Calculus of fractions #
Following the definitions by [Gabriel and Zisman][gabriel-zisman-1967],
given a morphism property W : MorphismProperty C on a category C,
we introduce the class W.HasLeftCalculusOfFractions. The main
result Localization.exists_leftFraction is that if L : C ⥤ D
is a localization functor for W, then for any morphism L.obj X ⟶ L.obj Y in D,
there exists an auxiliary object Y' : C and morphisms g : X ⟶ Y' and s : Y ⟶ Y',
with W s, such that the given morphism is a sort of fraction g / s,
or more precisely of the form L.map g ≫ (Localization.isoOfHom L W s hs).inv.
We also show that the functor L.mapArrow : Arrow C ⥤ Arrow D is essentially surjective.
Similar results are obtained when W has a right calculus of fractions.
References #
- [P. Gabriel, M. Zisman, Calculus of fractions and homotopy theory][gabriel-zisman-1967]
A left fraction from X : C to Y : C for W : MorphismProperty C consists of the
datum of an object Y' : C and maps f : X ⟶ Y' and s : Y ⟶ Y' such that W s.
- Y' : C
the auxiliary object of a left fraction
the numerator of a left fraction
the denominator of a left fraction
- hs : W self.s
the condition that the denominator belongs to the given morphism property
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The left fraction from X to Y given by a morphism f : X ⟶ Y.
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The left fraction from X to Y given by a morphism s : Y ⟶ X such that W s.
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If φ : W.LeftFraction X Y and L is a functor which inverts W, this is the
induced morphism L.obj X ⟶ L.obj Y
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A right fraction from X : C to Y : C for W : MorphismProperty C consists of the
datum of an object X' : C and maps s : X' ⟶ X and f : X' ⟶ Y such that W s.
- X' : C
the auxiliary object of a right fraction
the denominator of a right fraction
- hs : W self.s
the condition that the denominator belongs to the given morphism property
the numerator of a right fraction
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The right fraction from X to Y given by a morphism f : X ⟶ Y.
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The right fraction from X to Y given by a morphism s : Y ⟶ X such that W s.
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If φ : W.RightFraction X Y and L is a functor which inverts W, this is the
induced morphism L.obj X ⟶ L.obj Y
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A multiplicative morphism property W has left calculus of fractions if
any right fraction can be turned into a left fraction and that two morphisms
that can be equalized by precomposition with a morphism in W can also
be equalized by postcomposition with a morphism in W.
- exists_leftFraction ⦃X Y : C⦄ (φ : W.RightFraction X Y) : ∃ (ψ : W.LeftFraction X Y), CategoryStruct.comp φ.f ψ.s = CategoryStruct.comp φ.s ψ.f
- ext ⦃X' X Y : C⦄ (f₁ f₂ : X ⟶ Y) (s : X' ⟶ X) : W s → CategoryStruct.comp s f₁ = CategoryStruct.comp s f₂ → ∃ (Y' : C) (t : Y ⟶ Y') (_ : W t), CategoryStruct.comp f₁ t = CategoryStruct.comp f₂ t
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A multiplicative morphism property W has right calculus of fractions if
any left fraction can be turned into a right fraction and that two morphisms
that can be equalized by postcomposition with a morphism in W can also
be equalized by precomposition with a morphism in W.
- exists_rightFraction ⦃X Y : C⦄ (φ : W.LeftFraction X Y) : ∃ (ψ : W.RightFraction X Y), CategoryStruct.comp ψ.s φ.f = CategoryStruct.comp ψ.f φ.s
- ext ⦃X Y Y' : C⦄ (f₁ f₂ : X ⟶ Y) (s : Y ⟶ Y') : W s → CategoryStruct.comp f₁ s = CategoryStruct.comp f₂ s → ∃ (X' : C) (t : X' ⟶ X) (_ : W t), CategoryStruct.comp t f₁ = CategoryStruct.comp t f₂
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A choice of a left fraction deduced from a right fraction for a morphism property W
when W has left calculus of fractions.
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A choice of a right fraction deduced from a left fraction for a morphism property W
when W has right calculus of fractions.
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The equivalence relation on left fractions for a morphism property W.
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Auxiliary definition for the composition of left fractions.
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The equivalence class of z₁.comp₀ z₂ z₃ does not depend on the choice of z₃ provided
they satisfy the compatibility z₂.f ≫ z₃.s = z₁.s ≫ z₃.f.
The morphisms in the constructed localized category for a morphism property W
that has left calculus of fractions are equivalence classes of left fractions.
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The morphism in the constructed localized category that is induced by a left fraction.
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Auxiliary definition towards the definition of the composition of morphisms in the constructed localized category for a morphism property that has left calculus of fractions.
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Composition of morphisms in the constructed localized category for a morphism property that has left calculus of fractions.
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The constructed localized category for a morphism property that has left calculus of fractions.
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The localization functor to the constructed localized category for a morphism property that has left calculus of fractions.
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The morphism on Localization W that is induced by a left fraction.
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The morphism in Localization W that is the formal inverse of a morphism
which belongs to W.
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The isomorphism in Localization W that is induced by a morphism in W.
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The image by a functor which inverts W of an equivalence class of left fractions.
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The functor Localization W ⥤ E that is induced by a functor C ⥤ E which inverts W,
when W has a left calculus of fractions.
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The universal property of the localization for the constructed localized category when there is a left calculus of fractions.
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The right fraction in the opposite category corresponding to a left fraction.
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The left fraction in the opposite category corresponding to a right fraction.
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The right fraction corresponding to a left fraction in the opposite category.
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The left fraction corresponding to a right fraction in the opposite category.
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The equivalence relation on right fractions for a morphism property W.