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Mathlib.CategoryTheory.Endofunctor.Algebra

Algebras of endofunctors #

This file defines (co)algebras of an endofunctor, and provides the category instance for them. It also defines the forgetful functor from the category of (co)algebras. It is shown that the structure map of the initial algebra of an endofunctor is an isomorphism. Furthermore, it is shown that for an adjunction F ⊣ G the category of algebras over F is equivalent to the category of coalgebras over G.

TODO #

  • a : C

    carrier of the algebra

  • str : F.obj s.a s.a

    structure morphism of the algebra

An algebra of an endofunctor; str stands for "structure morphism"

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    A morphism between algebras of endofunctor F

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      To construct an isomorphism of algebras, it suffices to give an isomorphism of the As which commutes with the structure morphisms.

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        The forgetful functor from the category of algebras, forgetting the algebraic structure.

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          An algebra morphism with an underlying isomorphism hom in C is an algebra isomorphism.

          An algebra morphism with an underlying epimorphism hom in C is an algebra epimorphism.

          An algebra morphism with an underlying monomorphism hom in C is an algebra monomorphism.

          From a natural transformation α : G → F we get a functor from algebras of F to algebras of G.

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            If α and β are two equal natural transformations, then the functors of algebras induced by them are isomorphic. We define it like this as opposed to using eq_to_iso so that the components are nicer to prove lemmas about.

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              Naturally isomorphic endofunctors give equivalent categories of algebras. Furthermore, they are equivalent as categories over C, that is, we have equiv_of_nat_iso hforget = forget.

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                The inverse of the structure map of an initial algebra

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                  The structure map of the initial algebra is an isomorphism, hence endofunctors preserve their initial algebras

                  • V : C

                    carrier of the coalgebra

                  • str : s.V F.obj s.V

                    structure morphism of the coalgebra

                  A coalgebra of an endofunctor; str stands for "structure morphism"

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                    A morphism between coalgebras of an endofunctor F

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                      To construct an isomorphism of coalgebras, it suffices to give an isomorphism of the Vs which commutes with the structure morphisms.

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                        The forgetful functor from the category of coalgebras, forgetting the coalgebraic structure.

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                          A coalgebra morphism with an underlying isomorphism hom in C is a coalgebra isomorphism.

                          An algebra morphism with an underlying epimorphism hom in C is an algebra epimorphism.

                          An algebra morphism with an underlying monomorphism hom in C is an algebra monomorphism.

                          From a natural transformation α : F → G we get a functor from coalgebras of F to coalgebras of G.

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                            If α and β are two equal natural transformations, then the functors of coalgebras induced by them are isomorphic. We define it like this as opposed to using eq_to_iso so that the components are nicer to prove lemmas about.

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                              Naturally isomorphic endofunctors give equivalent categories of coalgebras. Furthermore, they are equivalent as categories over C, that is, we have equiv_of_nat_iso hforget = forget.

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                                Given an adjunction F ⊣ G, the functor that associates to an algebra over F a coalgebra over G defined via adjunction applied to the structure map.

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                                  Given an adjunction F ⊣ G, the functor that associates to a coalgebra over G an algebra over F defined via adjunction applied to the structure map.

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                                    Given an adjunction, assigning to an algebra over the left adjoint a coalgebra over its right adjoint and going back is isomorphic to the identity functor.

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                                      Given an adjunction, assigning to a coalgebra over the right adjoint an algebra over the left adjoint and going back is isomorphic to the identity functor.

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                                        If F is left adjoint to G, then the category of algebras over F is equivalent to the category of coalgebras over G.

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