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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The trivial representation of a group.
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- hom : M.V ⟶ N.V
- comm : ∀ (g : ↑G), CategoryTheory.CategoryStruct.comp (↑M.ρ g) s.hom = CategoryTheory.CategoryStruct.comp s.hom (↑N.ρ g)
A homomorphism of Action V G
s is a morphism between the underlying objects,
commuting with the action of G
.
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The identity morphism on an Action V G
.
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The composition of two Action V G
homomorphisms is the composition of the underlying maps.
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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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Auxilliary definition for functorCategoryEquivalence
.
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Auxilliary definition for functorCategoryEquivalence
.
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Auxilliary definition for functorCategoryEquivalence
.
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Auxilliary 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.
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The forgetful functor is intertwined by functorCategoryEquivalence
with
evaluation at PUnit.star
.
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Auxilliary construction for the Abelian (Action V G)
instance.
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Given an object X
isomorphic to the tensor unit of V
, X
equipped with the trivial action
is isomorphic to the tensor unit of Action V G
.
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When V
is monoidal the forgetful functor Action V G
to V
is monoidal.
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When V
is braided the forgetful functor Action V G
to V
is braided.
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Upgrading the functor Action V G ⥤ (SingleObj G ⥤ V)
to a monoidal functor.
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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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The natural isomorphism from the composition of restrictions along homomorphisms to the restriction along the composition of homomorphism.
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Given a family F
of types with G
-actions, this is the limit cone demonstrating that the
product of F
as types is a product in the category of G
-sets.
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We have fin 1 → G ≅ G
as G
-sets, with G
acting by left multiplication.
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Given X : Action (Type u) (Mon.of G)
for G
a group, then G × X
(with G
acting as left
multiplication on the first factor and by X.ρ
on the second) is isomorphic as a G
-set to
G × X
(with G
acting as left multiplication on the first factor and trivially on the second).
The isomorphism is given by (g, x) ↦ (g, g⁻¹ • x)
.
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The natural isomorphism of G
-sets Gⁿ⁺¹ ≅ G × Gⁿ
, where G
acts by left multiplication on
each factor.
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A functor between categories induces a functor between
the categories of G
-actions within those categories.
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A monoidal functor induces a monoidal functor between
the categories of G
-actions within those categories.