Kan extensions in the $2$-category of monoidal categories - MathOverflowmost recent 30 from mathoverflow.net2019-10-20T18:07:44Zhttps://mathoverflow.net/feeds/question/205838https://creativecommons.org/licenses/by-sa/4.0/rdfhttps://mathoverflow.net/q/2058386Kan extensions in the $2$-category of monoidal categoriesMartin Brandenburghttps://mathoverflow.net/users/28412015-05-06T07:46:35Z2015-06-24T22:04:14Z
<p>Kan extensions make sense in any $2$-category. But so far I have only really seen them in the case of the $2$-category of categories, functors, natural transformations and the $2$-category of $k$-linear categories, $k$-linear functors, natural transformations. Where have Kan extensions been studied and used explicitly for other $2$-categories (a related question was asked <a href="https://mathoverflow.net/questions/196351/kan-extensions-in-concrete-2-categories">here</a>, but this doesn't really go into examples)? In particular I am interested in the $2$-category of monoidal categories, lax monoidal functors and lax monoidal natural transformations, also with "strong" instead of "lax". For example, the left Kan extension of a lax monoidal functor $(f,\eta,\mu) : I \to A$ (between monoidal categories $I,A$) along the unique lax monoidal functor $I \to \{1\}$ is a universal monoid object $M=(X,\eta,\mu)$ in $A$ equipped with a cocone $\{\alpha_i : f(i) \to X\}_{i \in \mathrm{Ob}(I)}$ which is lax monoidal in the sense that (a) $1 \xrightarrow{\eta} f(1) \xrightarrow{\alpha_1} X$ equals $1 \xrightarrow{\eta} X$ and (b) the diagram
$$
\begin{array}{ccc}
f(i \otimes j) & \xrightarrow{\alpha_{i \otimes j}} & X \\ {\scriptsize\mu}\uparrow ~~&&~~\uparrow{\scriptsize\mu} \\ f(i) \otimes f(j) & \xrightarrow{\alpha_i \otimes \alpha_j} & X \otimes X
\end{array}
$$
commutes. Is there any interesting example for this kind of "lax monoidal colimit"? If yes, what does it mean for a monoidal category to be "lax monoidal cocomplete"? Consider for example the case that the underlying category of a monoidal category is cocomplete and the tensor product is cocontinuous in each variable (this is what I would call a cocomplete monoidal category, or perhaps more precisely, a monoidal cocomplete category, because this is precisely a monoid in the symmetric monoidal $2$-category of cocomplete categories), which is quite typical. Do "lax monoidal colimits" exist then? What about "lax monoidal left Kan extensions" in general or "strong monoidal left Kan extensions" in general?</p>
https://mathoverflow.net/questions/205838/kan-extensions-in-the-2-category-of-monoidal-categories/205909#2059096Answer by Damiano Mazza for Kan extensions in the $2$-category of monoidal categoriesDamiano Mazzahttps://mathoverflow.net/users/450272015-05-06T21:34:30Z2015-05-06T21:34:30Z<p>This does not answer the specific questions you ask but is a paper which is focused precisely on computing monoidal Kan extensions:</p>
<p>Paul-André Melliès and Nicolas Tabareau. <a href="http://hal.archives-ouvertes.fr/hal-00339331/en/" rel="noreferrer">Free models of T-algebraic theories computed as Kan extensions</a>. Preprint. (Presented at the International Category Theory Conference, Calais, 2008).</p>
<p>One of the main results is a sufficient condition for the usual end/coend formula (which computes the object part of Kan extensions in $\mathbf{Cat}$) to be also valid in $\mathbf{SymMonCat}$ (or just $\mathbf{MonCat}$ I guess). Their original motivation, if I remember correctly, is to understand the existence/non-existence of free commutative (co)monoids in a given category. Adapting Lawvere's approach to the monoidal case, a commutative monoid in a symmetric monoidal category $\mathcal C$ is a strict monoidal functor from the PROP $\mathbb F$ (finite ordinals and arbitrary functions between them) to $\mathcal C$, so the free commutative monoid on an object $A$ of $\mathcal C$ is the left Kan extension of $A$ (seen as a strict monoidal functor from the PROP $\mathbb B$ of finite ordinals and bijections) along the inclusion $\mathbb B\hookrightarrow\mathbb F$. However, the Kan extension must be computed in $\mathbf{SymMonCat}$ (with <em>strict</em> monoidal functors as $1$-cells), otherwise we do not even find a monoid. That's why the usual end formula may not work, so the authors try to explain when and why.</p>
<p>Of course, this not only fails to address your specific questions, but it may have nothing to do with what you need, because the above paper is mostly interested in <em>strict</em> monoidal functors. However, some of the definitions and results should work for strong functors too (I don't know about lax). The use of <em>symmetric</em> monoidal categories, on the other hand, is not essential (the reason for their interest in <em>commutative</em> (co)monoids is that, originally, the authors were trying to give an explicit formula for the free exponential modality $!A$ of linear logic, which is a free commutative comonoid). Anyway, I thought it would not hurt to give you this reference.</p>
https://mathoverflow.net/questions/205838/kan-extensions-in-the-2-category-of-monoidal-categories/210088#2100888Answer by Yonatan Harpaz for Kan extensions in the $2$-category of monoidal categoriesYonatan Harpazhttps://mathoverflow.net/users/511642015-06-24T22:04:14Z2015-06-24T22:04:14Z<p>I believe that this is a particular case of Lurie's "operadic left Kan extension". We may identify a monoidal $\infty$-category $\mathcal{C}$ with a coCartesian fibrations of $\infty$-operads $\mathcal{C}^{\otimes} \longrightarrow \mathcal{Ass}^{\otimes}$ where $\mathcal{Ass}^{\otimes}$ is the associative operad considered as an $\infty$-operad. If $\mathcal{A}^{\otimes} \longrightarrow \mathcal{Ass}^{\otimes}$ is another coCartesian fibration (i.e., another monoidal $\infty$-category $\mathcal{A}$), then the notion of a lax monoidal functor $\mathcal{A} \longrightarrow \mathcal{C}$ is given by the notion of a map of $\infty$-operads $\mathcal{A}^{\otimes} \longrightarrow \mathcal{C}^{\otimes}$ over $\mathcal{Ass}^{\otimes}$, and can be thought of as the data of an $\mathcal{A}$-algebra object in $\mathcal{C}$. For this reason the corresponding $\infty$-category of lax monoidal functors is sometimes denoted by $Alg_{\mathcal{A}}(\mathcal{C})$. Now if $\mathcal{A}^{\otimes} \longrightarrow \mathcal{B}^{\otimes} \longrightarrow \mathcal{Ass}^{\otimes}$ are maps of $\infty$-operads then the formation of operadic left Kan extension over $Ass^{\otimes}$ yields a left adjoint
$$ LK: Alg_{\mathcal{A}}(\mathcal{C}) \longrightarrow Alg_{\mathcal{B}}(\mathcal{C}) $$
to the forgetful functor $Alg_{\mathcal{B}}(\mathcal{C}) \longrightarrow Alg_{\mathcal{A}}(\mathcal{C})$. We may hence think about it as associating to an $\mathcal{A}$-algebra $X$ the free $\mathcal{B}$-algebra generated from $X$. This construction is studied in section 3.1.3 of "Higher Algebra". A general existence result for free algebras is given by Corollary 3.1.3.5. It essentially says exactly what you propose in your question: if a monoidal category $\mathcal{C}$ has colimits and these are preserved by the tensor product in each variable separately then $\mathcal{C}$ admits the formation of free algebras (and in particular the formation of lax monoidal Kan extensions as you describe). </p>