Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Definiton of a category question

  1. Nov 15, 2003 #1


    User Avatar
    Gold Member

    "for each object A there is an arrow id_A called the identity of A whose source and target are both A."

    wouldnt this definition imply that id_A is a point instead of an arrow?
    if not help me visualise this.

    btw this should be the notation id_A:A->A
  2. jcsd
  3. Nov 15, 2003 #2


    User Avatar
    Science Advisor

    Categories contain objects and "arrows" (I would call them "relations") but I've never heard of a "point" being defined for a general category.

    You may be thinking of the object as a single "point" and then identifying the arrow (from the object to itself) with that "point".

    Remember that in general, given two objects in a category, there may be many arrows from one to another so you cannot identify arrows with objects. In particular, there may be many arrows from a given category to itself. The arrow "id_A" is a specific one of those so you certainly cannot identify "id_A" with the object.
  4. Nov 16, 2003 #3
    i wouldn't call them relations because a relation is generally used in reference to being some subset of the cartesian product of sets. arrows are not sets in general.
  5. Nov 16, 2003 #4


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Objects and morphism is the usual terminology. If it helps, you can think of morphisms as arrows and objects as boxes, as in a flow chart.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook