Centre for Mathematical Structures! Exploring Mathematical Structures across Mathematics, Computer Science, Physics, Biology, and other disciplines!

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1 ! Centre for Mathematical Structures! Exploring Mathematical Structures across Mathematics, Computer Science, Physics, Biology, and other disciplines!! DUALITY IN NON-ABELIAN ALGEBRA I. FROM COVER RELATIONS TO GRANDIS EX2-CATEGORIES! Zurab Janelidze Stellenbosch University!!! CMS PREPRINT #1! 26 March 2014 Thomas Weighill Stellenbosch University

2 DUALITY IN NON-ABELIAN ALGEBRA I. FROM COVER RELATIONS TO GRANDIS EX2-CATEGORIES ZURAB JANELIDZE AND THOMAS WEIGHILL Abstract. The aim of this series of papers is to develop a self-dual categorical approach to some topics in non-abelian algebra, which is based on replacing the framework of a category with that of a category equipped with a functor to it. The present paper gives some preliminary steps in this direction, where several known structures on a category, which arise in the categorical treatment of these topics, are viewed as such functors; as a result, we obtain some new conceptual links between these structures. 1. Introduction In [15] it was shown that among all regular categories [2], the regular protomodular categories [3] and the semi-abelian categories [13] can be characterized by dual axioms on the bifibration of subobjects of a regular category. In fact, as reported by the first author in his talk at Category Theory 2013, similar characterizations are possible if we start with an arbitrary category rather than a regular one, equipped with a class M of monomorphisms which contains identity morphisms, and impose dual conditions on the codomain functor from C M to C, where C M is the full subcategory of the category C 2 of morphisms in C, consisting of elements of M. As remarked already in [15], such axioms are closely related to those used by M. Grandis in his work on non-abelian homological algebra [8, 9, 10]. Once again, the settings considered by M. Grandis can be obtained by imposing dual axioms on the same functor. In fact, the settings considered by M. Grandis are wholly self-dual, i.e. the codomain functor C M! C that we start with as an initial structure on C can be replaced with any functor to C. Structures consisting of a category and a functor to it appear of course implicitly or explicitly in many di erent areas of category theory. A fibered category in the sense of A. Grothendieck [11, 12] is perhaps the best explicit example of such a structure. Although at a later stage we will indeed work only with fibrations (and in fact, with bifibrations), aproperaxiomaticinvestigationnecessitatestobeginourstudywithfunctorswhichare not necessarily fibrations. However, so far we restrict ourselves to faithful functors, since those arising in the results mentioned above, e.g. the codomain functors C M! C, are such. In fact, for simplification of arguments it is better to restrict further to amnestic functors, which are functors that have skeletal fibres, i.e. any isomorphism in a fibre is Both authors research was partially supported by the South African National Research Foundation. Second author s research was also supported by the MIH Media Lab at Stellenbosch University Mathematics Subject Classification: 18D30, 18A32, 18A20, 18E10, 06A15, 06A75, 06F99. Key words and phrases: biform, cover relation, ex2-category, factorization system, faithful amnestic functor, form, Grothendieck fibration, ideal of null morphisms, subobject, universalizer. c Zurab Janelidze and Thomas Weighill, Permission to copy for private use granted. 1

3 an identity morphism note that this, together with faithfulness, forces the fibres to be ordered sets. Each faithful functor can be transformed into one which is amnestic by replacing its fibres with their skeletons this new functor, which is now both faithful and amnestic, will behave in the same way as the original one, up to the properties that are of interest to us. A functor F : B! C which is both faithful and amnestic, will be called a form over C. The main goal of this series of papers is to use forms for a self-dual treatment of various topics from non-abelian algebra, which mimics and generalizes the self-dual treatment of similar topics from abelian algebra achieved in the context of abelian categories. Among the topics we have mind are the non-abelian isomorphism theorems, and so our goal is very much along the first sentence of the following passage from the paper [19] by S. Mac Lane (the same paper where duality of the abelian context was first explored): A further development giving the first and second isomorphism theorems, and so on, can be made by introducing additional carefully chosen dual axioms. This will be done below only in the more symmetrical abelian case. In the present paper we only make preliminary steps, where several structures on a category, pertinent in this development, are represented by, and compared to each other via forms. In particular, we use forms to explore a passage from cover relations in the sense of [14] to ex2-category structures in the sense of M. Grandis [8, 9, 10] Preliminaries Note that a form is essentially the same as a forgetful functor in a concrete category over C, in the sense of [1]. Forms certainly abound in mathematics. Among them, the following three major classes of forms should be distinguished: the forgetful functors for categories of algebras over monads on a category C; we will refer to these as algebraic forms over C; the so-called topological functors to C, which in some sense play a similar role for topological and relational structures as the algebraic forms do for algebraic structures; we will refer to these as topological forms; the amnestic functors obtained from the codomain functors C M! C, where M is aclassofmonomorphismsinc, and where C M denotes the full subcategory of the category C 2 of morphisms in C, whose objects are the elements of M; we will refer to these as subobject forms. Many concrete instances of subobject forms are in fact topological forms, while they are hardly ever algebraic. Subobject forms are often fibrations, or opfibrations, and in fact, usually they are bifibrations. A form F which is a fibration will be called a right form, and when F is an opfibration, we call it a left form; thus, F is a right form if and only

4 if its dual form F op is a left form. When F is both a left form and a right form, we will say that F is a biform. As shown in [22], topological forms are the same as what can be called locally complete biforms, i.e. biforms whose fibres are complete lattices. Two forms F 1 : S 1! C and F 2 : S 2! C are said to be isomorphic when there is an isomorphism I : S 1! S 2 such that F 2 I = F 1. Equivalence classes of (left/right/bi-) forms under the equivalence relation F 1 is isomorphic to F 2 willbecalled isomorphism classes of(left/right/bi-)forms,althoughtheirsizesarebiggerthanthesizesof,say, classes of morphisms in a category. Recall that, given a functor F : S! C, thefibre F 1 (Y )atanobjecty in C is the subcategory of S consisting of those objects and morphisms which by F are mapped to Y and 1 Y, respectively. If F is faithful then each fibre is a preorder. For such F to be amnestic is equivalent to each fibre being an ordered set (class). Recall that in general, a functor is said to be amnestic when the only isomorphisms which are mapped by it to an identity morphism are the identity morphisms (see e.g. [1]). Just as any preorder can be turned into an ordered set (class) by identifying isomorphic objects in it, any faithful functor F : S! C gives rise to a form, by identifying isomorphic objects in each fibre. All forms that we consider in this paper can be obtained using the same general procedure which constructs a form over a category C from a binary relation C on the class of morphisms of C. First, we construct a category from C, denoted by Mor C (C). Its objects are pairs (B,f) whereb is an object in C and f is a morphism f : A! B having the following property: 3 (m) fcf and in any diagram f A A 0 X u / B v / B 0 f 0 if ucf and vfcf 0 then vucf 0. Before describing morphisms in Mor C (C), let us note the following: 2.1. Lemma. Every morphism f in C has the property (m), if and only if C is a reflexive and transitive relation such that (C 1 ) C has left preservation property [14], i.e. if fcg and the composites ef and eg are defined, then efceg. A morphism v :(B,f)! (B 0,f 0 )inmor C (C) is a morphism v : B! B 0 in C such that vfcf 0. Identity morphisms and composition of morphisms in Mor C (C) aredefined as in C. We then get a faithful functor Mor C (C)! C, which maps each morphism v :(B,f)! (B 0,f 0 ) to the morphism v : B! B 0 in C. The corresponding form will be called the form of the relation C. We now describe two special classes of such forms, which we use in the paper.

5 Let M be a class of monomorphisms in C. Define a relation C on the class of morphisms of C as follows: fcf 0 if and only if f 0 2 M and f factors through f 0, i.e. f = f 0 g for some morphism g. It is easy to see that a morphism f satisfies (m) forthisrelationifandonly if f 2 M. The form F of this relation will be called the form of M-subobjects. Objects in each fibre F 1 (X) arecalledm-subobjects of the object X. They can be represented by monomorphisms m 2 M with codomain X, with m and m 0 representing the same M-subobject if and only if m = m 0 i for some isomorphism i (which is unique once it exists). Thus, when M is the class of all monomorphisms, an M-subobject is the same as a subobject in the usual sense. Let N be any class of morphisms in C. Following [8], a sequence 4 A f / B g / C of morphisms in C is said to be N -exact (at B), when gf 2 N and for any two morphisms u and v such that vf 2 N and gu 2 N,wehavevu 2 N.Suchpair(g, f) ofmorphisms is then called an N -exact pair. Note that this notion is self-dual: (g, f) isann -exact pair in C if and only if (f,g) isann -exact pair in C op. Given a class N of morphisms in a category C, define a relation C on the class of morphisms of C as follows: fcf 0 when f 0 is part of an N -exact pair (g 0,f 0 )suchthat g 0 f 2 N. It is easy to see that a morphism f satisfies (m) forthisrelation,ifandonlyiff is part of an N -exact pair (g, f). The form of C will be called the form of N -exact pairs in C. The self-dual nature of the notion of an N -exact pair has the following manifestation: 2.2. Proposition. For any class N of morphisms in a category C, the form of N -exact pairs in C is isomorphic to the dual of the form of N -exact pairs in C op. Proof. The isomorphism can be established by mapping an object, represented by a morphism f : A! B, inthefibreatb of the form of N -exact pairs in C, the object in the fibre at B of the form of N -exact pairs in C op, which is represented by a morphism g : B! C such that (g, f) isann -exact pair. 3. Left forms, cover relations, and factorizations A reflexive and transitive cover relation in the sense of [14] is a reflexive and transitive binary relation on the class of morphisms of C which satisfies the condition (C 1 )(see Section 2 above) as well as the following two conditions: (C 0 )IffCg then f and g have the same codomain. (C 2 )Foranymorphismf : A! B we have fc1 B. We will now show that there is a bijection between reflexive and transitive cover relations on C and isomorphism classes of left forms F over C satisfying the following conditions:

6 (LF 1 ) F is locally bounded above, i.e. in each fibre there is a terminal object, that is, each ordered set F 1 (X) hasanupperbound(writtenas1 X ). (LF 2 ) F is conormal, i.e. for any object Y of C, and for any W 2 F 1 (Y ), there exists a morphism f : X! Y in C such that we have f F (1 X )=W. Here f F denotes the change-of-base functor f F : F 1 (X)! F 1 (Y ) itmapseach object V 2 F 1 (X) tothecodomainofthecocartesianliftingoff Theorem. For any category C, there is a bijection: reflexive and transitive cover relations C on C isomorphism classes of conormal locally bounded above left forms F over C Under this bijection, to an isomorphism class of a form F corresponds the cover relation C defined as follows: for any two morphisms f : X! Y and f 0 : X 0! Y in C we have fcf 0 if and only if f F (1 X ) 6 f 0 F (1X0 ). In the other direction, to a cover relation C corresponds the isomorphism class of the form of the relation C. Proof. Let F be a left form over C. It is easy to see that if F is locally bounded above, then the relation C = F defined in the theorem is a reflexive and transitive cover relation. If F 1 and F 2 are isomorphic then F1 = F2. Now, let C be a reflexive and transitive relation on the class of morphisms of C satisfying (C 1 ). Write F = C for the form of the relation C. It is easy to show, thanks to Lemma 2.1, that F is a left form which is locally bounded above and is conormal. When further (C 0 )holds,wehave(c ) = C. Finally, it is not di cult to verify that for any left form F which is locally bounded and conormal, the left form (F ) is isomorphic to F. Next, we characterize various properties of the form of M-subobjects, where M is aclassofmonomorphismsinacategory,viaconditionsontheclassm first studied in [5, 7, 20, 21, 16]; we will freely use/recall some of the well known results from these works. Consider any functor F : B! C and let W 2 F 1 (Y ). A left universalizer of W is amorphismf : X! Y in C with the universal property of being terminal among those morphisms f : X! Y which satisfy the following condition: (LU 0 )ForanyobjectX of C, and for any V 2 F 1 (X), there exists a (unique) morphism b : V! W in B with the property F (b) =f. Thus, f is a left universalizer of W when f satisfies the condition above and for any other morphism f 0 : X 0! Y satisfying the same condition, there is a unique morphism x : X 0! X such that fx = f Lemma. Let M be a class of monomorphisms in a category C, satisfying the following condition: (M 0 ) for any object C in C there exists an isomorphism which belongs to the class M and whose codomain is C. Then for any monomorphism m : X! Y from the class M, the morphism m is a left universalizer of the M-subobject of Y represented by m, for the form of M-subobjects. 5

7 Proof. Let F denote the form of M-subobjects. In it clear that if W denotes the M- subobject of Y represented by m, then (LU 0 )holdsforf = m. Suppose f 0 : X 0! Y also satisfies (LU 0 ). Then, taking f = f 0 in (LU 0 ), and taking V to be the M-subobject of X represented by an isomorphism with codomain X, we will conclude that f 0 factors through m, i.e. mx = f 0 for some morphism x. Since m is a monomorphism, such x is necessarily unique. The condition (M 0 ) in Lemma 3.2 is necessary. Indeed, for, if for example we take M to be the class of functions whose domain is the empty set, then the class of left universalizers for the form of M-subobjects over the category Set will be the class of isomorphisms, and so in this case the assertion of the lemma does not hold. In the case when the functor F above is a left form, the condition (LU 0 ) is equivalent to the following one: (LU 1 )ForanyobjectX of C, and for any V 2 F 1 (X), we have f F (V ) 6 W. When further F is a biform, (LU 0 )becomesthesameastorequirethatf 1 F (W ) is the terminal object in the fibre F 1 (X) atx, where f 1 F stands for the right adjoint in the change of base adjunction f F a f 1 F : F 1 (X) F 1 (Y ) 6 induced by f. Thus, when F is a biform, a left universalizer in the above sense is the same as a left universalizer in the sense of [15] (note that what we call in this paper a biform was called a form in [15]) Lemma. For any left form, every left universalizer is a monomorphism. Proof. This is a consequence of the fact that if a morphism f satisfies (LU 1 ), then so does any composite fg. Right universalizers are defined dually. By the dual of the above lemma, for a right form, every right universalizer is an epimorphism. Consider the following additional axiom on a cover relation: (C 3 ) C admits images [14], i.e. for any morphism w : W! Y there exists a morphism f : X! Y such that fcw and f is terminal with this property, i.e. if f 0 : X 0! Y is another morphism with f 0 Cw then f 0 = fx for a unique morphism x : X 0! X (then f is called a C-image of the morphism w). It was shown in [14] that the process of assigning to a cover relation C the class M of C-images defines a bijection between reflexive and transitive cover relations satisfying (C 3 ) and classes M of monomorphisms satisfying the following conditions, which are due to H. Ehrbar and O. Wyler [5] (see also [20, 21, 16]): (M 1 ) M is closed under composition with isomorphisms.

8 (M 2 )Everymorphismf in C has a factorisation f = me with m 2 M, such that for any commutative diagram of solid arrows below with n 2 M, there is a unique morphism g which makes the diagram commute: 7 e e 0 f g m h / / n We note immediately that already the first half of (M 2 )implies(m 0 ), when M is a class of monomorphisms. Under the bijection referred to above, the cover relation C corresponding to the class M is defined as follows: fcf 0 when m factors through m 0, where m and m 0 are part of factorizations of f and f 0, respectively, given by (M 2 ). Further, it is easy to see that the form of M-subobjects is isomorphic to the form of the corresponding cover relation. The images for a reflexive and transitive cover relation C are in fact the same as left universalizers for the form of the relation C. Thus, C satisfies (C 3 )ifandonlyifthe corresponding left form satisfies the following condition: (LF 3 ) F admits left universalizers, i.e. for any object Y of C, each W 2 F 1 (Y )hasaleft universalizer (which is written as lun F (W ):Lun F (W )! Y ). As (LF 3 )isinvariantunderisomorphismofleftforms,wegetthatthebijectionoftheorem 3.1 restricts to a bijection between isomorphism classes of left forms satisfying (LF 1 ), (LF 2 ), and (LF 3 ), and and reflexive and transitive cover relations admitting images. Combining this bijection with the one established in [14] and recalled above, we get: 3.4. Theorem. For any category C, there is a bijection: classes M of monomorphisms in C satisfying (M 1 ) and (M 2 ) isomorphism classes of conormal locally bounded above left forms F over C admitting left universalizers Under this bijection, to an isomorphism class of a form F corresponds the class of left universalizers for F. In the other direction, to a class M of monomorphisms corresponds the form of M-subobjects. It is well known that for a class M of morphisms which satisfies (M 0 )and(m 1 ), the following conditions are equivalent: the codomain functor C M! C is an opfibration (when M is a class of monomorphisms, this is the same as to say that the form of M-subobjects is a left form).

9 8 (M 2 )holds. Thus, the theorem above gives a characterization of left forms which are isomorphic to forms of M-subobjects, where M is a class of monomorphisms which satisfies (M 0 )and (M 1 ): such left forms are precisely those left forms which are locally bounded above, conormal, and admit left universalizers. It is also well known that for a class M of monomorphisms satisfying (M 1 )and(m 2 ), we have: the form of M-subobjects is a right form (and hence a biform) if and only if all pullbacks of morphisms along those in M exist (in which case M is even stable under pullbacks); the class M is part of a factorization system (E, M) inthesenseofp.freydand G. M. Kelly [7] if and only if M is closed under composition. This produces two important restrictions of the bijection described in Theorem 3.4: a bijection between classes M of monomorphisms, such that C is finitely M-complete in the sense of D. Dikranjan and W. Tholen [4], and isomorphisms classes of biforms over C which are locally bounded above, conormal and admit left universalizers; and a bijection between classes M of monomorphisms which are part of a factorization system, and isomorphisms classes of left forms over C which are locally bounded above, conormal, admit left universalizers, and for which the class of left universalizers is closed under composition. As it follows from Proposition in [14], the requirement above that the class of left universalizers is closed under composition can be replaced with the following condition: (LF 4 )ForanyobjectY of C, and for any W 2 F 1 (Y ), the change-of-base functor F 1 (Lun F (W ))! F 1 (Y )inducedbylun F (W )isfull. Note that when F is a biform, the condition above becomes equivalent to requiring that left universalizers are injective, i.e. the left adjoint in the corresponding change-of-base adjunction is injective on objects. In the language of the corresponding cover relation, the condition above evidently translates to the following one: (C 4 ) Any C-image f is C-reflecting [14], i.e. for any v and v 0,iffvCfv 0 then vcv 0. Thus, we have:

10 Corollary. There is a bijection classes M of monomorphisms in C which are part of a factorization system (E, M) isomorphism classes of conormal locally bounded above left forms F over C admitting injective left universalizers given by assigning to a class M of monomorphisms the isomorphism class of the form of M-subobjects. 4. Biforms and ideals of null morphisms A class N of morphisms in a category C satisfying the following conditions is called an ideal of null morphisms [6, 17, 18]: (LN 0 )Ifn 2 N and the composite nt is defined, then nt 2 N. (RN 0 )Ifn 2 N and the composite sn is defined, then sn 2 N. Note that (LN 0 )isdualto(rn 0 ), i.e. the first one holds for a class N in C if and only if the second one holds for the same class in C op. Consider the following additional pair of dual conditions on an ideal N : (LN 1 ) Any morphism g is part of an N -exact pair (g, f). (RN 1 ) Any morphism f is part of an N -exact pair (g, f). When stated for the dual F op : B op! C op of a right form F, the conditions (LF 1 )and (LF 2 )become: (RF 1 ) F is locally bounded below, i.e. in each fibre there is an initial object, that is, each ordered set F 1 (X) hasalowerbound(writtenas0 X ). (RF 2 ) F is normal, i.e. for any object X in C, and for any V 2 F 1 (X), there exists a morphism f : X! Y in C such that we have f 1 F (0Y )=V. We will now show that a biform F over a category C which is locally bounded (i.e. satisfies both (LF 1 )and(rf 1 )) and binormal (i.e. satisfies both (LF 2 )and(rf 2 )) is determined uniquely (up to an isomorphism of forms) by the class of those morphisms n : X! Y in C for which the change-of-base adjunction is trivial (i.e. the change of base maps n F : F 1 (X)! F 1 (Y )andn 1 F : F 1 (Y )! F 1 (X) are constant maps) we will call such morphisms F -trivial morphisms. Moreover, this gives a bijection between isomorphism classes of locally bounded binormal biforms over a category C and ideals N of null morphisms in C for which every morphism B! C is part of a sequence A! B! C! D which is exact relative to the ideal.

11 Theorem. For any category C, there is a bijection: ideals N of null morphisms in C satisfying (LN 1 ) and (RN 1 ) isomorphism classes of binormal locally bounded biforms F over C Under this bijection, to an isomorphism class of a biform F corresponds the class of F - trivial morphisms. In the other direction, to an ideal N corresponds the isomorphism class of the form of N -exact pairs. Proof. Let F be a biform over C. It is clear that the class N = F of F -trivial morphisms is an ideal of null morphisms. Note that when F is locally bounded, a composite fg of two morphisms f : A! B and g : B! C is F -trivial if and only if f F (1 A ) 6 g 1 F (0C ). When in addition F is normal, this gives that for two morphisms f : A! B and f 0 : A 0! B we have f F (1 A ) 6 ff 0 (1A0 )ifandonlyifforanymorphismg : B! C we have: gf 0 2 F implies gf 2 F. So the cover relation C corresponding to F is uniquely determined by the class of F -trivial morphisms, which after applying Theorem 3.1 implies that two locally bounded normal biforms are isomorphic provided they determine the same class of trivial morphisms. Next, we show that N = F satisfies (LN 1 )whenf is locally bounded and conormal. Given a morphism g : B! C, consider the morphism f : A! B such that f F (1 A )=g 1 F (0C )(suchf exists when the form is conormal). Then the composite gf is F -trivial. Moreover, if u : A 0! B and v : B! C 0 are morphisms such that vf and gu are F -trivial, then f F (1 A ) 6 v 1 F (0C0 )andu F (1 A0 ) 6 g 1 F (0C )whichwiththeprevious equality together give u F (1 A0 ) 6 v 1 F (0C0 )andhencevu is trivial. Thus the pair (g, f) is F -exact. This proves that the class N = F satisfies (LN 1 ), as desired. Dually, the class N = F satisfies (RN 1 )whenf is locally bounded and normal. It is not di cult to see that two isomorphic biforms will give rise to the same class of trivial morphisms. So, thus far we have shown that the assignment F 7! F determines a map from isomorphism classes of locally bounded binormal biforms to ideals of null morphisms satisfying (LN 1 ) and (RN 1 ), and that this map is injective. It is easy to see that when a class N of morphisms satisfies (LN 1 ), the form F = N of N -exact pairs is a right form. Dually, in view of Proposition 2.2, when N satisfies (RN 1 ), the same form is a left form. Further, once each identity morphism 1 B : B! B is part of an N -exact pair (g, 1 B ), and (LN 0 )holds,theformn is locally bounded above. Dually, once each identity morphism 1 B : B! B is part of an N -exact pair (1 B,f), and (RN 0 ) holds, the form N is locally bounded below. So when N is an ideal of null morphisms satisfying (LN 1 )and(rn 1 ), we get that N is a biform which is locally bounded. Moreover, in this case N is also binormal, which can be easily verified. For a locally bounded form F over C, a morphism n : X! Y in C is F -trivial if and only if there exists a morphism b :1 X! 0 Y in the domain of F such that F (b) =n. This fact can be used to verify straightforwardly that for an ideal N of null morphisms satisfying (LN 1 )and(rn 1 ), the class of N -trivial morphisms coincides with the class N. Consider now the following pair of dual conditions on a class N of morphisms in a category C:

12 (LN 2 ) N admits kernels, i.e. for any morphism g : B! C in C there exists a morphism k : A! B such that gk 2 N and k is terminal with this property, i.e. if gk 0 2 N for some morphism k 0 : A 0! B then k 0 = ku for a unique morphism u : A 0! A (such k is called an N -kernel of g). (RN 2 ) N admits cokernels, i.e. for any morphism f : A! B in C there exists a morphism c : B! C such that cf 2 N and c is initial with this property, i.e. if c 0 f 2 N for some morphism c 0 : B! C 0 then c 0 = uc for a unique morphism u : C! C 0 (such c is called an N -cokernel of f). It is not di cult to see if k is an N -kernel of a morphism g, then the pair (g, k) is N -exact, when N satisfies (LN 0 ). So under (LN 0 ), the condition (LN 2 )implies(ln 1 ). Dually, under (RN 0 ), the condition (RN 2 )implies(rn 1 ). When N is the class of trivial morphisms in a locally bounded below normal biform F, the class of N -kernels coincides with the class of left universalizers for F. In detail, an N -kernel of a morphism g : B! C is the same as a left universalizer of g 1 F (0C ). This and its dual observation allow us to restrict the bijection described in Theorem 4.1 to a bijection: 11 ideals N of null morphisms in C admitting kernels and cokernels isomorphism classes of binormal locally bounded biforms F over C admitting left and right universalizers (1) Now, as it follows by the remarks after Theorem 3.4 in Section 3, for an ideal of null morphisms admitting kernels and cokernels, the class of kernels is closed under composition if and only if the corresponding biform admits injective left universalizers. A dual result would state that for an ideal of null morphisms admitting kernels and cokernels, the class of cokernels is closed under composition if and only if the corresponding biform admits surjective right universalizers (i.e. the left adjoint in the change-of-base adjunction induced by any right universalizer is surjective on objects). Ideals N of null morphisms in a category C which admit kernels and cokernels, and for which the classes of kernels and cokernels are closed under composition, are precisely those for which the pair (C, N ) is an ex2-category in the sense of M. Grandis [8]. In fact, in the definition given in [8], the class N is also required to be a closed ideal, i.e. any morphism in N must factor through an identity morphism which belongs to N. However, this is a consequence of the other requirements: consider a morphism n : L! M from the class N. Let k : K! M be the kernel of 1 M. Then n factors though 1 K. To show that 1 K 2 N, consider the composite kk 0 where k 0 is a kernel of 1 K. Notice that kk 0 2 N. Then, 1 M is a cokernel of kk 0. Since the class of N -kernels is closed under composition, the composite kk 0 is a kernel of some morphism and hence of its cokernel 1 M. But k is also a kernel of 1 M. This implies that k 0 is an isomorphism, which forces 1 K 2 N. Thus, we have:

13 4.2. Corollary. For any category C, there is a bijection (1) under which, to an isomorphism class of a biform F corresponds the class of F -trivial morphisms. In the other direction, to an ideal N corresponds the isomorphism class of the form F of M-subobjects, where M is the class of N -kernels. Furthermore, such F admits injective left universalizers and surjective right universalizers if and only if the pair (C, N ) is an ex2-category. These results reveal a correspondence between a hierarchy of conditions on cover relations and a hierarchy of conditions on ideals of null morphisms encountered in the work of M. Grandis [8, 9, 10], which is given by the procedure of translating the condition into the language of the form F of the cover relation, imposing also the dual condition on the same form F, and then translating further the resulting condition into the language of the ideal of F -trivial morphisms. This correspondence, to the extent explored above, can be summarized by the following table: 12 Conditions on a cover relation C: C is reflexive and transitive + C admits images + C-images are C-reflecting Corresponding dual conditions on a biform F : F is locally bounded and binormal + left and right universalizers exist + left universalizers are injective and right universalizers are surjective Corresponding dual conditions on an ideal N : N is an ideal such that every morphism B! C is part of an exact sequence A! B! C! D N is an ideal admitting kernels and cokernels N makes an ex2-category A di erent presentation of the same correspondence is to view the resulting classes of ideals as classes of pairs of dual structures : For example, an ideal N which admits kernels and cokernels is the same as apair (E, M), where M is a class of monomorphisms satisfying (M 1 )and(m 2 ), and E is a class of epimorphisms satisfying dual conditions, such that the form of M- subobjects is isomorphic to the form of E-quotients (which is a form given by the dual of the construction for the form of M-subobjects construct the form of E-subobjects in the dual category and then take the dual of the resulting form). Above, is the same as means that there is a bijection between these two types of structures; namely, for an ideal N the corresponding pair (E, M) consistsofthe class E of N -cokernels and the class M of N -kernels. Note that the compatibility condition on the pair (E, M)whichsaysthattheform of M-subobjects is isomorphic to the form of E-quotients, can be also presented in the following elementary way: there is a relation R E M which induces a bijection between the class of all M-subobjects and the class of all E-quotients, and

14 13 is such that for any diagram A m / B e / C a s A 0 m 0 / B 0 e 0 / C 0 of solid arrows where (e, m) 2 R and (e 0,m 0 ) 2 R, the dotted arrow a exists making the left square commute, if and only if the dotted arrow c exists making the right square commute. The pairs (e, m) in this relation are those which are N -exact for the corresponding ideal N of null morphisms these in turn are those morphisms s for which the dotted arrows above always exist. It is worth noting that in a similar way, an ex2-category structure is the same as a pair of factorization systems such that the class E in the first factorization system is aclassofepimorphisms,theclassm in the second factorization system is a class of monomorphisms, and the form of M-subobjects for the second factorization system is isomorphic to the form of E-quotients for the first one. c References [1] J. Adámek, H. Herrlich, and G. E. Strecker, Abstract and concrete categories, Pure and Applied Mathematics (New York), A Wiley-Interscience Publication, John Wiley & Sons, Inc., New York, [2] M. Barr, P. A. Grillet, and D. H. van Osdol, Exact categories and categories of sheaves, Springer Lecture Notes in Mathematics 236, [3] D. Bourn, Normalization equivalence, kernel equivalence and a ne categories, Springer Lecture Notes in Mathematics 1488, 1991, [4] D. Dikranjan and W. Tholen, Categorical structure of closure operators, Mathematics and its Applications 346, Kluwer Academic Publishers, [5] H. Ehrbar and O. Wyler, On subobjects and images in categories, Technical Report, Department of Mathematical Sciences, Carnegie Mellon University, [6] C. Ehresmann, Sur une notion générale de cohomologie, C. R. Acad. Sci. Paris 259, 1964, [7] P. J. Freyd and G. M. Kelly, Categories of continuous functors I, J. Pure Appl. Algebra 2, 1972, [8] M. Grandis, On the categorical foundations of homological and homotopical algebra, Cah. Top. Géom. Di. Catég. 33, 1992, [9] M. Grandis, Homological Algebra, The interplay of homology with distributive lattices and orthodox semigroups, World Scientific Publishing Co., Singapore, 2012.

15 [10] M. Grandis, Homological Algebra In Strongly Non-Abelian Settings, World Scientific Publishing Co., Singapore, [11] A. Grothendieck, Technique de descente et théorèmes d existence en géométrie algébrique. I. Généralités. Descente par morphismes fidèlement plats, Séminaire N. Bourbaki 5, Exposé 190, 1959, [12] A. Grothendieck, Catégories fibrées et descente, Exposé VI, in: Revêtements Etales et Groupe Fondamental (SGA1), Springer Lecture Notes in Mathematics 224, 1971, [13] G. Janelidze, L. Márki and W. Tholen, Semi-abelian categories, J. Pure Appl. Algebra 168, 2002, [14] Z. Janelidze, Cover relations on categories, Appl. Categor. Struct. 17, 2009, [15] Z. Janelidze, On the form of subobjects in semi-abelian and regular protomodular categories, Appl. Categor. Struct., December [16] G. B. Im and G. M. Kelly, On classes of morphisms closed under limits, J. Korean Math. Soc. 23, 1986, [17] G. M. Kelly, Complete functors in homology. I. Chain maps and endomorphisms, Proc. Cambridge Philos. Soc. 60, 1964, [18] R. Lavendhomme, La notion d idéal dans la théorie des catégories, Ann. Soc. Sci. Bruxelles Sér. 1 79, 1965, [19] S. Mac Lane, Duality for groups, Bulletin of the American Mathematical Society 56, 1950, [20] J. MacDonald and W. Tholen, Decomposition of morphisms into infinitely many factors, Category Theory (Gummersbach, 1981), , Lecture Notes in Mathemtics 962, Springer, Berlin-New York, [21] W. Tholen, Factorizations, localizations, and the orthogonal subcategory problem, Math. Nachr. 114, 1983, [22] O. Wyler, On the categories of general topology and topological algebra, Arch. Math. (Basel) 22, 1971, Mathematics Division Department of Mathematical Sciences Stellenbosch University Private Bag X Matieland South Africa 14 Mathematics Division Department of Mathematical Sciences Stellenbosch University Private Bag X Matieland South Africa zurab@sun.ac.za, thomas.weighill@ml.sun.ac.za