The motivation for introducing 40 years ago the construction, of which "categories of elements" is a special case, was to make clear the elementary nature of the notion of adjointness. Given an opposed pair of functors between two arbitrary given categories, one obviously elementary way of providing them with an adjointness is to give two natural transformations satisfying two equations; but very useful also is the definition in terms of bijections of hom sets which should be equivalent. The frequent mode for expressing the latter in terms of presheaf categories involved the complicated logical notion of "smallness" and the additional axiom that a category of small sets actually exists, but had the disadvantage that it would therefore not apply to arbitrarily given categories. By contrast, a formulation of this bijection in terms of discrete fibrations required no such additional apparatus and was universally applicable. Unfortunately, since I had given the construction no name, people in reading it began to use the unfortunate term "comma". It would indeed be desirable to have a more objective name for such a basic construction. (The notation involving the comma was generalized from the very special case when the two functors to B, to which the construction is applied, both have the category 1 as domain, and the result of the construction is the simple hom set in B between the two objects, which is often denoted by placing a comma between the names of the objects and enclosing the whole in parentheses.) One habit which it would be useful to drop is that of agonizing over the true definition of elements. In any category the elements of an object B are the maps with codomain B, these elements having various forms which are their domains. For example, if the category has a terminal object, we have in particular the elements often called punctiform. On the other hand, it is often appropriate to apply the term point to elements more general than that, for example, in algebraic geometry over a non-algebraically closed field, points are the elements whose forms are the spectra of extensions of the ground field. As Volterra remarked already in the 1880s, the elements of a space are not only points, but also curves, etc.; it is often convenient to use the term "figure" for elements whose forms belong to a given subcategory. Bill Lawvere ************************************************************ F. William Lawvere Mathematics Department, State University of New York 244 Mathematics Building, Buffalo, N.Y. 14260-2900 USA Tel. 716-645-6284 HOMEPAGE: http://www.acsu.buffalo.edu/~wlawvere ************************************************************ On Thu, 25 Sep 2003 Jpdonaly@aol.com wrote:
To all category theorists:
In various textbooks, I see reference to the common comma category Elts(G), which is called the "category of elements of functor G". This category seems to be drastically misnamed. Does anyone agree? Here is my side of the story, beginning with a review of the nature of Elts(G) and some of its significance.
G is a functor from a small category C into the category F of small functions. Denoting the singleton {0} of the void set 0 by 1 (as usual), Elts(G) consists of all triples (g,a,f) with a in the domain category C and g:1--->G(codomain a), f:1--->G(domain a) such that g = G(a) o f, where "o" denotes function composition. (Warning: By my conventions, a.domain a = a; codomain a.a = a.) The composition of Elts(G) is defined by (h,b,g)(g,a,f) = (h,ba,f). The objects are the Elts(G)-morphisms of the form (f,u,f), where u is a C-object, and the map (f,u,f)-->f(0) identifies each of these with an element of the set G(u), so that the convention of naming categories after their objects (to the extent possible) is what presumably leads to calling Elts(G) "the category of elements of the values of G at objects" or, for short, "the category of elements of G".
Several basic features of Elts(G) are exposed by treating it as a subcategory of a product BxC of a transition category B with the domain category C of G. To define B, let X be the set of functions f:1--->G(u) as u varies over the objects of C; B is then the full transition category or groupoid of X, that is, the self-product XxX with the transition composition (h,g)(g,f) = (h,f). But rather than taking the ordinary cartesian product for BxC, one uses the attachment product consisting of those triples (g,a,f) with (g,f) in B and a in C, so that C-morphism a is viewed as being attached on its left to g and on its right to f. Then Elts(G) inherits by restriction the projection functor (g,a,f)-->a, which is reasonably called the detaching functor from Elts(G) into C---this functor will be generically denoted by "det". There is also the transition projection (g,a,f)-->(g,f) which maps Elts(G) functorially onto a transitive relation on X, and the rule (g,a,f)-->g defines the entwining function of the canonical natural transformation which entwines the constant functor (g,a,f)-->1 on Elts(G) with the function composite functor G o det: Elts(G)--->F. (A constant functor with value 1 will be denoted generically by "delta(1)".)
There are many important examples: If C is a group with object e so that G is group action with action set Y=G(e), then, to within the identification (g,a,f)-->(g(0),a,f(0)), Elts(G) is the traditional idea of a G-action as a function from CxY into Y after correction to remove the categorically problematic product CxY. If C is actually the group RxR, R being the additive group of real numbers and G the action of C on the real affine plane by translation, then Elts(G) is essentially the category of attached planar vectors as used in Engineering Statics 101, which is why it seems appropriate to continue to use the word "attach" in the context of a more general function-valued functor G:C--->F. If C is a certain type of monoid, then Elts(G) is a semiautomaton. Among its theoretical services is the fact that Elts(G) plays a role in the construction of Kan extensions along inclusion functors, thus in particular in the theory of induced group actions. It plays an analogous part in sheafification relative to a Grothendieck site, and it is used to show that representable functors are dense in the set of function-valued functors on C, providing, according to Mac Lane and Moerdijk, "a plethora of tensor products". Even more basically, if the domain C of G is discrete, then Elts(G) is a coproduct a.k.a. a disjoint union of (the object values) of G. As will be noticed in a moment, the set of functors A:C--->Elts(G) which are right inverse to the detaching functor det on Elts(G)---that is, the "attaching functors" into Elts(G)---constitute a (small) limit object of G, thus, in the case of discrete C, a product of the sets G(u). From these examples it appears that Elts(G) is sufficiently important to require a unique and unambiguous nomenclature, but, unfortunately, the things in Elts(G) are really not the elements of G.
The (global) elements of an object u in a category are generally agreed to be the morphisms from a given terminal object t to u. This convention terminologically extends the observation that the functions f from the terminal object 1 into a (small) set X can be identified with the elements of X by the mapping f-->f(0). The general definition has the virtue that each terminal object has only one element, as should surely be the case, and the representable functor of t provides a plausible (but not necessarily effective) attempt to convert a given category into a category of functions between sets of elements. In fact, this language seems to have found broad acceptance. But then the functor G is an object in the morphismwise (i.e. "vertical") composition category F^C of natural transformations whose (fully extended) entwining functions map from C into F, and, because 1 is terminal in F, the constant functor delta(1) on C is terminal in F^C. So G already has a set of elements, namely, those natural transformations which entwine delta(1) with G. Such elements of G are not in Elts(G) in any sense.
At first sight this terminological conflict might seem to be innocuous, since Elts(G) and global elements of G occur in somewhat disparate contexts, but the apparent separation does not hold up well when one considers how close the set of global elements of G is to being a limit object of G. The only problem with it is that it is not small; that is, it is not in the codomain category F of G, and the only reason for this defect is that F, the common codomain of the entwining functions of the things in F^C, is not small. Mac Lane in CWM gives an ad hoc workaround which replaces, for a given G, the category F with a small, G-dependent category of small functions, but this approach effectively isolates G by artificially depriving it of morphisms into functors which do not happen to map into Mac Lane's ad hoc replacement category; so one needs a more perspicuous method of eliminating F and its untoward largeness.
F. W. Lawvere was apparently motivated by such considerations to introduce comma categories in his thesis, an approach which works very well in addressing the present awkwardnesses. One defines a category Law(C) whose objects are the categories Elts(G) as G ranges through the functors in F^C and whose morphisms are the cocompatible functors S:Elts(G)--->Elts(H) between such objects. The composition is function composition of functors, and "cocompatible" means that S does not disturb middle components of attached C-morphisms or, alternatively put, det o S = det, where "det" continues to be the generic symbol for a detaching functor. Then, if s in F^C entwines functor G with functor H, there is a cocompatible functor S:Elts(G)--->Elts(H) which is evaluated at an attached C-morphism (y*,a,x*) by
S(y*,a,x*)=(s(codomain a)(y)*, a, s(domain a)(x)*),
where I use y*, for example, to denote that function f:1--->G(codomain a) whose value is y. Then the assignments s-->S define a functorial isomorphism---which I call the Lawvere isomorphism (but should this be attributed to someone else?) from F^C onto Law(C). Moreover, the objects Elts(G) of Law(C) are small. This implies, of course, that the homset of cocompatible functors from Elts(G) into Elts(H) is also small.
Elts(delta(1)) evidently consists of triples of the form (0*,a,0*) and can thus be identified with C by the detaching functor (0*,a,0*)-->a on Elts(delta(1)). With this identification, a cocompatible functor from Elts(delta(1)) into Elts(G) becomes an attaching functor into Elts(G), so that, in the Lawvere picture, the global elements of G are the attaching functors into the category of...uh...elements of G. The set of such global elements is plainly small and therefore must be what God intends to be the standard limit object of G, except that it is difficult to believe that God would use such a verbal collision to say what a global element is. These are my grounds for believing that Elts(G) has to be renamed and redenoted.
As a related suggestion, I might recommend dropping the habit of referring to categories by the names of their objects. This illogicality immediately inhibits use of the subcategory concept (I still don't know what categorists use to refer to the subcategory formed by the monomorphisms in an abstractly given category), and then it just goes looking for the sort of trouble which has turned up as "the category of elements of G". Besides this, the terminology "comma category" is disrespectful of category theory, itself, due to the inappropriateness of naming a fundamental, overarching categorical concept after a punctuation mark. ("Slice category" doesn't seem to be any better.) Given the precedent of attached vectors, which are used in a rough sense even by sophisticated diagrammaticists, the category Elts(G) is obviously some kind of attachment category, and since the transition components of any of its morphisms all have domain 1, it is a based attachment category with base 1 or just a basement category---or even just a basement denoted by something like G/1, if you're used to placing domains on the right. Anyway, this is approximately what I use in my study notes, and so far it works fine.
At the same time, I would be interested in seeing sharp, well reasoned criticisms of this note provided that they are written at about the same technical level so that I can understand them. I would like to emphasize that, aside from what may be terminologically or notationally novel here, I am not making a substantial research proposal or claiming priority for any discoveries. I have no reason at all to doubt that all of the mathematics here is well known.
Pat Donaly