Binary (co)products #
We define a category WalkingPair
, which is the index category
for a binary (co)product diagram. A convenience method pair X Y
constructs the functor from the walking pair, hitting the given objects.
We define prod X Y
and coprod X Y
as limits and colimits of such functors.
Typeclasses HasBinaryProducts
and HasBinaryCoproducts
assert the existence
of (co)limits shaped as walking pairs.
We include lemmas for simplifying equations involving projections and coprojections, and define braiding and associating isomorphisms, and the product comparison morphism.
References #
The type of objects for the diagram indexing a binary (co)product.
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An equivalence from WalkingPair
to Bool
, sometimes useful when reindexing limits.
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The function on the walking pair, sending the two points to X
and Y
.
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The diagram on the walking pair, sending the two points to X
and Y
.
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The natural transformation between two functors out of the walking pair, specified by its components.
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The natural isomorphism between two functors out of the walking pair, specified by its components.
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Every functor out of the walking pair is naturally isomorphic (actually, equal) to a pair
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The natural isomorphism between pair X Y ⋙ F
and pair (F.obj X) (F.obj Y)
.
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A binary fan is just a cone on a diagram indexing a product.
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The first projection of a binary fan.
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The second projection of a binary fan.
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A convenient way to show that a binary fan is a limit.
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A binary cofan is just a cocone on a diagram indexing a coproduct.
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The first inclusion of a binary cofan.
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The second inclusion of a binary cofan.
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A convenient way to show that a binary cofan is a colimit.
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A binary fan with vertex P
consists of the two projections π₁ : P ⟶ X
and π₂ : P ⟶ Y
.
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A binary cofan with vertex P
consists of the two inclusions ι₁ : X ⟶ P
and ι₂ : Y ⟶ P
.
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Every BinaryFan
is isomorphic to an application of BinaryFan.mk
.
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Every BinaryFan
is isomorphic to an application of BinaryFan.mk
.
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This is a more convenient formulation to show that a BinaryFan
constructed using
BinaryFan.mk
is a limit cone.
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This is a more convenient formulation to show that a BinaryCofan
constructed using
BinaryCofan.mk
is a colimit cocone.
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If s
is a limit binary fan over X
and Y
, then every pair of morphisms f : W ⟶ X
and
g : W ⟶ Y
induces a morphism l : W ⟶ s.pt
satisfying l ≫ s.fst = f
and l ≫ s.snd = g
.
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If s
is a colimit binary cofan over X
and Y
,, then every pair of morphisms f : X ⟶ W
and
g : Y ⟶ W
induces a morphism l : s.pt ⟶ W
satisfying s.inl ≫ l = f
and s.inr ≫ l = g
.
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Binary products are symmetric.
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If X' ≅ X
, then X × Y
also is the product of X'
and Y
.
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If Y' ≅ Y
, then X x Y
also is the product of X
and Y'
.
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Binary coproducts are symmetric.
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If X' ≅ X
, then X ⨿ Y
also is the coproduct of X'
and Y
.
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If Y' ≅ Y
, then X ⨿ Y
also is the coproduct of X
and Y'
.
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An abbreviation for HasLimit (pair X Y)
.
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An abbreviation for HasColimit (pair X Y)
.
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If we have a product of X
and Y
, we can access it using prod X Y
or
X ⨯ Y
.
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If we have a coproduct of X
and Y
, we can access it using coprod X Y
or
X ⨿ Y
.
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Notation for the product
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Notation for the coproduct
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The projection map to the first component of the product.
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The projection map to the second component of the product.
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The inclusion map from the first component of the coproduct.
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The inclusion map from the second component of the coproduct.
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The binary fan constructed from the projection maps is a limit.
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The binary cofan constructed from the coprojection maps is a colimit.
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If the product of X
and Y
exists, then every pair of morphisms f : W ⟶ X
and g : W ⟶ Y
induces a morphism prod.lift f g : W ⟶ X ⨯ Y
.
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diagonal arrow of the binary product in the category fam I
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If the coproduct of X
and Y
exists, then every pair of morphisms f : X ⟶ W
and
g : Y ⟶ W
induces a morphism coprod.desc f g : X ⨿ Y ⟶ W
.
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codiagonal arrow of the binary coproduct
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If the product of X
and Y
exists, then every pair of morphisms f : W ⟶ X
and g : W ⟶ Y
induces a morphism l : W ⟶ X ⨯ Y
satisfying l ≫ Prod.fst = f
and l ≫ Prod.snd = g
.
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If the coproduct of X
and Y
exists, then every pair of morphisms f : X ⟶ W
and
g : Y ⟶ W
induces a morphism l : X ⨿ Y ⟶ W
satisfying coprod.inl ≫ l = f
and
coprod.inr ≫ l = g
.
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If the products W ⨯ X
and Y ⨯ Z
exist, then every pair of morphisms f : W ⟶ Y
and
g : X ⟶ Z
induces a morphism prod.map f g : W ⨯ X ⟶ Y ⨯ Z
.
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If the coproducts W ⨿ X
and Y ⨿ Z
exist, then every pair of morphisms f : W ⟶ Y
and
g : W ⟶ Z
induces a morphism coprod.map f g : W ⨿ X ⟶ Y ⨿ Z
.
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If the products W ⨯ X
and Y ⨯ Z
exist, then every pair of isomorphisms f : W ≅ Y
and
g : X ≅ Z
induces an isomorphism prod.mapIso f g : W ⨯ X ≅ Y ⨯ Z
.
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If the coproducts W ⨿ X
and Y ⨿ Z
exist, then every pair of isomorphisms f : W ≅ Y
and
g : W ≅ Z
induces an isomorphism coprod.mapIso f g : W ⨿ X ≅ Y ⨿ Z
.
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HasBinaryProducts
represents a choice of product for every pair of objects.
See https://stacks.math.columbia.edu/tag/001T.
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HasBinaryCoproducts
represents a choice of coproduct for every pair of objects.
See https://stacks.math.columbia.edu/tag/04AP.
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If C
has all limits of diagrams pair X Y
, then it has all binary products
If C
has all colimits of diagrams pair X Y
, then it has all binary coproducts
The braiding isomorphism which swaps a binary product.
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The braiding isomorphism can be passed through a map by swapping the order.
The braiding isomorphism is symmetric.
The associator isomorphism for binary products.
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The left unitor isomorphism for binary products with the terminal object.
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The right unitor isomorphism for binary products with the terminal object.
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The braiding isomorphism which swaps a binary coproduct.
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The braiding isomorphism is symmetric.
The associator isomorphism for binary coproducts.
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The left unitor isomorphism for binary coproducts with the initial object.
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The right unitor isomorphism for binary coproducts with the initial object.
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The binary product functor.
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The product functor can be decomposed.
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The binary coproduct functor.
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The coproduct functor can be decomposed.
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The product comparison morphism.
In CategoryTheory/Limits/Preserves
we show this is always an iso iff F preserves binary products.
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Naturality of the prodComparison
morphism in both arguments.
The product comparison morphism from F(A ⨯ -)
to FA ⨯ F-
, whose components are given by
prodComparison
.
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If the product comparison morphism is an iso, its inverse is natural.
The natural isomorphism F(A ⨯ -) ≅ FA ⨯ F-
, provided each prodComparison F A B
is an
isomorphism (as B
changes).
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The coproduct comparison morphism.
In CategoryTheory/Limits/Preserves
we show
this is always an iso iff F preserves binary coproducts.
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Naturality of the coprod_comparison morphism in both arguments.
The coproduct comparison morphism from FA ⨿ F-
to F(A ⨿ -)
, whose components are given by
coprodComparison
.
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If the coproduct comparison morphism is an iso, its inverse is natural.
The natural isomorphism FA ⨿ F- ≅ F(A ⨿ -)
, provided each coprodComparison F A B
is an
isomorphism (as B
changes).
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Auxiliary definition for Over.coprod
.
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A category with binary coproducts has a functorial sup
operation on over categories.