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
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 .
Compact closed categories are *-autonomous, with the dual of the monoidal unit as the dualizing object, but *-autonomous categories need not to be compact closed: is not necessarily a dual of . However, if in a *-autonomous category we have
for each pair (A,B) of objects, then the category is compact closed.
A familiar example is given by matrix theory as finite-dimensional linear algebra, namely 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.
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.
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 which 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.
- Michael Barr (1979). "*-autonomous Categories". In Springer-Verlag. Lecture Notes in Mathematics 752.
- Michael Barr (1995). "Non-symmetric *-autonomous Categories". Theoretical Computer Science 139: 115–130. doi:10.1016/0304-3975(94)00089-2.
- Michael Barr (1999). "*-autonomous categories: once more around the track". Theory and Applications of Categories 6: 5–24.