The forgetful functor from (commutative) (additive) monoids preserves filtered colimits. #
Forgetful functors from algebraic categories usually don't preserve colimits. However, they tend to preserve filtered colimits.
In this file, we start with a small filtered category J and a functor F : J ⥤ MonCat.
We then construct a monoid structure on the colimit of F ⋙ forget MonCat (in Type), thereby
showing that the forgetful functor forget MonCat preserves filtered colimits. Similarly for
AddMonCat, CommMonCat and AddCommMonCat.
The colimit of F ⋙ forget MonCat in the category of types.
In the following, we will construct a monoid structure on M.
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The colimit of F ⋙ forget AddMon in the category of types.
In the following, we will construct an additive monoid structure on M.
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The canonical projection into the colimit, as a quotient type.
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- MonCat.FilteredColimits.M.mk F x = (F.comp (CategoryTheory.forget MonCat)).ιColimitType x.fst x.snd
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The canonical projection into the colimit, as a quotient type.
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- AddMonCat.FilteredColimits.M.mk F x = (F.comp (CategoryTheory.forget AddMonCat)).ιColimitType x.fst x.snd
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As J is nonempty, we can pick an arbitrary object j₀ : J. We use this object to define the
"one" in the colimit as the equivalence class of ⟨j₀, 1 : F.obj j₀⟩.
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- MonCat.FilteredColimits.colimitOne F = { one := MonCat.FilteredColimits.M.mk F ⟨⋯.some, 1⟩ }
As J is nonempty, we can pick an arbitrary object j₀ : J. We use this object to
define the "zero" in the colimit as the equivalence class of ⟨j₀, 0 : F.obj j₀⟩.
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- AddMonCat.FilteredColimits.colimitZero F = { zero := AddMonCat.FilteredColimits.M.mk F ⟨⋯.some, 0⟩ }
The definition of the "one" in the colimit is independent of the chosen object of J.
In particular, this lemma allows us to "unfold" the definition of colimit_one at a custom chosen
object j.
The definition of the "zero" in the colimit is independent of the chosen object
of J. In particular, this lemma allows us to "unfold" the definition of colimit_zero at
a custom chosen object j.
The "unlifted" version of multiplication in the colimit. To multiply two dependent pairs
⟨j₁, x⟩ and ⟨j₂, y⟩, we pass to a common successor of j₁ and j₂ (given by IsFiltered.max)
and multiply them there.
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The "unlifted" version of addition in the colimit. To add two dependent pairs
⟨j₁, x⟩ and ⟨j₂, y⟩, we pass to a common successor of j₁ and j₂
(given by IsFiltered.max) and add them there.
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Multiplication in the colimit is well-defined in the left argument.
Addition in the colimit is well-defined in the left argument.
Multiplication in the colimit is well-defined in the right argument.
Addition in the colimit is well-defined in the right argument.
Multiplication in the colimit. See also colimitMulAux.
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- MonCat.FilteredColimits.colimitMul F = { mul := fun (x y : MonCat.FilteredColimits.M F) => Quot.lift₂ (MonCat.FilteredColimits.colimitMulAux F) ⋯ ⋯ x y }
Addition in the colimit. See also colimitAddAux.
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- AddMonCat.FilteredColimits.colimitAdd F = { add := fun (x y : AddMonCat.FilteredColimits.M F) => Quot.lift₂ (AddMonCat.FilteredColimits.colimitAddAux F) ⋯ ⋯ x y }
Multiplication in the colimit is independent of the chosen "maximum" in the filtered category.
In particular, this lemma allows us to "unfold" the definition of the multiplication of x and y,
using a custom object k and morphisms f : x.1 ⟶ k and g : y.1 ⟶ k.
Addition in the colimit is independent of the chosen "maximum" in the filtered
category. In particular, this lemma allows us to "unfold" the definition of the addition of
x and y, using a custom object k and morphisms f : x.1 ⟶ k and g : y.1 ⟶ k.
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- AddMonCat.FilteredColimits.colimitAddMonoid F = { add := Add.add, add_assoc := ⋯, zero := Zero.zero, zero_add := ⋯, add_zero := ⋯, nsmul := nsmulRecAuto, nsmul_zero := ⋯, nsmul_succ := ⋯ }
The bundled monoid giving the filtered colimit of a diagram.
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The bundled additive monoid giving the filtered colimit of a diagram.
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The monoid homomorphism from a given monoid in the diagram to the colimit monoid.
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The additive monoid homomorphism from a given additive monoid in the diagram to the colimit additive monoid.
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The cocone over the proposed colimit monoid.
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- MonCat.FilteredColimits.colimitCocone F = { pt := MonCat.FilteredColimits.colimit F, ι := { app := MonCat.FilteredColimits.coconeMorphism F, naturality := ⋯ } }
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The cocone over the proposed colimit additive monoid.
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- AddMonCat.FilteredColimits.colimitCocone F = { pt := AddMonCat.FilteredColimits.colimit F, ι := { app := AddMonCat.FilteredColimits.coconeMorphism F, naturality := ⋯ } }
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Given a cocone t of F, the induced monoid homomorphism from the colimit to the cocone point.
As a function, this is simply given by the induced map of the corresponding cocone in Type.
The only thing left to see is that it is a monoid homomorphism.
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Given a cocone t of F, the induced additive monoid homomorphism from the colimit
to the cocone point. As a function, this is simply given by the induced map of the
corresponding cocone in Type. The only thing left to see is that it is an additive monoid
homomorphism.
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The proposed colimit cocone is a colimit in MonCat.
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- MonCat.FilteredColimits.colimitCoconeIsColimit F = { desc := MonCat.FilteredColimits.colimitDesc F, fac := ⋯, uniq := ⋯ }
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The proposed colimit cocone is a colimit in AddMonCat.
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- AddMonCat.FilteredColimits.colimitCoconeIsColimit F = { desc := AddMonCat.FilteredColimits.colimitDesc F, fac := ⋯, uniq := ⋯ }
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The colimit of F ⋙ forget₂ CommMonCat MonCat in the category MonCat.
In the following, we will show that this has the structure of a commutative monoid.
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The colimit of F ⋙ forget₂ AddCommMonCat AddMonCat in the category AddMonCat. In the
following, we will show that this has the structure of a commutative additive monoid.
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- CommMonCat.FilteredColimits.colimitCommMonoid F = { toMonoid := (CommMonCat.FilteredColimits.M F).str, mul_comm := ⋯ }
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- AddCommMonCat.FilteredColimits.colimitAddCommMonoid F = { toAddMonoid := (AddCommMonCat.FilteredColimits.M F).str, add_comm := ⋯ }
The bundled commutative monoid giving the filtered colimit of a diagram.
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The bundled additive commutative monoid giving the filtered colimit of a diagram.
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The cocone over the proposed colimit commutative monoid.
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The cocone over the proposed colimit additive commutative monoid.
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- One or more equations did not get rendered due to their size.
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The proposed colimit cocone is a colimit in CommMonCat.
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- One or more equations did not get rendered due to their size.
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The proposed colimit cocone is a colimit in AddCommMonCat.
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- One or more equations did not get rendered due to their size.