Module: Mathlib.RingTheory.GradedAlgebra.Radical

Goal state before:
pp: ι : Type u_1
σ : Type u_2
A : Type u_3
inst✝⁶ : CommRing A
inst✝⁵ : AddCommMonoid ι
inst✝⁴ : LinearOrder ι
inst✝³ : IsOrderedCancelAddMonoid ι
inst✝² : SetLike σ A
inst✝¹ : AddSubmonoidClass σ A
𝒜 : ι → σ
inst✝ : GradedRing 𝒜
I : Ideal A
hI : IsHomogeneous 𝒜 I
I_ne_top : I ≠ ⊤
homogeneous_mem_or_mem : ∀ {x y : A}, IsHomogeneousElem 𝒜 x → IsHomogeneousElem 𝒜 y → x * y ∈ I → x ∈ I ∨ y ∈ I
x y : A
rid₁ : x ∉ I
rid₂ : y ∉ I
set₁ : Finset ι := {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}
set₁_eq : set₁ = {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}
set₂ : Finset ι := {i ∈ DFinsupp.support ((decompose 𝒜) y) | (proj 𝒜 i) y ∉ I}
set₂_eq : set₂ = {i ∈ DFinsupp.support ((decompose 𝒜) y) | (proj 𝒜 i) y ∉ I}
nonempty : ∀ x ∉ I, {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}.Nonempty
max₁ : ι := set₁.max' ⋯
max₂ : ι := set₂.max' ⋯
mem_max₁ : max₁ ∈ set₁
mem_max₂ : max₂ ∈ set₂
hxy : (proj 𝒜 (max₁ + max₂)) (x * y) ∈ I
antidiag : Finset (ι × ι) :=
  {z ∈ DFinsupp.support ((decompose 𝒜) x) ×ˢ DFinsupp.support ((decompose 𝒜) y) | z.1 + z.2 = max₁ + max₂}
ha : antidiag = {z ∈ DFinsupp.support ((decompose 𝒜) x) ×ˢ DFinsupp.support ((decompose 𝒜) y) | z.1 + z.2 = max₁ + max₂}
mem_antidiag : (max₁, max₂) ∈ antidiag
⊢ (proj 𝒜 max₁) x * (proj 𝒜 max₂) y ∈ I

Tactic:
have eq_add_sum :=
          calc
            proj 𝒜 (max₁ + max₂) (x * y) = ∑ ij ∈ antidiag, proj 𝒜 ij.1 x * proj 𝒜 ij.2 y := by
              simp_rw [ha, proj_apply, DirectSum.decompose_mul, DirectSum.coe_mul_apply 𝒜]
            _ =
                proj 𝒜 max₁ x * proj 𝒜 max₂ y +
                  ∑ ij ∈ antidiag.erase (max₁, max₂), proj 𝒜 ij.1 x * proj 𝒜 ij.2 y :=
              (add_sum_erase _ _ mem_antidiag).symm

Goal state after:
ι : Type u_1
σ : Type u_2
A : Type u_3
inst✝⁶ : CommRing A
inst✝⁵ : AddCommMonoid ι
inst✝⁴ : LinearOrder ι
inst✝³ : IsOrderedCancelAddMonoid ι
inst✝² : SetLike σ A
inst✝¹ : AddSubmonoidClass σ A
𝒜 : ι → σ
inst✝ : GradedRing 𝒜
I : Ideal A
hI : IsHomogeneous 𝒜 I
I_ne_top : I ≠ ⊤
homogeneous_mem_or_mem : ∀ {x y : A}, IsHomogeneousElem 𝒜 x → IsHomogeneousElem 𝒜 y → x * y ∈ I → x ∈ I ∨ y ∈ I
x y : A
rid₁ : x ∉ I
rid₂ : y ∉ I
set₁ : Finset ι := {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}
set₁_eq : set₁ = {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}
set₂ : Finset ι := {i ∈ DFinsupp.support ((decompose 𝒜) y) | (proj 𝒜 i) y ∉ I}
set₂_eq : set₂ = {i ∈ DFinsupp.support ((decompose 𝒜) y) | (proj 𝒜 i) y ∉ I}
nonempty : ∀ x ∉ I, {i ∈ DFinsupp.support ((decompose 𝒜) x) | (proj 𝒜 i) x ∉ I}.Nonempty
max₁ : ι := set₁.max' ⋯
max₂ : ι := set₂.max' ⋯
mem_max₁ : max₁ ∈ set₁
mem_max₂ : max₂ ∈ set₂
hxy : (proj 𝒜 (max₁ + max₂)) (x * y) ∈ I
antidiag : Finset (ι × ι) :=
  {z ∈ DFinsupp.support ((decompose 𝒜) x) ×ˢ DFinsupp.support ((decompose 𝒜) y) | z.1 + z.2 = max₁ + max₂}
ha : antidiag = {z ∈ DFinsupp.support ((decompose 𝒜) x) ×ˢ DFinsupp.support ((decompose 𝒜) y) | z.1 + z.2 = max₁ + max₂}
mem_antidiag : (max₁, max₂) ∈ antidiag
eq_add_sum :
  (proj 𝒜 (max₁ + max₂)) (x * y) =
    (proj 𝒜 max₁) x * (proj 𝒜 max₂) y + ∑ ij ∈ antidiag.erase (max₁, max₂), (proj 𝒜 ij.1) x * (proj 𝒜 ij.2) y
⊢ (proj 𝒜 max₁) x * (proj 𝒜 max₂) y ∈ I