : ↑(Basis.ofVectorSpaceIndex K V), if x = j then 1 ⊗ₜ[K] (Basis.ofVectorSpace K V).coord x else 0) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).coord j

Proof state:
case H
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
⊢ ∀ (i : ↑(Basis.ofVectorSpaceIndex K V)),
    ((TensorProduct.mk K (Dual K V) K).compr₂
          (LinearMap.rTensor (Dual K V) (contractLeft K V) ∘ₗ
            ↑(TensorProduct.assoc K (Dual K V) V (Dual K V)).symm ∘ₗ LinearMap.lTensor (Dual K V) (coevaluation K V)))
        ((Basis.ofVectorSpace K V).dualBasis i) =
      ((TensorProduct.mk K (Dual K V) K).compr₂
          (↑(TensorProduct.lid K (Dual K V)).symm ∘ₗ ↑(TensorProduct.rid K (Dual K V))))
        ((Basis.ofVectorSpace K V).dualBasis i)

Tactic:
intro j

Result:
case H
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ((TensorProduct.mk K (Dual K V) K).compr₂
        (LinearMap.rTensor (Dual K V) (contractLeft K V) ∘ₗ
          ↑(TensorProduct.assoc K (Dual K V) V (Dual K V)).symm ∘ₗ LinearMap.lTensor (Dual K V) (coevaluation K V)))
      ((Basis.ofVectorSpace K V).dualBasis j) =
    ((TensorProduct.mk K (Dual K V) K).compr₂
        (↑(TensorProduct.lid K (Dual K V)).symm ∘ₗ ↑(TensorProduct.rid K (Dual K V))))
      ((Basis.ofVectorSpace K V).dualBasis j)

Proof state:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ (LinearMap.rTensor (Dual K V) (contractLeft K V))
      (∑ x : ↑(Basis.ofVectorSpaceIndex K V),
        ((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] (Basis.ofVectorSpace K V) x) ⊗ₜ[K]
          (Basis.ofVectorSpace K V).coord x) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j

Tactic:
rw [map_sum]

Result:
case H.h
K : Type u
inst✝³ : Field K
V : Type v
inst✝² : AddCommGroup V
inst✝¹ : Module K V
inst✝ : FiniteDimensional K V
this : DecidableEq ↑(Basis.ofVectorSpaceIndex K V) := Classical.decEq ↑(Basis.ofVectorSpaceIndex K V)
j : ↑(Basis.ofVectorSpaceIndex K V)
⊢ ∑ x : ↑(Basis.ofVectorSpaceIndex K V),
      (LinearMap.rTensor (Dual K V) (contractLeft K V))
        (((Basis.ofVectorSpace K V).dualBasis j ⊗ₜ[K] (Basis.ofVectorSpace K V) x) ⊗ₜ[K]
          (Basis.ofVectorSpace K V).coord x) =
    1 ⊗ₜ[K] (Basis.ofVectorSpace K V).dualBasis j