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Mathlib.CategoryTheory.Localization.SmallShiftedHom

Shrinking morphisms in localized categories equipped with shifts #

If C is a category equipped with a shift by an additive monoid M, and W : MorphismProperty C is compatible with the shift, we define a type-class HasSmallLocalizedShiftedHom.{w} W X Y which says that all the types of morphisms from X⟦a⟧ to Y⟦b⟧ in the localized category are w-small for a certain universe. Then, we define types SmallShiftedHom.{w} W X Y m : Type w for all m : M, and endow these with a composition which transports the composition on the types ShiftedHom (L.obj X) (L.obj Y) m when L : C ⥤ D is any localization functor for W.

@[reducible, inline]

Given objects X and Y in a category C, this is the property that all the types of morphisms from X⟦a⟧ to Y⟦b⟧ are w-small in the localized category with respect to a class of morphisms W.

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    Given f : SmallHom W X Y and a : M, this is the element in SmallHom W (X⟦a⟧) (Y⟦a⟧) obtained by shifting by a.

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    • f.shift a = (W.shiftLocalizerMorphism a).smallHomMap f
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      The type of morphisms from X to Y⟦m⟧ in the localized category with respect to W : MorphismProperty C that is shrunk to Type w when HasSmallLocalizedShiftedHom.{w} W X Y holds.

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        Given f : SmallShiftedHom.{w} W X Y a, this is the element in SmallHom.{w} W (X⟦n⟧) (Y⟦a'⟧) that is obtained by shifting by n when a + n = a'.

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          The canonical map (X ⟶ Y) → SmallShiftedHom.{w} W X Y m₀ when m₀ = 0 and [HasSmallLocalizedShiftedHom.{w} W M X Y] holds.

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            The bijection SmallShiftedHom.{w} W X Y m ≃ ShiftedHom (L.obj X) (L.obj Y) m for all m : M, and X and Y in C when L : C ⥤ D is a localization functor for W : MorphismProperty C such that the category D is equipped with a shift by M and L commutes with the shifts.

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