nLab
Tall-Wraith monoid

Idea

Given an algebraic theory $V$, a $V$-algebra is a model of $V$ in the category $\mathrm{Set}$. A Tall–Wraith $V$-monoid is the kind of thing that acts on $V$-algebras.

Definition

Definition

Let $V$ be an algebraic theory and let $V\mathrm{Alg}$ be the category of models of this theory in $\mathrm{Set}$. Then a Tall–Wraith $V$-monoid is a monoid object in the category of co-$V$-objects in $V\mathrm{Alg}$.

To see why these are what acts on $V$-algebras one needs to understand what a co-$V$-object in $V\mathrm{Alg}$ actually is. A co-$V$-object in some category $D$ is a representable covariant functor from $D$ to $V\mathrm{Alg}$. To give a particular $D$-object, $d$, the structure of a co-$V$-object is to give a lift of the $\mathrm{Set}$-valued $\mathrm{Hom}$-functor $D(d,-)$ to $V\mathrm{Alg}$. Thus a co-$V$-object in $V\mathrm{Alg}$ is a representable covariant functor from $V\mathrm{Alg}$ to itself.

One can therefore consider composition of such representable covariant functors. The main result of this can be simply stated:

Proposition

The composition of representable covariant functors $V\mathrm{Alg}\to V\mathrm{Alg}$ 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 $V\mathrm{Alg}$ 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-$V$-algebra objects in $V$, say $R_1$ and $R_2$, there is a product $R_1\odot R_2$ and a natural isomorphism

$${\mathrm{Hom}}_V(R_1\odot R_2,A)\cong {\mathrm{Hom}}_V(R_1,{\mathrm{Hom}}_V(R_2,A))$$Hom_V(R_1 \odot R_2,A) \cong Hom_V(R_1, Hom_V(R_2,A))

for any $V$-algebra, $A$.

A Tall–Wraith $V$-monoid is thus a triple $(P,\mu ,\eta )$ with $\mu :P\odot P\to P$ and $\eta :I\to P$ (where $I$ is the free $V$-algebra on one element — this represents the identity functor), satisfying the obvious coherence diagrams. An action of $P$ on a $V$-algebra, say $A$, is then a morphism $\rho :P\odot A\to A$ again satisfying certain coherence diagrams.

Ah, but I have not told you what $P\odot A$ is! At the moment, one can take the “product” of two co-$V$-algebra objects in $V\mathrm{Alg}$ but now I want to take the product of a co-$V$-algebra object with a $V$-algebra. How do I do this? I do this by observing that a $V$-algebra is a co-$\mathrm{Set}$-algebra object in $V\mathrm{Alg}$! That’s a complicated way of saying that $V$ represents a covariant functor $V\mathrm{Alg}\to \mathrm{Set}$. Precomposing this with the functor represented by $P$ yields again a covariant functor $V\mathrm{Alg}\to \mathrm{Set}$. This is again representable and we write its representing object $P\odot A$.

As an aside, we note a consequence. As we’ve seen, the category of co-$V$-algebra objects in $V$ is a monoidal category, with the tensor product $\odot$. Now we’re seeing this monoidal category acts on the category of $V$-algebras. Indeed, it acts on the categories of $V$-algebra and co-$V$-algebra objects in a reasonably arbitrary base category.

One postscript to this is that although the category of co-$V$-algebra objects in $V\mathrm{Alg}$ is not a variety of algebras, for a specific Tall–Wraith $V$-monoid $P$, the category of $P$-modules is a variety of algebras.

Examples

• If $V$ is the theory of commutative unital rings, a $V$-algebra is a commutative unital ring, a co-$V$-algebra object in $V$ is a biring and the corresponding sort of Tall–Wraith $V$-monoid is called, in Tall and Wraith’s original paper, a biring triple.

• If $V$ is the theory of commutative associative algebras over a field $k$, then a $V$-algebra is a commutative associative algebra over $k$, and the corresponding sort of Tall–Wraith $V$-monoid is called a plethory.

• If $V$ is the theory of abelian groups, than a $V$-algebra is an abelian group, and the corresponding sort of Tall–Wraith $V$-monoid is a ring.

  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 ${\mathrm{AbGp}}^{\mathrm{op}}$ (though this picture gets the morphisms the wrong way around; in full abstraction then the category of co-abelian group objects in $\mathrm{AbGrp}$ is the opposite category of the category of abelian group objects in ${\mathrm{AbGrp}}^{\mathrm{op}}$). Concretely, such a thing is an abelian group $A$ together with group homomorphisms

  $$\begin{array}{rl}\mu & :A\to A\coprod A,\\ ϵ& :A\to I\\ \iota & :A\to A\end{array}$$\begin{aligned} \mu &: A \to A \coprod A, \\ \epsilon &: A \to I \\ \iota &: A \to A \end{aligned}

  where $I$ 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, $I=\{0\}$. Thus $ϵ$ is forced to be the map that sends everything to $0$: we have no choice here.

  We also have that $A\coprod A=A\oplus A$. That means that for $a\in A$, $\mu (a)=(a_1,a_2)$ for some $a_1,a_2\in A$. Now, one of the laws says that $ϵ$ is a counit for $\mu$. This means that $(ϵ\oplus 1)\mu =1$ and similarly for $1\oplus ϵ$. Thus $a_1=a_2=a$ and $\mu$ is the diagonal map. So, we have no choice here either.

  The diagram for $\iota$ (representing the inverse map) is a little more complicated. As $I$ is the initial object in $\mathrm{AbGrp}$, there is a unique morphism $I\to A$ (inclusion of the zero). Composing this with $ϵ$ yields a morphism $A\to A$ which maps every element to the zero in $A$. Using $\mu$ and $\iota$ we can construct another morphism $A\to A$ as

  $$A\stackrel{\mu }{\to }A\coprod A\stackrel{1\coprod \iota }{\to }A\coprod A\stackrel{{\Delta }^c}{\to }A$$A \overset{\mu}\rightarrow A \coprod A \overset{1 \coprod \iota}\rightarrow A \coprod A \overset{\Delta^c}\rightarrow A

  where ${\Delta }^c$ is the co-diagonal. The relations for abelian groups say that this morphism must be the same as the zero morphism $A\to A$. Using the fact that $A\coprod A\cong A\oplus A$ and that $\mu$ is the diagonal, this says that $a+\iota (a)=0$. Hence, by the uniqueness of inverses for abelian groups, $\iota (a)=-a$.

  Thus if $(A,\mu ,\iota ,ϵ)$ is a co-abelian group object in $\mathrm{AbGrp}$ then $\mu$ is the diagonal, $\iota$ the inverse from abelian groups, and $ϵ$ the zero morphism.

  However, that is still not quite the same as saying that $(A,\mu ,\iota ,ϵ)$ is a co-abelian group object in $\mathrm{AbGrp}$. Certainly, $(A,\mu ,ϵ)$ is a co-commutative co-monoid object in $\mathrm{AbGrp}$ since $\mu$ is the diagonal, which is automatically co-commutative and co-associative, and $ϵ$ the zero map, which is the co-unit for the diagonal. What remains is to fit $\iota$ into the structure.

  The first issue is that $\iota$ is not automatically a morphism in $\mathrm{AbGrp}$. 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 $\iota$ is a morphism of abelian groups and so $(A,\mu ,\iota ,ϵ)$ 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 $\iota$ is the inverse for $\mu$. The relation that $\iota$ is the inverse for addition (let us write it as, say, $\alpha$) is that

  $$A\stackrel{\Delta }{\to }A\times A\stackrel{1\times \iota }{\to }A\times A\stackrel{\alpha }{\to }A$$A \overset{\Delta}\rightarrow A \times A \overset{1 \times \iota}\rightarrow A \times A \overset{\alpha}\rightarrow A

  is the zero map $A\to I\to A$. This is precisely the relation that $\iota$ is the inverse for $\mu$ since we have the following identifications: $\mu =\Delta$, $A\coprod A=A\times A$, and ${\Delta }^c=\alpha$. Also, $ϵ=0$ and $\eta :I\to A$ is the initial morphism in $\mathrm{AbGrp}$.

  Thus the fact that $\iota$ is the inverse for the diagonal+zero co-monoidal structure is due to the fact that $\iota$ is the inverse for $(\alpha ,\eta )$ and $\alpha :A\oplus A\to A$ is the co-diagonal in $\mathrm{AbGrp}$ and $\eta :I\to A$ is the unit.

  It is part of the general theory that the category of co-$V$-objects in $V$ 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!

References

• D. Tall, G. Wraith, Representable functors and operations on rings, Proc. London Math. Soc. (3), 1970, 619–643, MR265348, doi

• J. Borger, B. Wieland, Plethystic algebra, Adv. Math. 194 (2005), no. 2, 246–283, doi, pdf, MR2006i:13044

• A. Stacey and S. Whitehouse, The Hunting of the Hopf Ring, Homology, Homotopy and Applications 11(2), 2009, 75–132, online, arXiv/0711.3722.

An old and long query-discussion has been archived starting here.