A comma double category is a generalization of a comma category to double categories and virtual double categories.
There are two apparently-maximal levels of generality of this construction, which intersect nontrivially but do not coincide. The first is an ordinary comma object in the 2-category of virtual double categories (which includes as a full sub-2-category the 2-category of pseudo double categories and lax functors), which produces another virtual double category. The second is a colax/lax comma object relative to the 2-monad whose algebras are pseudo double categories. The two constructions intersect in the case of the comma object of a pseudo double functor over a lax one, between pseudo double categories.
For virtual double categories, the comma virtual double category has the universal property of a comma object in the 2-category of virtual double categories, functors and vertical transformations.
Explicitly, it is constructed as follows. Let be virtual double categories and be functors of virtual double categories. The comma double category is defined as
Let be the 2-category of directed graphs internal to Cat. There is a 2-monad on whose algebras are (pseudo) double categories, and whose lax and colax morphisms are lax and colax double functors respectively. The oplax/lax comma double category is then an oplax/lax comma object for this 2-monad.
Next, we consider what properties are required of the input data (in the virtual case) to determine that a comma virtual double category has units and composites. An analogy with the double category case gives some guidance. Since functors of virtual double categories correspond to lax functors of double categories, we don’t have any requirements for the functor on top of having composites or units. On the other hand, for to be “oplax”, we require that it be normal for units or furthermore strong for composites.
If , and have units and is a normal functor, then has units.
If , and have composites and is a strong functor, then has composites.
Next, the comma has restrictions whenever the constituent categories do and the functors preserve them.
If , and have restrictions and preserve them, then has restrictions.
In practice this proof burden can be reduced if we are interested in virtual equipments (i.e. having units and restrictions) because in that case restrictions are automatically preserved. We summarize this as follows:
If and are virtual equipments and is a normal functor, then is a virtual equipment.
A monad in the horizontal bicategory of a double category is equivalent to a lax functor from the terminal double category. In this case we might call the slice double category?.
generalized multicategories can be constructed using a slice category when the monad is a polynomial monad. Specifically, let be the double category of polynomial functors in some locally cartesian closed category ; then a polynomial monad on can be identified with a horizontal monad in on the terminal object . The slice is then equivalent to the “horizontal Kleisli category” presented in Cruttwell-Shulman; -multicategories are then monads in that comma double category.
The double category of decorated cospans is naturally constructed as a comma double category. Given a symmetric lax monoidal functor , there is an associated lax double functor from where is the delooping of into a double category whose horizontal category is and vertical category is the terminal category. Then there is a colax (pseudo even) double functor that picks out the unique object of . Then the double category of decorated cospans is .
poset-valued sets given by an endofunctor on and a poset can be viewed as the comma double category from to , since a poset is a monad in , and is a colax endofunctor of . The “morphisms” of poset-valued sets are the horizontal morphisms in the resulting comma double category.
The Dialectica construction associated to an internal poset in a monoidal category with pullbacks can be obtained as a comma double category. Let be the double category whose horizontal morphisms are spans in , regard as a double category in the horizontal direction, and let be the colax functor defined on objects by and taking the pair (so that and ) to the span . The internal poset is a monad in , so we have a comma double category , whose horizontal category is the Dialectica construction .
The double gluing construction relative to a pair of functors and can be phrased as a comma double category of the cospan , where and are regarded as vertically discrete double categories and is the double Chu construction. To obtain the relevant monoidal structures we can consider this instead to be a comma object in double polycategories?.
An explicit description of the comma double category from an oplax to a lax functor is given in
Last revised on August 1, 2024 at 16:01:55. See the history of this page for a list of all contributions to it.