Mark Gritter (markgritter) wrote,
Mark Gritter

The Puppy Functor

We introduce a finite category Pup. A standard representation is the objects {inside, outside} and in addition to the identities there are two morphisms that form a bijection:

door: inside -> outside
door -1: outside -> inside

The literature conventionally refers to these operations as "let the dog out" and "let the dog in", respectively.

A "puppy functor" P is defined as follows, and is applicable only to categories in which products exist:

P(S) = S x ( inside or outside )

P( 1S : S->S ) = f x 1

P(f : S->T ) = f x ( door or door-1 ) for all other morphisms

In the resulting category, any action must be accompanied by letting the dog outside or letting the dog back inside, as no function maps to the inside or outside identity. Further, any bijective morphism involves letting the dog out and then back in again (or the reverse), because

P(1) = P( ff -1 ) = P( f )P( f -1 )

Categories with endomorphisms (other than the identity, of course) cannot have a puppy functor. In fact, the only categories which have a puppy functor are those whose graphs are bipartite. The practical aspect of this restriction implies that dog owners cannot expect to perform any third task between letting the dog out and letting the dog back in.
Tags: category theory, geek, mathematics

  • Predictions for 2018

    Tintri asked me to put together some technology predictions for 2018. You can access the "on-demand webinar" here:…

  • Back from Tintri sales kickoff

    Tintri has historically held its sales kickoff in the Bay Area to be close to headquaters. This year we went to Las Vegas instead, at the Hard Rock…

  • Retiring another interview question

    I've been talking to people who have built cloud services that communicate with on-premises equipment (like storage arrays or routers.) One of my…

  • Post a new comment


    default userpic

    Your reply will be screened

    Your IP address will be recorded 

    When you submit the form an invisible reCAPTCHA check will be performed.
    You must follow the Privacy Policy and Google Terms of use.