category with duals (list of them)
dualizable object (what they have)
A monoidal category is semicartesian if the unit for the tensor product is a terminal object. This a weakening of the concept of cartesian monoidal category, which might seem like pointless centipede mathematics were it not for the existence of interesting examples and applications.
Many semicartesian monoidal categories are also symmetric, and sometimes that is included in the definition.
Some examples of semicartesian monoidal categories that are not cartesian include the following.
The category of Poisson manifolds with the usual product of Poisson manifolds as its tensor product.
The category of convex spaces, also known as ‘barycentric algebras’, made into a closed monoidal category where the internal hom has being the set of convex linear maps from to , made into an barycentric algebra via pointwise operations.
The thin category associated to the linear order of extended nonnegative real numbers with addition as the tensor product and internal hom as truncated subtraction.
If is any monoidal category, being the monoidal unit, the slice category inherits a monoidal product given by
where the isomorphism displayed is the canonical one. This monoidal product is semicartesian. The forgetful functor is strong monoidal, and is universal in the sense of exhibiting the fact that semicartesian monoidal functors and strong monoidal functors form a coreflective sub-bicategory of the bicategory of monoidal categories and strong monoidal functors. (Check this.)
The internal logic of a (symmetric) semicartesian monoidal category is affine logic, which is like linear logic but permits the weakening rule (and also the exchange rule, if the monoidal structure is symmetric).
In a semicartesian monoidal category, any tensor product of objects comes equipped with morphisms
respectively, where stands for the unique morphism to the terminal object and , are the right and left unitors. We can thus ask whether and make into the product of and . If so, it is a theorem that is a cartesian monoidal category. (This theorem is probably in Eilenberg and Kelly’s paper on closed categories, but they may not have been the first to note it.)
Alternatively, suppose that is a monoidal category equipped with monoidal natural transformations and such that
are identity morphisms. Then is a cartesian monoidal category.
So, suppose is a semicartesian monoidal category. The unique map is a monoidal natural transformation. Thus, if there exists a monoidal natural transformation obeying the above two conditions, is cartesian. The converse is also true.
The characterization of cartesian monoidal categories in terms of and , apparently discovered by Robin Houston, is mentioned here:
and as of 2014, Nick Gurski plans to write up the proof in a paper on semicartesian monads.
It is well-known that any functor between cartesian monoidal categories is automatically and uniquely colax monoidal; the colax structure maps are the comparison maps for the cartesian product. (This also follows from abstract nonsense given that the 2-monad for cartesian monoidal categories is colax-idempotent.) An inspection of the proof reveals that this property only requires the domain category to be semicartesian monoidal, although the codomain must still be cartesian.
The notion of semicartesian operad? is a type of generalized multicategory which corresponds to semicartesian monoidal categories in the same way that operads correspond to (perhaps symmetric) monoidal categories and Lawvere theories correspond to cartesian monoidal categories. Applications of semicartesian operads include:
Generalized operads in classical algebraic topology (blog post) – this also uses the above fact about colax functors
Characterizing finite measure spaces (blog comment)
A relevance monoidal category is the “dual” of a semicartesian monoidal category, with diagonals but not projections.