Processing math: 100%

Product . category theory

Collection

context F:ObC2
range a,b:Ob2, ab
definition Fa×Fb,π:=limF

Discussion

Elaboration

C2 is the functor category with objects being functors in 2C 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:NA and ψB:NB. 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 AB 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 AB.

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

Parents

Context

Refinement of

Link to graph
Log In
Improvements of the human condition