Difference between product and pullback

  • Context: Graduate 
  • Thread starter Thread starter espen180
  • Start date Start date
  • Tags Tags
    Difference Product
Click For Summary
SUMMARY

The discussion clarifies the distinction between the categorical product and the pullback of two morphisms in category theory. It establishes that while both concepts can yield similar diagrams, they are fundamentally different, particularly in their dependence on morphisms. In the category of abelian groups, the nature of the morphisms—whether trivial or injective—affects the resulting pullback, which can either be the trivial group or the regular product of the domains. The product of two objects is defined as the pullback of their respective morphisms to the terminal object.

PREREQUISITES
  • Understanding of category theory concepts, specifically products and pullbacks.
  • Familiarity with morphisms and their roles in categorical definitions.
  • Knowledge of abelian groups and their properties in category theory.
  • Basic comprehension of slice categories and terminal objects.
NEXT STEPS
  • Study the properties of pullbacks in various categories, including abelian groups.
  • Explore the concept of slice categories and their applications in category theory.
  • Investigate the implications of morphism types (trivial vs. injective) on pullbacks.
  • Learn about the relationship between products and pullbacks in different categorical contexts.
USEFUL FOR

Category theorists, mathematicians studying algebraic structures, and students seeking to deepen their understanding of categorical concepts and their applications.

espen180
Messages
831
Reaction score
2
What exactly is the difference between the categorical product of two objects and the fiber product (pullback) of two morphisms with the same two objects as domains? However I look at it, the morphisms in the definition appear superfluous.

Can anyone display a category where products and pullbacks generally are not equal?

Thanks in advance.
 
Physics news on Phys.org
What is your definition of the product of two morphisms?
 
I guess a reasonable definition is to do it in a slice category.
When I work it out, I get the same diagram as with a pullback, but with two extra morphisms, but these are superfluous due to commutativity, so the pullback is the product of two objects in a slice category, or of two mophisms with a shared codomain.

From wiki:
225px-Categorical_pullback_%28expanded%29.svg.png


But it seems that this forces the domain of the pullback to be the product of X and Y with p1 and p2 beign the usual projection morphisms. Is this not the case?

Edit: I was mistaken. Of course, the pullback depends on the morphisms. For example in the category of abelian groups, if the two morphisms are trivial, the pullback is the trivial group, but if they are injective, the pullback is the regular product of the domains, correct?
 
Last edited:
espen180 said:
Edit: I was mistaken. Of course, the pullback depends on the morphisms. For example in the category of abelian groups, if the two morphisms are trivial, the pullback is the trivial group, but if they are injective, the pullback is the regular product of the domains, correct?
No. If the morphisms are trivial (i.e. send everything to the identity) then the pullback is the regular direct product of the domains. If they are injective the pullback varies, depending on what the morphisms are. For example consider the pullback along id:G->G and id:G->G. As a set, it's ##\{(g_1,g_2) \colon g_1=g_2\}##, i.e., it's the diagonal in ##G\times G##.
 
The pullback of f: X → Z and g: Y → Z is their product in the slice category of objects over Z.

The product of X and Y is the pullback of X → 1 and Y → 1, where 1 is the terminal object.
 

Similar threads

  • · Replies 10 ·
Replies
10
Views
1K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 43 ·
2
Replies
43
Views
7K
  • · Replies 1 ·
Replies
1
Views
5K
Replies
5
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 0 ·
Replies
0
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K