Question of Tom Leinster of 26 Nov: one-object closed categories
Tom observed that an abelian monoid is a symmetric monoidal closed category with one object, and asked whether anyone had studied categories enriched in such a closed category. Eilenberg and I, in our long article [Closed categories, in Proc. Conf. on Categorical Algebra (La Jolla, 1965), Springer-Verlag 1966, 421 - 562] remarked in our "Examples" section (Ch.4, Section3, page 553) that these are, to within isomorphism, the only one-object s.m.closed categories. The odd thing is that we don't seem to have looked at V-categories for such a V - perhaps we did not want to give trivial-looking examples, although we gave other little examples such as Heyting algebras. The use we made of such a V arising from an abelian monoid M was to give an interesting but unusual example of a monoidal functor. We observed that a monoidal functor f: V --> Ab was the same thing as an M-algebra, commutative precisely when the monoidal functor f is symmetric. Anyway, I had a brief look at V-categories for such a V tonight, but with too few details so far to say much about them before bedtime. Queer little creatures, aren't they? A V-category A has objects a, b. c. and so on, but each A(a,b) is the unique object * of V. All the action takes place at the level of j: I --> A(a,a) and M: A(b,c) o A(a,b) --> A(a,c). Sorry to use the traditional M for composition, when it was the monoid. Let the monoid be G. When G is an abelian group, the M and j seem to be determined by elements N_a,b depending on two objects of A. There is more meat in a V-functor. I look forward to working through this - as I suppose Tom has done - and looking especially at the cases where G is a two-element monoid. The underlying ordinary category A_o of the V-category A seems to be an odd beast. While I don't remember ever working through this, it still rings a bell. Somewhere I have seen arrows decorated with something like numbers (elements of G ?). Ross Street and his colleagues at Macquarie often use the word "suspension" for the process of seeing an abelian as a one-object monoidal category, a monoidal category as a one-object bicategory, and such things - I don't know the full definition of "suspension"; but they and numerous others are well aware of, for instance, the tricategory with one object, whose 1-cells are commutative rings, whose 2-cells are two-sided algebras, whose 3-cells are bimodules, and whose 4-cells are bimodule-homomorphisms. Clearly I got that wrong; I think the objects should have been the commutative rings. Anyway, people doing this work are very likely to have seen and used such one-object V. Still, it looks like fun; and I've never seen an exposition of V-Cat for the case where G is the two-element group. I'm taking it to bed. Max Kelly.
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maxk@maths.usyd.edu.au