In this article we establish a connection between pretabular unitary allegories and bicategories of relations, and also between tabular unitary allegories and regular categories. The material is entirely adapted from Categories, Allegories?; we have merely changed some details of arrangement, notation, and terminology.
Preliminaries
We write the composite of morphisms , as .
An allegory is a -enriched -category where each hom-poset has binary meets, and the modular law is satisfied. The modular law takes two forms, whenever the left sides of the inequalities make sense:
(and of course there are variations, using commutativity of ). Each of these forms can be derived from the other, using the -structure.
The -operation, which we henceforth denote by , preserves meets since it preserves order and is an involution.)
Lemma
For any , we have .
Proof
We have where the middle inequality uses the modular law.
Recall that a map in an allegory is a morphism such that . A relation is well-defined if we merely have a counit inclusion , and is total if we merely have a unit inclusion . Clearly maps are closed under composition, as are total relations and well-defined relations.
Lemma
If is a map, then for any we have .
Proof
The non-trivial inclusion follows from
where the first inequality is an instance of the modular law, and the second holds for any well-defined relation .
Lemma
If are maps and , then .
Since the dagger operation preserves order, we have . But also the inclusion between left adjoints is mated to an inclusion between right adjoints. Hence , and therefore .
Domains and coreflexives
Definition
Let be a morphism. We define the domain to be . A morphism is coreflexive if ; in particular, domains are coreflexive. denotes the poset of coreflexives in .
Lemma
Coreflexives are symmetric and transitive. (Symmetric and transitive imply idempotent.)
Proof
We have where the first inequality is lemma . Of course also . (If is symmetric and transitive, then as well.)
Lemma
For , we have .
Proof
One inclusion is trivial:
The other inclusion follows from fairly tricky applications of the modular law:
Lemma
For and coreflexives , we have if and only if . In particular, .
Proof
If , then
If , then
where the antepenultimate inequality uses the modular law, and the last uses lemma .
Corollary
For and , we have .
By lemma , it suffices that . But this follows from the last sentence of lemma .
Units
Definition
An object is a unit in an allegory if is maximal in and if for every object there is such that (i.e., is total).
Of course by maximality we also have , so such must also be a map.
We say is unital (Freyd-Scedrov say unitary) if has a unit.
Proposition
Let be a unit. Then is an injective order-preserving function.
Proof
Order-preservation is clear. If and , then :
where the first inequality uses lemma , and the last inequality follows from (since is maximal in ). Therefore implies .
Corollary
For a unit and any , there is at most one total relation = map (because in that case , and we apply the previous proposition), and this is maximal in . Thus is terminal in .
Let denote the maximal element of . For any , we then have since this is mated to the inequality . Therefore is the maximal element of .
Tabulations in allegories
Recall from Categories, Allegories that a tabulation of is a pair of maps , such that and .
Preliminaries on tabulations
Lemma
For maps , , the condition implies is a jointly monic pair in .
Proof
Let be maps, and suppose and . If , then
where the second and fourth equations use lemma .
Proposition
Suppose is tabulated by , and suppose , are maps. We have if and only if there exists a map such that and (this is unique by lemma ).
Proof
One direction is easy: if and for some map , then
In the other direction: suppose . Put . First we check that is a map. We have from
using lemma . We have from
where the last step uses one of the tabulation conditions. So is a map.
Finally, we have , which implies (lemma ). Similarly .
Corollary
Tabulations are unique up to unique isomorphism.
Proposition
A diagram in
commutes if and only if .
Proof
An identity inclusion is certainly mated to an inclusion . Conversely, an inclusion is mated to , and we can use lemma .
Corollary
Given and in , a tabulation of provides a pullback of .
Proof
Indeed, by definition of tabulation we then have , so . For the universality of : if , then , and we can apply proposition to finish.
Corollary
If is a monomorphism in and has a tabulation, then .
Proof
The tabulation of gives a pullback of the pair , but since this pullback is already , the conclusion is clear.
Lemma
If is coreflexive, then for any tabulation of , we have and is monic.
Proof
The inclusion is mated to which must be an identity . If is jointly monic, then is monic.
Pretabularity and tabularity
Definition
An allegory is tabular if every morphism admits a tabulation. A unital allegory is pretabular if for all , the maximal morphism (see the last sentence of the Units section) admits a tabulation.
It is immediate from corollary that if is tabular, then has pullbacks.
Likewise, it is immediate from the preceding corollary that if is unital and pretabular, then has products, because we can form the pullback of the maps , to the terminal object .
Lemma
In a tabular allegory , a map is a strong epi in if .
Proof
If , then it is first of all clear that is an epi in because it retracts in . We show is orthogonal to monomorphisms in . That is, consider a commutative diagram
where is monic. We wish to show there exists a filler map such that and . The uniqueness of a filler map is clear since is monic.
Put . We first check that is a map. Notice that the identity inclusion is mated to an inclusion , so we have . In that case we have
where the last inclusion holds because is monic (corollary ). This gives the counit for . For the unit, use
So is a map. We also have
and finally we have
so that by lemma . This completes the proof.
Theorem
If is tabular, then has equalizers. Moreover, every map has a (strong epi)-mono factorization, and strong epis are preserved by pullbacks. In short, is a locally regular category. If is moreover unital, then is regular.
Proof
The equalizer of a pair of maps may be constructed as a tabulation of the coreflexive arrow . By lemma , the tabulation is of the form where is monic, and by an application of proposition , one sees it is the universal map that equalizes and .
For any map , consider a tabulation of the coreflexive by a pair of maps . Notice that is monic in , and . By proposition , there exists a unique such that . Following the proof of proposition , the map is constructed as . We have
i.e., ( is total). By lemma , this means is a strong epi in . Thus we have factored into a strong epi followed by a mono.
Now suppose is any strong epi and is a map, and that is a pullback of . Then is a tabulation of , so . Now the left side of this equation is total, and therefore so is the right: . But then
where the second inequality uses corollary . This means is strong epi, and we are done.
Revised on August 27, 2012 at 20:46:33
by
Todd Trimble