**3**

votes

**1**answer

195 views

### Generalisation of the Grothendieck construction for presheaves as a lax pullback

It is well-known that for any presheaf $F \colon \mathcal{C}^{\mathrm{op}} \to \mathrm{Set}$, the category of elements (obtained by the so-called Grothendieck construction) of $F$ is a comma category ...

**5**

votes

**1**answer

153 views

### Finitely presentable objects in functor categories

Given a locally finitely presentable category $\mathcal{C}$ it is well-known that every functor category $[\mathcal{A},\mathcal{C}]$ (where $\mathcal{A}$ is a small category) is also locally finitely ...

**1**

vote

**1**answer

153 views

### Natural bijection between sets with coloured elements?

In Andreas Blass's famous paper `Seven Trees in One', the existence of a natural bijection between binary trees and 7-tuples of binary trees is related to the equation $T^7 = T$ being satisfied by a ...

**12**

votes

**3**answers

1k views

### History and motivation for Tannaka, Krein, Grothendieck, Deligne et al. works on Tannaka-Krein theory?

I am trying to wrap my mind around Tannaka-Krein duality and it seems quite mysterious for me, as well, as its history. So let me ask:
Question: What was the motivation and historical context for ...

**3**

votes

**1**answer

154 views

### Coequalizers in the category of algebras of the double power locale monad

$\mathbf{Loc}$ is the category of locales, and $\mathbb{P}$ is the double power locale monad on it. Consider the category $\mathbf{Loc}^{\mathbb{P}}$, of algebras of this monad. Does anyone know ...

**12**

votes

**9**answers

1k views

### What are your favorite concrete examples of limits or colimits that you would compute during lunch?

(The title was initially "What are your favorite concrete examples that you would compute on the table during lunch to convince a working mathematician that the notions of limits and colimits are not ...

**1**

vote

**1**answer

147 views

### Varieties generated by a two element algebra

I have two questions regarding universal algebra, and also its ordered version.
If a variety $\mathcal{V}$ is generated by a specific two element algebra $2 = \{0,1\}$, then is that the only ...

**1**

vote

**0**answers

110 views

### What is the connection between direct/inverse image of set maps and direct/inverse image functors of sheaves?

Given a function of sets $f:X\to Y$, one defines the direct image and inverse image maps:
$$
f_*:\mathcal{P}(X)\to\mathcal{P}(Y)
$$
$$
f^{-1}:\mathcal{P}(Y)\to\mathcal{P}(X)
$$
In the usual way.
...

**1**

vote

**0**answers

180 views

### Fusion categories with permutation “associativity matrices”

Let $\mathcal{C}$ be a fusion category and let $(H_1,...,H_r)$ be its simple objects.
$\mathcal{C}$ is non-pointed if at least one of its simple object has Perron-Frobenius dimension $ \neq 1$.
...

**2**

votes

**1**answer

322 views

### What are the uses of Limits and Colimits of Category Theory in every day problems? [closed]

I am interested in knowing how we can use the concepts of Limits and Colimits in modeling problems in every day life? Could anyone provide (Software) engineering examples, perhaps? Or describe ...

**3**

votes

**2**answers

159 views

### A variety of algebras satisfying some dual conditions

I would like prove that, under the conditions described below, no non-trivial variety exists.
Let $\mathcal{V}$ be a variety of algebras e.g. rings, semigroups, semilattices.
Further suppose ...

**3**

votes

**2**answers

219 views

### Intersection of free objects

I am aware that the following question is a very basic one and therefore I would not be at all offended if it were to be closed. Moreover, I am not familiar at all with category theory.
Let ...

**7**

votes

**0**answers

166 views

### Two questions on the fibered category of enriched categories

Given a bicomplete closed symmetric monoidal category $\mathcal V$, denote by $\operatorname{Cat}(\mathcal V)$ the category of small $\mathcal V$-enriched categories. The object set functor
...

**8**

votes

**1**answer

551 views

### Analogy between topology and algebraic geometry

In topos theory, there are many generalizations of topological concepts. For example, open, closed, proper and etale morphisms between toposes. However, there are also such analogous concepts in ...

**4**

votes

**0**answers

181 views

### Limits and colimits of A_{\infty} categories

I have a question related to the discussion (Coequalizer in category of dg-algebras). How do you prove that the category of (small) dg-categories and the category of (small) A_{\infty} categories are ...

**4**

votes

**2**answers

360 views

### Is a composite of (co)monadic adjunctions (co)monadic?

I think this is probably elementary, but some searching (and asking on the chatroom) hasn't turned up a result. Could anyone point me to a reference for (or counterexample to) the following statement? ...

**2**

votes

**0**answers

98 views

### quantum deformations of tensor category

I was told that, if I understand correctly, that the enveloping algebra of semisimple Lie algebra admits one family of quantum deformation as Hopf algebra, which was proved by Drinfeld. Anyone can ...

**1**

vote

**0**answers

115 views

### Understanding the small objects argument via the orth. subcat problem

I suspect there is a strong connection between these two results, but I'm not able to work out the details; in particular I sense that there is a strong analogy between the SOA, stated in the ...

**4**

votes

**1**answer

270 views

### How to characterize flasque sheaves in more functorial way?

The motivation to ask this question is some proposition of flasque sheaves.
Let's recall the definition of flasque sheaf:A sheaf $F$ on a topological space $X$ is flasque if for every inclusion ...

**6**

votes

**1**answer

456 views

### Is “stackiness” transitive? (and a couple other basic questions about stacks)

Say, $B$ is a category fibered in groupoids over some category $C$, and $A$ is a category fibered in groupoids over $B$.
Suppose $A$ is a stack (over whatever site) over $B$, and $B$ is a stack over ...

**3**

votes

**0**answers

199 views

### Categorical proof for Chavelley theorem on affiness of scheme

The question is related to another question asked here a couple of minutes ago:
Does vanishing of cohomology of locally free sheaves imply affiness of scheme
In Hartshorne Exercise 4.2,we have the ...

**4**

votes

**0**answers

143 views

### Reconstruction of noncommutative scheme

It is known that a quasi compact scheme(even quasi separated scheme)can be determined uniquely by the category of quasi coherent sheaves on it by Gabriel-Rosenberg reconstruction theorem
The ...

**4**

votes

**2**answers

363 views

### Small objects in categories

I would like to pick out small objects from a category. I would like to find such a notion which
Dream 1. Picks out the schemes of finite type over $k$ from the category of $k$-schemes. Or at least ...

**2**

votes

**1**answer

276 views

### It looks so coKleisli, but it's not. What is it?

Fix a symmetric monoidal category $(M,\otimes,I)$ and a small discrete monoidal subcategory $M'\subseteq M$. Define a new symmetric monoidal category $C:=CoKl(M,M')$ as follows: $Ob(C):=Ob(M)$, and ...

**0**

votes

**0**answers

154 views

### A potential definition of weak $\omega$-categories

This question was inspired by the Homotopy Type Theory Book.
Might we define a weak $\omega$-category as described below?
Is any similar approach already considered in the literature?
Let ...

**1**

vote

**0**answers

141 views

### Terminology question in model category theory

This is a terminology question. That will help me to write down my papers. In Marc Olschok's PhD available here (I cannot find it anymore on the Internet so it is in my webpage, the original URL is ...

**6**

votes

**2**answers

237 views

### Is “being a modular category” a universal or categorical/algebraic property?

A semisimple braided category with duals is called modular when a certain matrix $S$ is invertible. The components $S_{AB}$ are indexed by (isomorphism classes of) simple objects of the category and ...

**0**

votes

**1**answer

126 views

### Generators in the sense of Freyd and Kelly

I am stuck in trying to interpret a definition in the paper "Categories of continuous functors" by P. Freyd and M. Kelly (click).
They say:
A category $\cal A$ with a proper factorization system ...

**2**

votes

**1**answer

176 views

### Functors with an epi-mono factorization property

This is a simple question about terminology and a request for any related references. Specifically, what would you call a functor $F:\mathbf{D}\rightarrow\mathbf{C}$ with the following property?
...

**1**

vote

**0**answers

83 views

### Which functors $\bf Cat \to Cat$ preserve $\mathbb D$-filtrancy?

In the following, I refer to [ABLR] as the paper A classification of accessible categories by Adamek, Borceux, Lack and Rosicki.
Doodling with symbols, and studying the aforementioned article, I ...

**7**

votes

**1**answer

193 views

### Correspondence between operads and monads requires tensor distribute over coproduct?

In checking the details of the correspondence between operads over a symmetric monoidal category and monads on some associated endofunctor of the category, I cannot make the obvious proof work without ...

**3**

votes

**1**answer

276 views

### Are multicolimits suitable colimits?

Today I encountered the notion of multicolimit.
Lacking a standard reference for this notion, let me give a self-contained definition of this gadget.
If $S\colon \cal K\to E$ is a diagram, we ...

**1**

vote

**0**answers

209 views

### Tensor product of d.g-algebras

I'd like to prove that the tensor product functor $- \otimes Y$, where $Y$ is a d.g-algebra over a field of characteristic 0, preserves finite products of d.g-algebras. This statement is in a paper by ...

**3**

votes

**1**answer

175 views

### Exact squares containing a cospan: what is known about this category?

Suppose I have a cospan of categories $X\xrightarrow{F} Z\xleftarrow{G}Y$ and I'm looking for exact squares containing it. The category $Exact_{F,G}$ of such things has a terminal object, namely the ...

**15**

votes

**29**answers

721 views

### Formalizations of category theory in proof assistants

What are the existing formalizations of category theory in proof assistants?
I'm primarily interested in public-domain code implementing category theory in a proof assistant (Coq, Agda, ...

**0**

votes

**1**answer

202 views

### right adjoint for pullback along fibration

Let $Grpd$ be the category of groupoids and $p:E\rightarrow B$ a fibration in the standard model structure on $Grpd$ (ie an isofibration). How do you prove that the pullback functor $p^{\star}:Grpd/B ...

**2**

votes

**1**answer

279 views

### Homotopy groups of filtered homotopy limits

Let $X$ be the homotopy limit of a filtered system of simplicial sets $X_i$. When are the morphisms $\pi_j(X)\to \varprojlim \pi_j(X_i)$ surjective for all $j\ge 0$? This seems to be no problem when ...

**6**

votes

**1**answer

262 views

### Are $(\infty,1)$-categories $A_\infty$ categories?

Let $X$ be a set. One can define a non-symmetric colored version of the non-unital $A_\infty$ operad as follows. The set of colors is the set of ordered pairs in $X$. Let ...

**2**

votes

**0**answers

113 views

### What are the Čech-local equivalences of (simplicial pre)sheaves?

Let $\mathcal{C}$ be small category and let $J$ be a Grothendieck topology on $\mathcal{C}$. The Čech model structure on $[\mathcal{C}^\mathrm{op}, \mathbf{sSet}]$ is defined to be the left Bousfield ...

**3**

votes

**0**answers

108 views

### Reference request: colimits of locally presentable categories

Consider the 2-category of locally presentable categories, cocontinuous functors, and natural transformations. I believe that this 2-category is 2-cocomplete in the sense of containing all small ...

**13**

votes

**2**answers

362 views

### Does base extension reflect the property of being isomorphic?

Let L/K be a (separable?) field extension, let A be a finite dimensional algebra over K, and let M and N be two A-modules. Let $A' = L \otimes_K A$ be the algebra given by extension of scalars, and ...

**4**

votes

**0**answers

115 views

### Construction of 3D Topological Quantum Field Theory from a 2D Modular Functor

I have been reading Bakalov and Kirillov's Lectures on Tensor Categories and Modular Functors in which the authors state that a direct construction of a $C$-extended 3D TQFT from a $C$-extended 2D ...

**1**

vote

**1**answer

275 views

### A $2$-torsion version of the motivic stable homotopy category?

For a field $k$ there exists the motivic stable homotopy $SH(k)$; it is compactly generated. My question: does there exist a 'reasonable' functor $p$ from $SH(k)$ to a certain triangulated category ...

**4**

votes

**1**answer

229 views

### Is a group scheme determined by its category of representations?

More precisely, let $G$ be an affine group scheme over a field $k$, $Rep_k(G)$ be the category of finite dimensional representations of G, and $\omega_0$ be the forgetful functor from $Rep_k(G)$ to ...

**1**

vote

**1**answer

224 views

### Is this square commutative?

Suppose that the following commutative diagram of $R$-modules is given.
$ \newcommand{\ra}[1]{\kern-1.5ex\xrightarrow{\ \ #1\ \ }\phantom{}\kern-1.5ex} \newcommand{\ras}[1]{\kern-1.5ex\xrightarrow{\ ...

**6**

votes

**2**answers

257 views

### Properness of the category of modules over a spectrum (that represents algebraic cobordism or motivic cohomology)

The abstract form of the question: let $C$ be a closed proper stable model category, $R$ is a ring object in it. Which conditions ensure that the category $R-mod$ is also proper?
Since weak ...

**6**

votes

**2**answers

644 views

### If $G \times G \cong H \times H$, then is $G \cong H$? [duplicate]

Let $G,H$ groups. Suppose that $G \times G \cong H \times H$. Then is necessarily $G \cong H$?
I know that if $G,H$ are finite groups then it is true (you show that the map $FinGrp \rightarrow ...

**3**

votes

**2**answers

297 views

### 2 and 3 pullbacks

If $F:A\to C$ and $G:B\to C$ are morphisms in $Cat$, then their pseudo-pullback (I hope it's the right notion) can be calculated as the strict limit $A\times_C C^I \times_C B$, where $I$ is the ...

**5**

votes

**0**answers

143 views

### Nice references for injective model structures and Quillen functors between motivic homotopy categories

It seems that for the injective model structures for the categories simplicial presheaves and simplicial Nisnevich sheaves (on the category of smooth varieties over a perfect field $k$) there exist ...

**1**

vote

**1**answer

155 views

### “order two sequence” in a paper of Waldhausen

In F. Waldhausen's paper "Algebraic K-theory of generalized free products, Part I", page 142,line 19, there is a term "order two sequence". Can anyone explain its meaning to me?
According to the ...