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There are three different kinds of finite limits in categories: terminal objects, binary products, and equalizers. In a category $C$, these define functors $1_{C},\times,\mathrm{Eq}\colon\mathrm{Fun}(I,C)\to C$ where $I=\emptyset,\{\bullet\ \ \bullet\}$, and $\{\bullet\rightrightarrows\bullet\}$ respectively.

Monoidal categories generalise the first two in that we now have a functor $1_{C}$ from $\mathcal{C}^{\emptyset}=*$ to $\mathcal{C}$ and a functor $\otimes_C$ from $C^{\{\bullet\ \ \bullet\}}=C$ to $C$, i.e.\ functors \begin{align*} 1_C &\colon * \to \mathcal{C}\\ \otimes_C &\colon \mathcal{C}\times\mathcal{C} \to \mathcal{C} \end{align*} together with associativity and unitality natural isomorphisms satisfying compatibility conditions.

What about equalizers? Has the notion of a category $C$ equipped with a functor $\rm{Eq}\colon\rm{Fun}(\{\bullet\rightrightarrows\bullet\},C)\to C$, a unit functor, and unitality/associativity natural isomorphisms satisfying coherence conditions been studied before? Moreover, are there any examples of such structures "found in nature"?

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  • $\begingroup$ How do you formulate associativity for this kind of structure? I think the concept of a 2-monad is relevant here - basically you see monoidal categories as algebras for the 2-monad you have sketched, and want to change that 2-monad so that the "cartesian algebras" are not categories with finite products, but rather categories with binary equalizers (maybe more?). But right now your description has no evident 2-monad structure, at least I don't see it. $\endgroup$ May 20, 2021 at 10:36
  • $\begingroup$ In my brain, the vague idea is that product maps have something to do with the fact the three-element set is a pushout of a span of two-element sets. One can imagine analogues of this for the walking parallel pair of arrows, for instance to get a parallel triple, or to get a three-object version, with two sets of parallel pairs that are composable. Just something to consider! $\endgroup$
    – David Roberts
    May 20, 2021 at 10:48

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Here is a way to write down what a symmetric monoidal category is starting just from the idea of finite (co)products. We have $$SymMonCat = Fun^\times_{(2,1)}(Span^{fin}_{(2,1)}, Cat_{(2,1)})$$ That is, a symmetric monoidal category is equivalent to the data of a finite-product-preserving $(2,1)$-functor from the $(2,1)$-category $Span^{fin}_{(2,1)}$ of finite sets and spans between them, to the $(2,1)$-category $Cat_{(2,1)}$ of categories.

I can think of at least two ways to understand the significance of $Span^{fin}_{(2,1)}$ from the point of view of finite (co)products.

  1. There is a functor $Span^{fin}_{(2,1)} \to Cat^{\amalg}_{(2,1)}$ (where $Cat^{\amalg}_{(2,1)}$ is the $(2,1)$-category of categories with finite coproducts and finite-coproduct preserving functors). The functor sends $n \mapsto Set^{fin}/n$. This functor is fully faithful. Moreover, $Set^{fin}/n$ is itself the free coproduct completion of the discrete category $n$.

So from this perspective, we have boiled it all down to finite (co)products, but the picture is a bit odd and I'm not sure how to complete the thought from this perspective. Alternatively,

  1. Note that the $(2,2)$-category $Span^{fin}_{(2,2)}$ of finite sets, spans between them, and maps of spans has the following universal property. For any 2-category $\mathcal K$ with finite products, we have that the 2-category $Fun_{(2,2)}^\times(Span^{fin}_{(2,2)}, \mathcal K)$ of all finite-product preserving 2-functors, is equivalent to $FinProd(\mathcal K)$, the 2-category of objects of $\mathcal K$ which internally have finite products.

(2) suggests the following generalization. Let $J$ be a set of small categories, and let $2Cat(J)$ be the category of 2-categories with $J$-limits. I believe that the functor $Comp_J: 2Cat(J) \to 2Cat$ carrying $\mathcal K$ to the 2-category of objects in $\mathcal K$ which internally have $J$-limits, is corepresentable by some $\mathcal J \in 2Cat(J)$. Define a $J$-symmetric monoidal category to be an object of $Fun^J_{(2,1)}(\mathcal J_{(2,1)}, Cat)$, where $\mathcal J_{(2,1)}$ is the maximal sub $(2,1)$-category of the $(2,2)$-category $\mathcal J$, and $Fun^J_{(2,1)}$ means we take $(2,1)$-functors which preserve $J$-limits in the $(2,1)$-categorical sense.

Then in the case where $J$ consists of the finite discrete categories, a $J$-symmetric monoidal category is a usual symmetric monoidal category. When $J$ consists of just the empty category, a $J$-symmetric monoidal category is an $E_0$-category. You could try out other classes of $J$, such as equalizers, but I'm not sure what you get!

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  • $\begingroup$ There's an issue with this answer: in order to say that $K \in \mathcal K$ internally has, say, equalizers, you need $\mathcal K$ to have certain $Cat$-cotensors -- a 2-categorical limit rather than a 1-categorical one. After modifying the above story to account for this, the analogy with symmetric monoidal categories is no longer perfect. $\endgroup$
    – Tim Campion
    Dec 27, 2021 at 19:07

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