symmetric monoidal (∞,1)-category of spectra
Given an algebraic theory , a -algebra is a model of in the category . A Tall–Wraith -monoid is the kind of thing that acts on -algebras.
To see why these are what acts on -algebras one needs to understand what a co--object in actually is. A co--object in some category is a representable covariant functor from to . To give a particular -object, , the structure of a co--object is to give a lift of the -valued -functor to . Thus a co--object in is a representable covariant functor from to itself.
One can therefore consider composition of such representable covariant functors. The main result of this can be simply stated:
The composition of representable covariant functors is again representable.
This is a basic result in general algebra, and is not stated here in its full generality.
An almost corollary of this is that the category of representable covariant functors from to itself is monoidal (the “almost” refers to the fact that you have to show that the identity functor is representable, but this is not hard).
Thus for two co--algebra objects in , say and , there is a product and a natural isomorphism
for any -algebra, .
A Tall–Wraith -monoid is thus a triple with and (where is the free -algebra on one element — this represents the identity functor), satisfying the obvious coherence diagrams. An action of on a -algebra, say , is then a morphism again satisfying certain coherence diagrams.
Ah, but I have not told you what is! At the moment, one can take the “product” of two co--algebra objects in but now I want to take the product of a co--algebra object with a -algebra. How do I do this? I do this by observing that a -algebra is a co--algebra object in ! That’s a complicated way of saying that represents a covariant functor . Precomposing this with the functor represented by yields again a covariant functor . This is again representable and we write its representing object .
As an aside, we note a consequence. As we’ve seen, the category of co--algebra objects in is a monoidal category, with the tensor product . Now we’re seeing this monoidal category acts on the category of -algebras. Indeed, it acts on the categories of -algebra and co--algebra objects in a reasonably arbitrary base category.
One postscript to this is that although the category of co--algebra objects in is not a variety of algebras, for a specific Tall–Wraith -monoid , the category of -modules is a variety of algebras.
If is the theory of commutative unital rings, a -algebra is a commutative unital ring, a co--algebra object in is a biring and the corresponding sort of Tall–Wraith -monoid is called, in Tall and Wraith’s original paper, a biring triple.
To understand the last example, we need to think about co-abelian group objects in the category of abelian groups. Abstractly, such a thing is an abelian group object internal to (though this picture gets the morphisms the wrong way around; in full abstraction then the category of co-abelian group objects in is the opposite category of the category of abelian group objects in ). Concretely, such a thing is an abelian group together with group homomorphisms
where is the initial object in the category of abelian groups. These homomorphisms must satisfy certain laws: just the abelian group axioms written out diagrammatically, with all the arrows turned around.
In fact, . Thus is forced to be the map that sends everything to : we have no choice here.
We also have that . That means that for , for some . Now, one of the laws says that is a counit for . This means that and similarly for . Thus and is the diagonal map. So, we have no choice here either.
The diagram for (representing the inverse map) is a little more complicated. As is the initial object in , there is a unique morphism (inclusion of the zero). Composing this with yields a morphism which maps every element to the zero in . Using and we can construct another morphism as
where is the co-diagonal. The relations for abelian groups say that this morphism must be the same as the zero morphism . Using the fact that and that is the diagonal, this says that . Hence, by the uniqueness of inverses for abelian groups, .
Thus if is a co-abelian group object in then is the diagonal, the inverse from abelian groups, and the zero morphism.
However, that is still not quite the same as saying that is a co-abelian group object in . Certainly, is a co-commutative co-monoid object in since is the diagonal, which is automatically co-commutative and co-associative, and the zero map, which is the co-unit for the diagonal. What remains is to fit into the structure.
The first issue is that is not automatically a morphism in . That is to say, when defining an algebraic theory then the operations are defined on the underlying objects. It is a consequence of the relations of abelian groups that the operations lift to morphisms of abelian groups (algebraic theories where this happens for all operations are sometimes called commutative). Thus is a morphism of abelian groups and so is a co-commutative co-monoid with an extra unary co-operation. In fact, it is an involution from the relations for abelian groups.
The final relation is that is the inverse for . The relation that is the inverse for addition (let us write it as, say, ) is that
is the zero map . This is precisely the relation that is the inverse for since we have the following identifications: , , and . Also, and is the initial morphism in .
Thus the fact that is the inverse for the diagonal+zero co-monoidal structure is due to the fact that is the inverse for and is the co-diagonal in and is the unit.
It is part of the general theory that the category of co--objects in is monoidal (though not, in general, symmetric). For details on this see The Hunting of the Hopf Ring, referred to belelow. This monoidal structure for abelian groups turns out to be the tensor product.
Thus a Tall–Wraith monoid for abelian groups is actually an ordinary monoid in the category of abelian groups: in other words, a ring!
An old and long query-discussion has been archived starting here.