Bicategories of spans in a category #
In this file, given a category C and two morphism properties
Wₗ and Wᵣ in C that are stable under compositions, contain identities and
such that for any morphism b : x₃ ⟶ x₄ in Wₗ and any morphism r : x₂ ⟶ x₃ in Wᵣ,
there exists a pullback square
t
x₁ --> x₂
| |
l | | r
v v
x₃ --> x₄
b
in C such that t satisfies Wₗ and l satisfies Wᵣ,
we construct the bicategory of spans in C with left morphism in Wₗ and right morphism
in Wᵣ.
A (Wₗ, Wᵣ)-span from c to c' is the data of an
object a : C, together with a morphism a ⟶ c in Wₗ,
and a morphism a ⟶ c' in Wᵣ.
Instances For
A morphism of spans is a morphism between the apices compatible with the projections.
the map between the apices
Instances For
Equations
- One or more equations did not get rendered due to their size.
Construct an isomorphism of spans from an isomorphism between the apices that is compatible with the projections.
Equations
Instances For
The identity span, where both legs are identity morphisms.
Equations
- CategoryTheory.Span.id c = { apex := c, l := CategoryTheory.CategoryStruct.id c, r := CategoryTheory.CategoryStruct.id c, wl := ⋯, wr := ⋯ }
Instances For
The composition of two spans: if the relevant pullback exists and if the morphism properties are stable under the relevant base change, it is given by the total span
P
/ \
/ \
X₁ X₂
/ \ / \
c c' c''
where the top diamond is a pullback square
Equations
- One or more equations did not get rendered due to their size.
Instances For
The bicategory of spans of C with left/right legs satisfying given
morphism properties. This is a one-field structure wrapper around C.
- of : C
the underlying object of
Cof a term inSpanBicat C _ _
Instances For
Equations
- One or more equations did not get rendered due to their size.
Equations
- One or more equations did not get rendered due to their size.
Constructor for 1-morphisms in SpanBicat C _ _
Equations
- CategoryTheory.Span.SpanBicat.mkHom l r wl wr = { apex := apex, l := l, r := r, wl := wl, wr := wr }
Instances For
Constructor for 2-morphisms in SpanBicat C _ _
Equations
- CategoryTheory.Span.SpanBicat.mkHom₂ e hₗ hᵣ = { hom := e, hom_l := ⋯, hom_r := ⋯ }
Instances For
Constructor for 2-isomorphisms in SpanBicat C _ _
Equations
Instances For
The goal of this section is to abstract as much as possible the fact that the composition uses an arbitrary pullback, and provides some "proxy" for working with the fact that apices of compositions of spans are pullbacks.
This way, if spans ever get refactored in a way that uses chosen pullbacks instead of arbitrary ones, most downstream applications will not be affected as long as they are careful to use the API provided here.
The primitives of this API are the data of the two projections
πₗ : (S₁ ≫ S₂).apex ⟶ S₁.apex and πᵣ : (S₁ ≫ S₂).apex ⟶ S₂.apex, the
equalities (S₁ ≫ S₂).l = πₗ ≫ S₁.l and (S₁ ≫ S₂).r = πᵣ ≫ S₂.r,
the commutative square πₗ ≫ S₁.r = πᵣ ≫ S₂.l and the fact that this defines a pullback square.
The left projection πₗ : (S₁ ≫ S₂).apex ⟶ S₁.apex.
Equations
Instances For
The right projection πᵣ : (S₁ ≫ S₂).apex ⟶ S₂.apex.
Equations
Instances For
The pullback cone that defines the apex for the composition of spans.
Equations
Instances For
The pullback cone that defines the apex for the composition of spans is a limit cone.
Equations
- One or more equations did not get rendered due to their size.
Instances For
A restatement of the universal property of (S₁ ≫ S₂).apex as coming from a pullback.
This is the main intended way to produce morphisms towards the apex of a composition of spans.
Equations
Instances For
A restatement of the universal property of S₁ ≫ S₂ as coming from a pullback. This is the main intended way to produce morphisms towards a composition of spans.
Equations
- CategoryTheory.Span.SpanBicat.compLift fₗ fᵣ hₗ hₘ hᵣ = { hom := CategoryTheory.Span.SpanBicat.compLiftApex fₗ fᵣ ⋯, hom_l := ⋯, hom_r := ⋯ }
Instances For
The associator isomorphisms for the bicategory structure on spans.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The right unitor for the bicategory structure on spans.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The left unitor for the bicategory structure on spans.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Equations
- One or more equations did not get rendered due to their size.
Extract the isomorphism between the apices from the data of an isomorphism of 1-morphisms
in SpanBicat C _ _.