Definiton of a category question

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Discussion Overview

The discussion revolves around the definition of identity arrows in category theory, specifically questioning whether the identity arrow can be considered a point rather than an arrow. Participants explore the implications of this definition and the terminology used in category theory.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that the definition of the identity arrow id_A implies it is a point instead of an arrow, seeking clarification on this visualization.
  • Another participant argues that while categories contain objects and arrows, the term "point" is not typically used in this context, emphasizing that arrows cannot be identified with objects.
  • A third participant challenges the use of the term "relations" to describe arrows, noting that relations are generally subsets of Cartesian products, while arrows do not fit this definition in general.
  • One participant introduces the terminology of objects and morphisms, suggesting that morphisms can be thought of as arrows and objects as boxes, akin to a flow chart.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and conceptualization of identity arrows, with no consensus reached on whether id_A can be considered a point.

Contextual Notes

There are unresolved issues regarding the definitions and terminology used in category theory, particularly the distinction between arrows and objects, as well as the implications of identifying identity arrows with points.

MathematicalPhysicist
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"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
 
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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.
 
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.
 
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.
 

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