This entry is about a concept of duality in general category theory. For the concept of dualizing objects in a closed category as used in homological algebra and stable homotopy theory see at dualizing object in a closed category.
A dualizing object is an object which can be regarded as being an object of two different categories and , such that the concrete duality which is induced by homming into that object induces dual adjunctions between and , schematically:
Many famous dualities are induced this way, for instance Stone duality and Gelfand-Naimark duality.
There are various different terms for “dualizing objects”. As recalled on p. 112 of the article by Porst and Tholen below
Isbell speaks of objects keeping summer and winter homes;
Lawvere speaks of objects sitting in two categories;
Simmons speaks of schizophrenic objects.
It has been convincingly argued by Tom Leinster (blog comment here) that the term “schizophrenic” should not be used. Todd Trimble then suggested the term “ambimorphic object.” Another suggestion was “Janusian object.”
Let and be two categories over , given by functors
(Traditionally in what follows, one assumed and to be faithful, i.e., one considered and to be categories of sets with extra structure, aka concrete categories, although we won’t actually need this. Or, sometimes one sees the hypothesis that are representable, but we won’t require this either.)
The situation described in the Idea section asks us to consider contravariantly adjoint functors , such that and are representable, i.e., are given by isomorphisms (with representing element ) and (with representing element ). Let and denote the universal maps of the adjunction (so that if we write and in covariant form as and , we would have with as unit and as counit).
The representing data induce a canonical isomorphism . It is given by the evident composite
The inverse is given by a similar composite but switching the roles of and , and , and , and , and and .
Letting be an element of , the element is by definition where is the unique map (by representability) such that . The alleged inverse takes an element to , where is the unique map such that .
To check these maps are inverse, one may check simply for any ; the other equation follows by symmetry. With notation as above, put , so . Now we embark on a diagram chase, starting with
where the top and right quadrilaterals commute by naturality, and is the unique arrow making the left quadrilateral commute. In fact is the structure map for a -algebra structure on , although for our purposes we will only need the unit axiom for such a structure. Following the perimeter of this diagram, we see
is the unique arrow such that . Continuing on our way, applying to and evaluating at , we have only to check that the composite
equals . But already we said , so the last composite is
and all we need to do now is check the unit equation . But this follows from representability of , applied to the diagram
where the top horizontal composite is an identity, according to a triangular equation.
The canonical identification of sets means that we have an object “sitting in two categories” (Lawvere), namely an -structure on this set and a -structure on the same set, with providing a lift of through , and lifting through .
Given functors , , we define to be the category whose objects are contravariantly adjoint pairs such that and are representable. Morphisms are pairs of natural transformations , such that the diagrams
commute. We define to be the 2-pullback of and in (i.e., the bi-iso-comma-object). In conjunction with the Yoneda lemma, the preceding proposition can be read as giving the construction of a functor .
Here then is one key definition.
Given , , a -dualizing object (or -ambimorphic object) is a triple in the essential image of .
Naturally represented adjunctions
Definition 1 is reasonably general and is one of several notions of “schizophrenic object” given in Dimov-Tholen93. A somewhat tighter notion, also considered in Dimov-Tholen89 and Porst-Tholen and which covers many cases that arise in practice, involves initial lifts through the functors , .
Again, suppose given contravariant adjoint functors with and representable as above. We have maps
so that for each object of and , we have a corresponding map (playing the role of “” in the proof of Proposition 1), and similarly for each object of and , we have a map (playing the role of “” in Proposition 1).
The adjunction between and is naturally represented if the family is -initial for each , and the family is -initial for each .
The restriction of to the full subcategory whose objects are naturally represented adjoint pairs is full and faithful.
See Dimov-Tholen, Proposition 2.3, where the proof is sketched. Thus naturally represented adjoint pairs could be equivalently described as certain types of ambimorphic objects. We now describe these.
Let us denote the action of on the module , with components , by the notation . Thus each induces a map . Similarly, each induces a map .
A -ambimorphic object is natural if for every , the -structured source diagram
admits an initial lift targeted at (so a suitable diagram of the form indexed over , for some object of ), and for every , the -structured source diagram
admits an initial lift (a diagram of type ), targeted at .
Thus the category of naturally represented adjoint pairs relative to is equivalent to the category of natural -dualizing objects. This is the notion of “schizophrenic object” given by Porst-Tholen; a reasonably detailed proof of the equivalence with naturally represented adjoint pairs is given in their Theorem 1.7.
One easy general example of the notion given in Definition 1 is where is a symmetric monoidal closed category and is an object therein. Here we may take where is the monoidal unit, and the contravariant representable functor lifts to a contravariant enriched hom ; the symmetry isomorphism can be exploited to show how is adjoint to itself. See here for more.
Further examples appear at
- G. D. Dimov, W. Tholen, A Characterization of Representable Dualities, In: Categorical Topology and its Relation to Analysis, Algebra and Combinatorics, Prague, Czechoslovakia 22-27 August 1988, J. Adamek and S. MacLane (eds.), World Scientific, Singapore, New Jersey, London, Hong Kong, 1989, pp. 336-357.
- G. D. Dimov, W. Tholen, Groups of Dualities, Trans. Amer. Math. Soc., 336 (2), 901-913, 1993. (pdf)
- H.-E. Porst, W. Tholen, Concrete Dualities in Category Theory at Work, Herrlich, Porst (eds.) (pdf)
- Michael Barr, John F. Kennison, R. Raphael, Isbell Duality (pdf)