Module: Mathlib.Algebra.Category.Ring.Under.Property

Goal state before:
pp: R S : CommRingCat
inst✝ : Algebra ↑R ↑S
A B : Under R
f g : A ⟶ B
c : Fork f g := Under.equalizerFork f g
hc : IsLimit c := Under.equalizerForkIsLimit f g
ι : R.mkUnder ↥((toAlgHom f).equalizer (toAlgHom g)) ⟶ A := ((toAlgHom f).equalizer (toAlgHom g)).val.toUnder
h' : (R.tensorProd S).obj (R.mkUnder ↥((toAlgHom f).equalizer (toAlgHom g))) ⟶ (R.tensorProd S).obj A :=
  (R.tensorProd S).map ι
w' : h' ≫ (R.tensorProd S).map f = h' ≫ (R.tensorProd S).map g
e : IsLimit ((R.tensorProd S).mapCone c) ≃ IsLimit (Fork.ofι h' w') := isLimitMapConeForkEquiv (R.tensorProd S) ⋯
heq :
  (Under.equalizerForkIsLimit ((R.tensorProd S).map f) ((R.tensorProd S).map g)).lift (Fork.ofι h' w') =
    (AlgHom.tensorEqualizer (↑S) (↑S) (toAlgHom f) (toAlgHom g)).toUnder ≫
      CategoryTheory.Under.homMk
        (ofHom
          (RingHom.id
            ↥((Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom f)).equalizer
                (Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom g)))))
        ⋯
⊢ Function.Bijective
      ⇑(ConcreteCategory.hom
          ((CategoryTheory.Under.forget S).map
            ((AlgHom.tensorEqualizer (↑S) (↑S) (toAlgHom f) (toAlgHom g)).toUnder ≫
              CategoryTheory.Under.homMk
                (ofHom
                  (RingHom.id
                    ↥((Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom f)).equalizer
                        (Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom g)))))
                ⋯))) ↔
    Function.Bijective ⇑(AlgHom.tensorEqualizer (↑R) (↑S) (toAlgHom f) (toAlgHom g))

Tactic:
]

Goal state after:
R S : CommRingCat
inst✝ : Algebra ↑R ↑S
A B : Under R
f g : A ⟶ B
c : Fork f g := Under.equalizerFork f g
hc : IsLimit c := Under.equalizerForkIsLimit f g
ι : R.mkUnder ↥((toAlgHom f).equalizer (toAlgHom g)) ⟶ A := ((toAlgHom f).equalizer (toAlgHom g)).val.toUnder
h' : (R.tensorProd S).obj (R.mkUnder ↥((toAlgHom f).equalizer (toAlgHom g))) ⟶ (R.tensorProd S).obj A :=
  (R.tensorProd S).map ι
w' : h' ≫ (R.tensorProd S).map f = h' ≫ (R.tensorProd S).map g
e : IsLimit ((R.tensorProd S).mapCone c) ≃ IsLimit (Fork.ofι h' w') := isLimitMapConeForkEquiv (R.tensorProd S) ⋯
heq :
  (Under.equalizerForkIsLimit ((R.tensorProd S).map f) ((R.tensorProd S).map g)).lift (Fork.ofι h' w') =
    (AlgHom.tensorEqualizer (↑S) (↑S) (toAlgHom f) (toAlgHom g)).toUnder ≫
      CategoryTheory.Under.homMk
        (ofHom
          (RingHom.id
            ↥((Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom f)).equalizer
                (Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom g)))))
        ⋯
⊢ Function.Bijective
      ⇑(ConcreteCategory.hom
          ((CategoryTheory.Under.forget S).map
            ((AlgHom.tensorEqualizer (↑S) (↑S) (toAlgHom f) (toAlgHom g)).toUnder ≫
              CategoryTheory.Under.homMk
                (ofHom
                  (RingHom.id
                    ↥((Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom f)).equalizer
                        (Algebra.TensorProduct.map (AlgHom.id ↑S ↑S) (toAlgHom g)))))
                ⋯))) ↔
    Function.Bijective ⇑(AlgHom.tensorEqualizer (↑R) (↑S) (toAlgHom f) (toAlgHom g))