-- Mathlib/LinearAlgebra/TensorProduct/Graded/External.lean
theorem gradedComm_of_tmul_of (i j : ι) (a : 𝒜 i) (b : ℬ j) :
    gradedComm R 𝒜 ℬ (lof R _ 𝒜 i a ⊗ₜ lof R _ ℬ j b) =
      (-1 : ℤˣ)^(j * i) • (lof R _ ℬ _ b ⊗ₜ lof R _ 𝒜 _ a)

Proof state:
R : Type u_1
ι : Type u_2
inst✝⁷ : CommSemiring ι
inst✝⁶ : Module ι (Additive ℤˣ)
inst✝⁵ : DecidableEq ι
𝒜 : ι → Type u_3
ℬ : ι → Type u_4
inst✝⁴ : CommRing R
inst✝³ : (i : ι) → AddCommGroup (𝒜 i)
inst✝² : (i : ι) → AddCommGroup (ℬ i)
inst✝¹ : (i : ι) → Module R (𝒜 i)
inst✝ : (i : ι) → Module R (ℬ i)
i j : ι
a : 𝒜 i
b : ℬ j
⊢ (gradedComm R 𝒜 ℬ) ((lof R ι 𝒜 i) a ⊗ₜ[R] (lof R ι ℬ j) b) = (-1) ^ (j * i) • (lof R ι ℬ j) b ⊗ₜ[R] (lof R ι 𝒜 i) a

Tactic:
rw [gradedComm]

Result:
R : Type u_1
ι : Type u_2
inst✝⁷ : CommSemiring ι
inst✝⁶ : Module ι (Additive ℤˣ)
inst✝⁵ : DecidableEq ι
𝒜 : ι → Type u_3
ℬ : ι → Type u_4
inst✝⁴ : CommRing R
inst✝³ : (i : ι) → AddCommGroup (𝒜 i)
inst✝² : (i : ι) → AddCommGroup (ℬ i)
inst✝¹ : (i : ι) → Module R (𝒜 i)
inst✝ : (i : ι) → Module R (ℬ i)
i j : ι
a : 𝒜 i
b : ℬ j
⊢ (((TensorProduct.directSum R R 𝒜 ℬ).trans
            (LinearEquiv.ofLinear (gradedCommAux R 𝒜 ℬ) (gradedCommAux R ℬ 𝒜) ⋯ ⋯)).trans
        (TensorProduct.directSum R R ℬ 𝒜).symm)
      ((lof R ι 𝒜 i) a ⊗ₜ[R] (lof R ι ℬ j) b) =
    (-1) ^ (j * i) • (lof R ι ℬ j) b ⊗ₜ[R] (lof R ι 𝒜 i) a

Proof state:
R : Type u_1
ι : Type u_2
inst✝⁷ : CommSemiring ι
inst✝⁶ : Module ι (Additive ℤˣ)
inst✝⁵ : DecidableEq ι
𝒜 : ι → Type u_3
ℬ : ι → Type u_4
inst✝⁴ : CommRing R
inst✝³ : (i : ι) → AddCommGroup (𝒜 i)
inst✝² : (i : ι) → AddCommGroup (ℬ i)
inst✝¹ : (i : ι) → Module R (𝒜 i)
inst✝ : (i : ι) → Module R (ℬ i)
i j : ι
a : 𝒜 i
b : ℬ j
⊢ (TensorProduct.directSum R R ℬ 𝒜).symm
      ((gradedCommAux R 𝒜 ℬ) ((TensorProduct.directSum R R 𝒜 ℬ) ((lof R ι 𝒜 i) a ⊗ₜ[R] (lof R ι ℬ j) b))) =
    (-1) ^ (j * i) • (lof R ι ℬ j) b ⊗ₜ[R] (lof R ι 𝒜 i) a

Tactic:
rw [TensorProduct.directSum_lof_tmul_lof, gradedCommAux_lof_tmul, Units.smul_def,
  ← Int.cast_smul_eq_zsmul R, LinearEquiv.map_smul, TensorProduct.directSum_symm_lof_tmul,
  Int.cast_smul_eq_zsmul, ← Units.smul_def]

Result:
no goals

Proof state:
R : Type u_1
ι : Type u_2
inst✝⁷ : CommSemiring ι
inst✝⁶ : Module ι (Additive ℤˣ)
inst✝⁵ : DecidableEq ι
𝒜 : ι → Type u_3
ℬ : ι → Type u_4
inst✝⁴ : CommRing R
inst✝³ : (i : ι) → AddCommGroup (𝒜 i)
inst✝² : (i : ι) → AddCommGroup (ℬ i)
inst✝¹ : (i : ι) → Module R (𝒜 i)
inst✝ : (i : ι) → Module R (ℬ i)
i j : ι
a : 𝒜 i
b : ℬ j
⊢ (((TensorProduct.directSum R R 𝒜 ℬ).trans
            (LinearEquiv.ofLinear (gradedCommAux R 𝒜 ℬ) (gradedCommAux R ℬ 𝒜) ⋯ ⋯)).trans
        (TensorProduct.directSum R R ℬ 𝒜).symm)
      ((lof R ι 𝒜 i) a ⊗ₜ[R] (lof R ι ℬ j) b) =
    (-1) ^ (j * i) • (lof R ι ℬ j) b ⊗ₜ[R] (lof R ι 𝒜 i) a

Tactic:
dsimp only [LinearEquiv.trans_apply, LinearEquiv.ofLinear_apply]

Result:
R : Type u_1
ι : Type u_2
inst✝⁷ : CommSemiring ι
inst✝⁶ : Module ι (Additive ℤˣ)
inst✝⁵ : DecidableEq ι
𝒜 : ι → Type u_3
ℬ : ι → Type u_4
inst✝⁴ : CommRing R
inst✝³ : (i : ι) → AddCommGroup (𝒜 i)
inst✝² : (i : ι) → AddCommGroup (ℬ i)
inst✝¹ : (i : ι) → Module R (𝒜 i)
inst✝ : (i : ι) → Module R (ℬ i)
i j : ι
a : 𝒜 i
b : ℬ j
⊢ (TensorProduct.directSum R R ℬ 𝒜).symm
      ((gradedCommAux R 𝒜 ℬ) ((TensorProduct.directSum R R 𝒜 ℬ) ((lof R ι 𝒜 i) a ⊗ₜ[R] (lof R ι ℬ j) b))) =
    (-1) ^ (j * i) • (lof R ι ℬ j) b ⊗ₜ[R] (lof R ι 𝒜 i) a