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Mathlib.CategoryTheory.Monoidal.End

Endofunctors as a monoidal category. #

We give the monoidal category structure on C ⥤ C, and show that when C itself is monoidal, it embeds via a monoidal functor into C ⥤ C.

TODO #

Can we use this to show coherence results, e.g. a cheap proof that λ_ (𝟙_ C) = ρ_ (𝟙_ C)? I suspect this is harder than is usually made out.

The category of endofunctors of any category is a monoidal category, with tensor product given by composition of functors (and horizontal composition of natural transformations).

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@[simp]
theorem CategoryTheory.endofunctorMonoidalCategory_tensorMap_app (C : Type u) [Category.{v, u} C] {F G H K : Functor C C} {α : F G} {β : H K} (X : C) :

Tensoring on the right gives a monoidal functor from C into endofunctors of C.

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  • One or more equations did not get rendered due to their size.
theorem CategoryTheory.μ_naturality₂ {C : Type u} [Category.{v, u} C] {M : Type u_1} [Category.{u_2, u_1} M] [MonoidalCategory M] (F : Functor M (Functor C C)) {m n m' n' : M} (f : m m') (g : n n') (X : C) [F.LaxMonoidal] :
noncomputable def CategoryTheory.unitOfTensorIsoUnit {C : Type u} [Category.{v, u} C] {M : Type u_1} [Category.{u_2, u_1} M] [MonoidalCategory M] (F : Functor M (Functor C C)) (m n : M) (h : MonoidalCategoryStruct.tensorObj m n 𝟙_ M) [F.Monoidal] :

If m ⊗ n ≅ 𝟙_M, then F.obj m is a left inverse of F.obj n.

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If m ⊗ n ≅ 𝟙_M and n ⊗ m ≅ 𝟙_M (subject to some commuting constraints), then F.obj m and F.obj n forms a self-equivalence of C.

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  • One or more equations did not get rendered due to their size.