Closure Operators with Respect to a Functor Dedicated to Lamar Bentley on his 60th birthday G. CASTELLINI Department of Mathematics, University of Puerto Rico, Mayag¨uez campus, P.O. Box 9018, Mayag¨uez, PR 00681-9018. and E. GIULI∗ Dipartimento di Matematica, Universit´adi L’Aquila, 67100 L’Aquila, ITALY Abstract. A notion of closure operator with respect to a functor U is introduced. This al- lows us to describe a number of mathematical constructions that could not be described by means of the already existing notion of closure operator. Some basic results and examples are provided. Mathematics Subject Classification (1991). 18A22, 18A32, 06A15. Key words: Closure operator, faithful functor, factorization structure. Introduction The notion of closure operator in an arbitrary category A introduced in [7] depends on one parameter: subobjects. Precisely, for every object A ∈A, a class of subobjects subA of A is given and then a closure operator is defined on these classes. This notion has lead to a useful theory. For more details on closure operators see, e.g., [2], [6-7], [8-9], [15] and [16]. However, many very natural and useful closures cannot be described by means of the previous notion, since they do not act on the given subobjects of the category A, but rather on subobjects of another category X related to A via a functor U A −→ X . More precisely, for a given class of subobjects in the category X U and for a given functor A −→ X , for each A in the category A, we close f the subobjects of UA in such a way that, for each A-morphism A −→ B, Uf preserves the closure. The main motivating example is the Kuratowski ∗ Both authors acknowledge partial support from the University of Puerto Rico, Maya- g¨uez Campus. The second author also acknowledges partial support from NATO Collab- orative Grant #940847 and from MURST. 2 G. CASTELLINI, E. GIULI closure in the category of topological groups: with the previous notion of closure operator we can only close subgroups while the Kuratowski closure acts on every subset of a given group. Other examples of this phenomenon are: the subgroup generated by a subset of a given group, the convex hull and the radial hull of a subset of a convex space, the down closure and the Scott closure of a subset of a directed-complete partially ordered set. It is worth to observe that the move from (ordinary) closure operators to the types of operators considered in this paper is analogous to the move from factorization structures to factorizations along a functor, as considered by Herrlich ([13]) and Tholen ([21]), which (in Proposition 12) also provides an analogue of Proposition 1.5. The aim of this paper is first of all to introduce the notion of closure operator with respect to a functor U (also called U-closure operator) that, besides containing the previous notion as a special case, is general enough to include all the above mentioned examples and more. Then some basic results that were widely used with the previous notion of closure operator can be extended to this setting as well (e.g., every idempotent and weakly hered- itary U-closure operator induces, under suitable conditions, a factorization structure on A). The U-closure operator induced by a suitable subclass of the class M of all subobjects of all objects of the category X is defined, and a notion of regular U-closure operator is derived as a special case. We provide conditions under which a class of A-morphisms N can be used to construct a U-closure operator (called the hull operator induced by N ). In the case when N is the second factor of a proper factorization structure for morphisms of the category A then, under suitable conditions, the hull operator induced by N composed with every closure operator of A with respect to N gives a U-closure operator. U-closure operators that can be obtained this way are called standard. It turns out that for these U-closure operators compactness (cf. [3], [4]), connectedness in the sense of [23] and [5] and Hausdorffness, are only depending on their second component (which is a usual closure operator of A) so that the investigation goes back to the classical case. For non-standard U-closure operators (e.g., the Kuratowski closure in topological groups) the situation is different. For instance for the Kuratowski U-closure in topological groups it is not known whether the corresponding class of compact objects (which are the usual compact topological groups by an adaptation of the Kuratowski-Mrowka theorem given for topological spaces) coincides or is properly contained in the class of compact objects with respect to the restriction of the above closure to subgroups (see [10] and [4]). Many examples that support the theory are included at the end of the paper. CLOSUREOPERATORSWITHRESPECTTOAFUNCTOR 3 We use the terminology of [1] throughout. We also acknowledge that Paul Taylor’s commutative diagrams macro package was used to typeset most of the diagrams in this paper. 1. U-closure operators Throughout we consider a finitely complete category X with a proper (E, M)- factorization structure for morphisms. For every object X ∈X , the class MX of morphisms of M with codomain X is preordered by m ≤ n if there is a morphism t such that n ◦ t = m. We ′ ′ define the following equivalence relation in MX : m ≃ m if m ≤ m and m′ ≤ m. The quotient class will be denoted by subX and called the subob- ject class of X with respect to (E, M). Since subX may be large, we assume that X has multiple pullbacks of arbitrary large families of M-morphisms with common codomain, with the pullback in M. Consequently, subX forms a (possibly large) complete lattice. In what follows we will not distinguish between m ∈ MX and the M- subobject that it defines (i.e., m = n means m ≃ n when m and n are taken as subobjects). f m If X −→ Y is an X -morphism, M −→ X is a subobject of X and e f(m) f(m) M −→ f(M) −→ Y is the (E, M)-factorization of f ◦ m, then f(M) −→ Y n is the image of m along f. Moreover, if N −→ Y is a subobject of Y , then −1 f (n) the pullback f −1(N) −→ X of n along f is the inverse image of n along f. Clearly f(−) and f −1(−) yield an adjoint situation between subY and subX. For the general theory of factorization structures for morphisms we refer to [11], [22] and [1]. U Let A be any category, let X be as above and let A −→ X be a given m functor. A pair (A, m) with A object of A and M −→ UA in M will be called U-subobject of A. We will write subU A for the class of all U-subobjects of A (i.e., subU A = {A}×sub UA) and we will simply refer to m for a U-subobject (A, m) of A when no confusion is possible. 1.1. DEFINITION. A U-closure operator c of A (with respect to (E,M)) is a family of functions cA (subU A −→ subU A)A∈A 4 G. CASTELLINI, E. GIULI with the following properties that hold for each A ∈A and U-subobjects m and n of A: (a) m ≤ cA (m); (b) m ≤ n ⇒ cA (m) ≤ cA (n); f (c) For each A-morphism A −→ B, Uf(cA (m)) ≤ cB (Uf(m)). Condition (a) implies that for every U-closure operator c on A, every m U-subobject M −→ UA of any object A has a canonical factorization A c (m) M −→ cA (M) m ց cA (m) y UA where cA (m) is called the U-closure of the U-subobject m of A. 1.2. REMARK. (1) Notice that under condition (b) of the above definition, condition (c) is f equivalent to the following statement concerning inverse images: if A −→ −1 B is an A-morphism and m is a U-subobject of B, then cA ((Uf) (m)) ≤ −1 (Uf) (cB (m)), i.e., the U-closure of the inverse image of m is less than or equal to the inverse image of the U-closure of m. (2) Under condition (a), both conditions (b) and (c) together are equivalent to the following: given m and n U-subobjects of A and B, respectively, if f is an X -morphism and g is an A-morphism such that n◦f = Ug ◦m, then there exists a unique morphism d such that the following diagram f M - N A B c m c n @@ ( ) n @@ ( ) @R @R d - m cA (M) cB (N) ?© cA (m) ?© cB (n) UA - UB Ug commutes. (3) The notion of closure operator of a category A with respect to a class CLOSUREOPERATORSWITHRESPECTTOAFUNCTOR 5 of monomorphisms M introduced in [7] can be easily seen as a special case of U-closure operator by simply taking U = idA. f We recall that an A-morphism A −→ B is U-initial if for every A- h g morphism D −→ B and X -morphism UD −→ UA such that (Uf) ◦ g = Uh, ′ g there exists a unique A-morphism D −→ A such that Ug′ = g and f ◦g′ = h. (cf. [14, Definition 2.1(1)]). 1.3. DEFINITION. Given a U-closure operator c of A, we say that a U- A subobject m of an object A is c-closed (c-dense) if m = cA(m) (m = c (m), f equivalently, cA(m) = idUA).