# Generalization of analytic functors

A functor $F\colon \bf Sets\to Sets$ is said to be analytic if it results from the left Kan extension of a functor $f\colon \mathbf{Bij}(\mathbb N)\to \bf Sets$ (the "species" of the functors $F$) along the natural inclusion $\mathbf{Bij}(\mathbb N)\to \bf Sets$, where $\mathbf{Bij}(\mathbb N)$ is the category having objects natural numbers and where $\mathbf{Bij}(\mathbb N)(m,n)$ are the bijective functions $\{1,\dots,m\}\to \{1,\dots,n\}$ (empty if $n\neq m$). Representing a left Kan extension as a coend it means that $$F(T)\cong \int^n T^n\times f(n)$$ (the most of you will recognize the fact that a functor is "anaytic" if it can be written in Taylor form, and the coend is in a suitable sense exactly that Taylor series) This can be expressed replacing $\bf Sets$ with any symmetric monoidally cocomplete category: there is a functor $\mathbf{Bij}(\mathbb N)^\text{op}\times \mathcal V\to \mathcal V\colon (n,V)\mapsto V\otimes\dots\otimes V=V^{\otimes n}$, which allows to define $$\int^n V^{\otimes n}\otimes f(n)$$ for any "species" $f\colon \mathbf{Bij}(\mathbb N)\to \mathcal V$.

In the case of $\bf Sets$, it seems to be possible to extend the definition of an analytic functor to the case of an arbitrary cardinal $\kappa$:

1. the category $\mathbf{Bij}(\kappa)$ is defined to be the category having objects cardinals $<\kappa$ and $\mathbf{Bij}(\kappa)(\mu,\nu)$ bijections between those cardinals (hence empty if $\mu\neq\nu$).
2. I say that a functor $F\colon\bf Sets\to Sets$ is $\kappa$-analytic if it results from $\text{Lan}_jf$ in the diagram $$\begin{array}{ccc} \mathbf{Bij}(\kappa) &\to& \bf Sets \\ \downarrow&\nearrow_F&\\ \bf Sets & \end{array}$$ $F(T)\cong \int^{\mu}T^\mu\times f(\mu)$ where $T^\mu$ is the product of $\mu$ copies of $T$, in the obvious sense.

Now my question: can I define something analogous in the case of a generic monoidal(ly cocomplete) category, maybe imposing some additional property? In the end all boils down to the possibility of defining a $\kappa$-fold tensor product $\otimes\colon \mathcal V^\kappa\to \mathcal V$, for any cardinal $\kappa$. Modules over a ring $R$ seem to have this "extension property", together with sets.

Being these two very explicit and natural examples of a monoidal category where "tensoring has an arbitrarily large arity", I think the problem has its own interest, and I would be surprised if the topic was new.

• See mathoverflow.net/questions/11767/infinite-tensor-products for the case of modules. The infinite tensor product of modules is rather ugly. Lots of the usual properties fail. The simplest example $K \otimes_K K \otimes_K \dotsc$ is a twisted(!) group algebra for the monstrous group $(K^*)^I / (K^*)^{(I)}$. – Martin Brandenburg Jul 3 '13 at 21:42
• Thank for your attention, I was motivated exactly by that topic to post the question in the form of a "k-ary tensor product". I think there is no hope to recover "usual properties"; it's better to think about an axiomatization of the structure I need, but for the moment I don't have the slightest idea. I repeat myself: on the one side I find quite astounding the lack of a theory of those monoidal categories where tensoring has an arbitrarily large arity, and on the other, it's obviously due to the absence of "good" properties for such a structure. – Fosco Loregian Jul 4 '13 at 9:07
• (...) I mean that in the end, even monoids where the operation has an arbitrary arity are quite strange animals. – Fosco Loregian Jul 4 '13 at 9:09
• This question now appears on MO: mathoverflow.net/questions/138518/… hence this topic can be closed, if you want. – Fosco Loregian Aug 4 '13 at 12:01