Product . category theory
Collection
context | F:ObC2 |
range | a,b:Ob2, a≠b |
definition | ⟨Fa×Fb,π⟩:=limF |
Discussion
Elaboration
C2 is the functor category with objects being functors in 2⟶C and the morphisms are natural transformations, i.e. families of 2-indexed arrows in C.
So
π:∏x:Ob2(Fa×Fb)→Fx
I.e. πa:(Fa×Fb)→Fa and πb:(Fa×Fb)→Fb.
Idea
We first discuss the concept in the category of sets.
Say we want to specify the binary product of A and B in C. We can do this in the language of cones, by considering the discrete category with only two objects a,b and no non-identity arrows, and define a functor Fa:=A and F:=B. A cone is any object N with two arrows ψA:N→A and ψB:N→B. If there is a limit cone, let's call its tip A×B, then you can put the two arrows together to define a map u(n):=⟨ψA(n),ψB(n)⟩ from N to A×B, and then ψA(n)=πA(u(n)) and ψB(n)=πB(u(n)).
If the objects of a category are propositions, then the product is ∧ i.e. 'and': from A∧B you can derive A as well as B. The coproduct turns out to be ∨, i.e. 'or': From both A or B, you can derive A∨B.
While the category C might have a billion ways to “look at” A and B, category theory works out that these will always just be some arrow concatenated with the limit cones binoculars - that's sort of the “why” answer to why projection operators are an ubiquitous concept.
Alternative definitions
If C has a terminal object T, the product A×B is the pullback A×TB.
Reference
Wikipedia: Product (category theory)