Universal Property
Explicitly, the pullback of the morphisms f and g consists of an object P and two morphisms p1 : P → X and p2 : P → Y for which the diagram
commutes. Moreover, the pullback (P, p1, p2) must be universal with respect to this diagram. That is, for any other such triple (Q, q1, q2) for which the following diagram commutes, there must exist a unique u : Q → P (called a mediating morphism) such that and
As with all universal constructions, the pullback, if it exists, is unique up to a unique isomorphism.
Read more about this topic: Pullback (category Theory)
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