Pullback (category theory)
In category theory, a branch of mathematics, a pullback (also called a fiber product, fibre product, fibered product or Cartesian square) is the limit of a diagram consisting of two morphisms f : X → Z and g : Y → Z with a common codomain; it is the limit of the cospan . The pullback is often written
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 required to be unique up to isomorphism. It follows that this isomorphism is unique.
The pullback is similar to the product, but not the same. One may obtain the product by "forgetting" that the morphisms f and g exist, and forgetting that the object Z exists. One is then left with a discrete category containing only the two objects X and Y, and no arrows between them. This discrete category may be used as the index set to construct the ordinary binary product. Thus, the pullback can be thought of as the ordinary (Cartesian) product, but with additional structure.
1. In the category of commutative rings (with identity), denoted CRing, the pullback is called the fibered product. Let and , that is, A, B, and C are commutative rings with identity and and are ring homomorphisms. Then the pullback of these objects and morphisms is defined to be the subset of the Cartesian product defined by
along with the morphisms and such that .
2. In the category of sets, a pullback of f and g is given by the set
together with the restrictions of the projection maps and to X ×Z Y .
Alternatively one may view the pullback in Set asymmetrically:
where is the disjoint (tagged) union of sets (the involved sets are not disjoint on their own unless f resp. g is injective). In the first case, the projection extracts the x index while forgets the index, leaving elements of Y.
- This example motivates another way of characterizing the pullback: as the equalizer of the morphisms f o p1, g o p2 : X × Y → Z where X × Y is the binary product of X and Y and p1 and p2 are the natural projections. This shows that pullbacks exist in any category with binary products and equalizers. In fact, by the existence theorem for limits, all finite limits exist in a category with a terminal object, binary products and equalizers.
3. Another example of a pullback comes from the theory of fiber bundles: given a bundle map π : E → B and a continuous map f : X → B, the pullback X ×B E is a fiber bundle over X called the pullback bundle. The associated commutative diagram is a morphism of fiber bundles.
Preimages of sets under functions can be described as pullbacks as follows: Suppose
- f : A → B
- B0 ⊆ B.
Let g be the inclusion map B0 ↪ B.
Then a pullback of f and g (in Set) is given by the preimage f-1 [ B0 ] together with the inclusion of the preimage in A
- f-1 [ B0 ] ↪ A
and the restriction of f to f-1 [ B0 ]
- f-1 [ B0 ] → B0.
- Whenever X ×Z Y exists, then so does Y ×Z X and there is an isomorphism X ×Z Y Y ×Z X.
- Monomorphisms are stable under pullback: if the arrow f above is monic, then so is the arrow p2. For example, in the category of sets, if X is a subset of Z, then, for any g : Y → Z, the pullback X ×Z Y is the inverse image of X under g.
- Isomorphisms are also stable, and hence, for example, X ×X Y Y for any map Y → X.
- Any category with pullbacks and products has equalizers.
- Adámek, p. 197.
- Adámek, Jiří, Herrlich, Horst, & Strecker, George E.; (1990). Abstract and Concrete Categories (4.2MB PDF). Originally publ. John Wiley & Sons. ISBN 0-471-60922-6. (now free on-line edition).
- Cohn, Paul M.; Universal Algebra (1981), D.Reidel Publishing, Holland, ISBN 90-277-1213-1 (Originally published in 1965, by Harper & Row).