there exists a unique arrow such that for all and . Clearly, if this reduces to the usual notion of orthogonality for morphisms.
Let be an (orthogonal) factorization system; we say it extends to a factorization structure for sinks if every sink can be factored as , where and the sink is orthogonal to . Note there is no restriction on the sinks involved to be small.
Some authors write for the collection of sinks orthogonal to , and say that is a factorization structure for sinks, or that the category is an “-category”. However, since is uniquely determined by , and is precisely the 1-ary sinks in , there is little harm in saying that is a factorization structure for sinks.
The dual notion is a factorization structure for cosinks (“sources”).
Observe that if is large, then the collection contains proper classes as elements. Therefore, in some foundations such as ZF, it is not definable as a single thing. In NBG one may treat it as a “hyperclass” defined by a first-order formula, in the same way that one treats classes in ZF. If we use a Grothendieck universe to define smallness, of course, there is no problem.
Many well-known factorization systems, and ways to construct factorization systems, extend to factorization structures for sinks and/or cosinks.
This theorem implies that if a category is M-complete for a class of morphisms which consists of monomorphisms and is closed under composition and pullback, then is the right class in a factorization structure for sinks.
If is a Grothendieck fibration, then factorization structures for sinks can be lifted from to . Dually, if is an opfibration, we can lift factorization structures for cosinks. For the “mismatched” types of lifting, we require more: must be a topological functor.
In Set, the factorization system (epi, mono) extends to both a factorization structure for sinks and one for cosinks. The epi-sinks are those that are jointly epimorphic, and the mono-sinks are those that are jointly monomorphic.
By lifting (epi,mono) to Top (or any other topological category over ), we obtain two factorization structures for sinks: (jointly surjective, subspace inclusions) and (final topologies, injections). Dually, we have two factorization structures for cosinks: (surjections, initial topologies) and (quotient maps, jointly injective).
Recall that a factorization system is called proper if and . In the case of a factorization structure for sinks, the second of these is automatic.
If is a factorization structure for sinks, then consists of monomorphisms.
Let be in , and suppose that for some , but . Consider the sink consisting of one copy of () for each arrow of the ambient category , and factor this sink as an -sink followed by an -morphism .
Now there are at least different sinks such that , since we may take each to be either or . Therefore, by orthogonality, there are at least different morphisms , a contradiction to Cantor's theorem, since there can be at most .
This proof is of course quite reminiscent of Freyd’s theorem that any complete small category is a preorder. In fact, Freyd’s theorem is a consequence of this one (or at least of its proof). For given a complete small category, there is a factorization structure acting at least on small sinks, where is the class of all morphisms and the class of families of injections into coproducts. (Any complete small category is also cocomplete, by the adjoint functor theorem.) Therefore, all morphisms in a complete small category are monic, including the unique maps to the terminal object; hence the category is a preorder.