Crans-Gray tensor product


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The Crans-Gray tensor product is a tensor product on the category of strict omega-categories which is analogous to (the lax version of) the Gray tensor product for 2-categories.

This tensor product makes the category StrωCatStr\omega Cat of strict omega-categories into a (non-symmetric) biclosed monoidal structure; thus in particular it has two internal-homs Hom rHom_r and Hom lHom_l. Both contain strict ω\omega-functors as their objects, and their kk-cells for k>0k\gt 0 are k-transfors which are lax or oplax in all dimensions (one internal-hom contains lax transfors and the other the oplax ones).

Construction from cubical sets

One abstract way to construct the Crans-Gray tensor product is as follows.

There is an obvious monoidal structure on the cube category obtained from the obvious product of cellular cubes which is analogous to the cartesian product of the topological cubes [0,1] n[0,1]^n. By Day convolution this naturally induces a monoidal structure on cubical sets.

Now strict ω\omega-categories, although usually defined as globular sets with extra structure, can also be regarded as cubical sets with extra structure. More precisely, there is a forgetful functor from StrωCatStr\omega Cat to CubSetCubSet which is monadic, and the monad is a monoidal monad. It follows by further general results of Day that the tensor product on CubSetCubSet lifts to StrωCatStr\omega Cat.

It is also possible to give a direct construction of this tensor product; see Crans’ thesis below.

Relation to the Gray tensor product

Strict nn-categories for n<ωn\lt\omega can of course be regarded as strict ω\omega-categories with all kk-cells identities for k>nk\gt n. In general, the Crans-Gray tensor product of an nn-category with an mm-category will be an (n+m)(n+m)-category, since the tensor product of the nn-cube and the mm-cube is the (n+m)(n+m)-cube. (Compare, for example, the fact that the Gray tensor product of two 1-categories is no longer, in general, a 1-category.)

However, we can obtain a tensor product on strict nn-categories for any nn by applying the reflector (or “cotruncation”) functor for the inclusion of strict nn-categories into strict ω\omega-categories. When n=1n=1, this produces the cartesian product of categories, while when n=2n=2 it reproduces the lax version of the Gray tensor product.

Sjoerd Crans has also defined a tensor product of Gray-categories, but the relationship between it and the tensor product of strict ω\omega-categories is not entirely clear, since neither StrωCatStr\omega Cat nor GrayCatGrayCat contains the other (their “intersection” is Str3CatStr3Cat).


The Crans-Gray tensor product makes StrωCatStr\omega Cat into a biclosed monoidal category. So in particular for X,YStrωCatX, Y \in Str\omega Cat two strict ω\omega-categories, there is an ω\omega-functor category between them defined by

[X,Y]=StrωCat(XG ,Y), [X,Y] = Str\omega Cat( X \otimes G^\bullet, Y) \,,

where \otimes is the CG-tensor product and G :GωCatG^\bullet : G \to \omega Cat is the globular object of standard globes. The objects in [X,Y][X,Y] are the (strict) ω\omega-functors, while the k-morphisms are a lax sort of kk-transfors between these. A dual ω\omega-functor category can be defined by X,Y=StrωCat(G X,Y)\langle X,Y\rangle = Str\omega Cat(G^{\bullet}\otimes X, Y); this has the same objects but its k-morphisms are oplax kk-transfors.

Or maybe lax and oplax should be switched here? Can someone verify?

Further Remarks


A detailed discussion is in

Some helpful remarks and diagrams are in

which is however mainly concerned with a slightly different topic.

A collection of Crans’s papers, including those on teisi, can be found here:

The modern version, incorporating many new primitives is given in

A general theory of lax tensor products is in