In mathematics, it can be shown that every function can be written as the composite of a surjective function followed by an injective function. Factorization systems are a generalization of this situation in category theory.

Definition

A factorization system (E, M) for a category C consists of two classes of morphisms E and M of C such that:

  1. E and M both contain all isomorphisms of C and are closed under composition.
  2. Every morphism f of C can be factored as for some morphisms and .
  3. The factorization is functorial: if and are two morphisms such that for some morphisms and , then there exists a unique morphism making the following diagram commute:


Remark: is a morphism from to in the arrow category.

Orthogonality

Two morphisms and are said to be orthogonal, denoted , if for every pair of morphisms and such that there is a unique morphism such that the diagram

commutes. This notion can be extended to define the orthogonals of sets of morphisms by

and

Since in a factorization system contains all the isomorphisms, the condition (3) of the definition is equivalent to

(3') and


Proof: In the previous diagram (3), take (identity on the appropriate object) and .

Equivalent definition

The pair of classes of morphisms of C is a factorization system if and only if it satisfies the following conditions:

  1. Every morphism f of C can be factored as with and
  2. and

Weak factorization systems

Suppose e and m are two morphisms in a category C. Then e has the left lifting property with respect to m (respectively m has the right lifting property with respect to e) when for every pair of morphisms u and v such that ve = mu there is a morphism w such that the following diagram commutes. The difference with orthogonality is that w is not necessarily unique.

A weak factorization system (E, M) for a category C consists of two classes of morphisms E and M of C such that:[1]

  1. The class E is exactly the class of morphisms having the left lifting property with respect to each morphism in M.
  2. The class M is exactly the class of morphisms having the right lifting property with respect to each morphism in E.
  3. Every morphism f of C can be factored as for some morphisms and .

This notion leads to a succinct definition of model categories: a model category is a pair consisting of a category C and classes of (so-called) weak equivalences W, fibrations F and cofibrations C so that

  • C has all limits and colimits,
  • is a weak factorization system,
  • is a weak factorization system, and
  • satisfies the two-out-of-three property: if and are composable morphisms and two of are in , then so is the third.[2]

A model category is a complete and cocomplete category equipped with a model structure. A map is called a trivial fibration if it belongs to and it is called a trivial cofibration if it belongs to An object is called fibrant if the morphism to the terminal object is a fibration, and it is called cofibrant if the morphism from the initial object is a cofibration.[3]

References

  1. Riehl (2014, §11.2)
  2. Riehl (2014, §11.3)
  3. Valery Isaev - On fibrant objects in model categories.
  • Peter Freyd, Max Kelly (1972). "Categories of Continuous Functors I". Journal of Pure and Applied Algebra. 2.
  • Riehl, Emily (2014), Categorical homotopy theory, Cambridge University Press, doi:10.1017/CBO9781107261457, ISBN 978-1-107-04845-4, MR 3221774
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