*-autonomous category
In mathematics, a *-autonomous (read "star-autonomous") category C is a symmetric monoidal closed category equipped with a dualizing object . The concept is also referred to as Grothendieck—Verdier category in view of its relation to the notion of Verdier duality.
Definition
Let C be a symmetric monoidal closed category. For any object A and , there exists a morphism
defined as the image by the bijection defining the monoidal closure
of the morphism
where is the symmetry of the tensor product. An object of the category C is called dualizing when the associated morphism is an isomorphism for every object A of the category C.
Equivalently, a *-autonomous category is a symmetric monoidal category C together with a functor such that for every object A there is a natural isomorphism , and for every three objects A, B and C there is a natural bijection
- .
The dualizing object of C is then defined by . The equivalence of the two definitions is shown by identifying .
Properties
Compact closed categories are *-autonomous, with the monoidal unit as the dualizing object. Conversely, if the unit of a *-autonomous category is a dualizing object then there is a canonical family of maps
- .
These are all isomorphisms if and only if the *-autonomous category is compact closed.
Examples
A familiar example is the category of finite-dimensional vector spaces over any field k made monoidal with the usual tensor product of vector spaces. The dualizing object is k, the one-dimensional vector space, and dualization corresponds to transposition. Although the category of all vector spaces over k is not *-autonomous, suitable extensions to categories of topological vector spaces can be made *-autonomous.
On the other hand, the category of topological vector spaces contains an extremely wide full subcategory, the category Ste of stereotype spaces, which is a *-autonomous category with the dualizing object and the tensor product .
Various models of linear logic form *-autonomous categories, the earliest of which was Jean-Yves Girard's category of coherence spaces.
The category of complete semilattices with morphisms preserving all joins but not necessarily meets is *-autonomous with dualizer the chain of two elements. A degenerate example (all homsets of cardinality at most one) is given by any Boolean algebra (as a partially ordered set) made monoidal using conjunction for the tensor product and taking 0 as the dualizing object.
The formalism of Verdier duality gives further examples of *-autonomous categories. For example, Boyarchenko & Drinfeld (2013) mention that the bounded derived category of constructible l-adic sheaves on an algebraic variety has this property. Further examples include derived categories of constructible sheaves on various kinds of topological spaces.
An example of a self-dual category that is not *-autonomous is finite linear orders and continuous functions, which has * but is not autonomous: its dualizing object is the two-element chain but there is no tensor product.
The category of sets and their partial injections is self-dual because the converse of the latter is again a partial injection.
The concept of *-autonomous category was introduced by Michael Barr in 1979 in a monograph with that title. Barr defined the notion for the more general situation of V-categories, categories enriched in a symmetric monoidal or autonomous category V. The definition above specializes Barr's definition to the case V = Set of ordinary categories, those whose homobjects form sets (of morphisms). Barr's monograph includes an appendix by his student Po-Hsiang Chu that develops the details of a construction due to Barr showing the existence of nontrivial *-autonomous V-categories for all symmetric monoidal categories V with pullbacks, whose objects became known a decade later as Chu spaces.
Non symmetric case
In a biclosed monoidal category C, not necessarily symmetric, it is still possible to define a dualizing object and then define a *-autonomous category as a biclosed monoidal category with a dualizing object. They are equivalent definitions, as in the symmetric case.
References
- Barr, Michael (1979). *-Autonomous Categories. Lecture Notes in Mathematics. Vol. 752. Springer. doi:10.1007/BFb0064579. ISBN 978-3-540-09563-7.
- Barr, Michael (1995). "Non-symmetric *-autonomous Categories". Theoretical Computer Science. 139: 115–130. doi:10.1016/0304-3975(94)00089-2. S2CID 14721961.
- Barr, Michael (1999). "*-autonomous categories: once more around the track" (PDF). Theory and Applications of Categories. 6: 5–24. CiteSeerX 10.1.1.39.881.
- Boyarchenko, Mitya; Drinfeld, Vladimir (2013). "A duality formalism in the spirit of Grothendieck and Verdier". Quantum Topology. 4 (4): 447–489. arXiv:1108.6020. doi:10.4171/QT/45. MR 3134025. S2CID 55605535.
- star-autonomous category at the nLab