The colimit module of a presheaf of modules on a cofiltered category #
Given a colimit cocone cR for a presheaf of rings R on a cofiltered category C,
M a presheaf of modules over R, and a colimit cocone cM for the underlying
functor Cᵒᵖ ⥤ AddCommGrpCat of M, we define a structure of module over cR.pt
on a type-synonym PresheafOfModules.ModuleColimit for cM.pt. This extends to
a functor PresheafOfModules.colimitFunctor : PresheafOfModules R ⥤ ModuleCat cR.pt.
TODO (@joelriou) #
- Define fiber functors on categories of (pre)sheaves of modules
- Refactor
Mathlib/Algebra/Category/ModuleCat/Stalk.leanso that it uses this slightly more general construction.
Given a cocone cR for a functor R : Cᵒᵖ ⥤ RingCat, this is the
functor ModuleCat cR.pt ⥤ PresheafOfModules R which sends a module M
over cR.pt to a presheaf of modules whose underlying presheaf of
abelian groups is the constant functor Cᵒᵖ ⥤ AddCommGrpCat with value M.
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Given a colimit cocone for a presheaf of rings R on a cofiltered category C,
M a presheaf of modules over R, and a colimit cocone cM for the underlying
functor Cᵒᵖ ⥤ AddCommGrpCat of M, this is the type cM.pt on which we define
a module structure below.
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- PresheafOfModules.ModuleColimit x✝¹ x✝ = ↑cM.pt
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The cocone for R ⋙ forget _ ⊗ M.presheaf ⋙ forget _ with point
ModuleColimit hcR hcM which allows to define the scalar multiplication
by cR.pt on ModuleColimit hcR hcM.
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The "inclusion" maps to the colimit ring.
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- PresheafOfModules.ModuleColimit.ιR cR = RingCat.Hom.hom (cR.ι.app U)
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The "inclusion" maps to the colimit module, as an additive map.
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Auxiliary definition for homEquiv. This is the universal property
of PresheafOfModules.ModuleColimit, as an abelian group.
Equations
- PresheafOfModules.ModuleColimit.homEquiv' hcR hcM = { toEquiv := CategoryTheory.ConcreteCategory.homEquiv.symm.trans hcM.homEquiv, map_add' := ⋯ }
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This is the universal property of PresheafOfModules.ModuleColimit as a module.
See also PresheafOfModules.colimitAdjunction.
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The linear map between the colimit modules induced by a morphism of modules.
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The colimit module functor from the category of presheaves of modules
over a presheaf of rings R on a cofiltered category to the category
of modules over a colimit of R.
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Given a presheaf of rings R on a cofiltered category, this is the
adjunction between colimitFunctor : PresheafOfModules R ⥤ ModuleCat cR.pt
and the constant functor.
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