Action V G
, the category of actions of a monoid G
inside some category V
. #
The prototypical example is V = ModuleCat R
,
where Action (ModuleCat R) G
is the category of R
-linear representations of G
.
We check Action V G ≌ (singleObj G ⥤ V)
,
and construct the restriction functors res {G H : Mon} (f : G ⟶ H) : Action V H ⥤ Action V G
.
When a group acts, we can lift the action to the group of automorphisms.
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Instances For
Equations
- Action.inhabited' G = { default := { V := PUnit.{u + 1} , ρ := 1 } }
The trivial representation of a group.
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- Action.trivial G = { V := AddCommGroupCat.of PUnit.{u + 1} , ρ := 1 }
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A homomorphism of Action V G
s is a morphism between the underlying objects,
commuting with the action of G
.
- hom : M.V ⟶ N.V
- comm : ∀ (g : ↑G), CategoryTheory.CategoryStruct.comp (M.ρ g) self.hom = CategoryTheory.CategoryStruct.comp self.hom (N.ρ g)
Instances For
The identity morphism on an Action V G
.
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Instances For
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- Action.Hom.instInhabitedHom M = { default := Action.Hom.id M }
The composition of two Action V G
homomorphisms is the composition of the underlying maps.
Equations
- Action.Hom.comp p q = Action.Hom.mk (CategoryTheory.CategoryStruct.comp p.hom q.hom)
Instances For
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- Action.instCategoryAction = CategoryTheory.Category.mk
Construct an isomorphism of G
actions/representations
from an isomorphism of the underlying objects,
where the forward direction commutes with the group action.
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- Action.mkIso f = CategoryTheory.Iso.mk (Action.Hom.mk f.hom) (Action.Hom.mk f.inv)
Instances For
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- (_ : CategoryTheory.IsIso f) = (_ : CategoryTheory.IsIso (Action.mkIso (CategoryTheory.asIso f.hom)).hom)
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- (_ : CategoryTheory.IsIso (Action.Hom.mk f)) = (_ : CategoryTheory.IsIso (Action.mkIso (CategoryTheory.asIso f)).hom)
Auxiliary definition for functorCategoryEquivalence
.
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Auxiliary definition for functorCategoryEquivalence
.
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Auxiliary definition for functorCategoryEquivalence
.
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Auxiliary definition for functorCategoryEquivalence
.
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The category of actions of G
in the category V
is equivalent to the functor category singleObj G ⥤ V
.
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(implementation) The forgetful functor from bundled actions to the underlying objects.
Use the CategoryTheory.forget
API provided by the ConcreteCategory
instance below,
rather than using this directly.
Equations
- Action.forget V G = CategoryTheory.Functor.mk { obj := fun (M : Action V G) => M.V, map := fun {X Y : Action V G} (f : X ⟶ Y) => f.hom }
Instances For
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- (_ : CategoryTheory.Faithful (Action.forget V G)) = (_ : CategoryTheory.Faithful (Action.forget V G))
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- Action.instConcreteCategoryActionInstCategoryAction V G = CategoryTheory.ConcreteCategory.mk (CategoryTheory.Functor.comp (Action.forget V G) CategoryTheory.ConcreteCategory.forget)
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The forgetful functor is intertwined by functorCategoryEquivalence
with
evaluation at PUnit.star
.
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Actions/representations of the trivial group are just objects in the ambient category.
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The "restriction" functor along a monoid homomorphism f : G ⟶ H
,
taking actions of H
to actions of G
.
(This makes sense for any homomorphism, but the name is natural when f
is a monomorphism.)
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The natural isomorphism from restriction along the identity homomorphism to
the identity functor on Action V G
.
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- Action.resId V = CategoryTheory.NatIso.ofComponents fun (M : Action V G) => Action.mkIso (CategoryTheory.Iso.refl ((Action.res V (CategoryTheory.CategoryStruct.id G)).toPrefunctor.obj M).V)
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The natural isomorphism from the composition of restrictions along homomorphisms to the restriction along the composition of homomorphism.
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A functor between categories induces a functor between
the categories of G
-actions within those categories.
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