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Enriched categories, topoi, abelian categories, monoidal categories, homological algebra.

34 votes
4 answers
5k views

Is there a categorical treatment of dynamical systems?

Let $X$ be a set and $(T,\cdot)$ an abelian group. Is there a category of $T$-dynamical systems on $X$ which yields useful information about $X$ and $T$? More precisely, is there a category whose obj …
Vidit Nanda's user avatar
  • 15.5k
22 votes
3 answers
6k views

Why are ring actions much harder to find than group actions?

I admit freely that the following question is a bit of a fishing expedition inspired by this lovely "definition" of a module as found on Wikipedia: A module is a ring action on an abelian group. …
Vidit Nanda's user avatar
  • 15.5k
21 votes
2 answers
1k views

How does it End?

A recent project has forced my colleague and me to take a rather abstract approach to dynamical systems, and the following definition arose naturally in that context. Let $\mathcal{C}$ be a category. …
Vidit Nanda's user avatar
  • 15.5k
21 votes
3 answers
1k views

Are functor categories with triangulated codomains themselves triangulated?

I'm fairly confident that the following assertion is true (but I will confess that I did not verify the octahedral axiom yet): Let $T$ be a triangulated category and $C$ any category (let's say small …
Vidit Nanda's user avatar
  • 15.5k
15 votes

What is the intuitive meaning of the coskeleton of a simplicial set?

A simplicial set $X$ is $k$-coskeletal iff the following condition holds: a simplex of dimension $\geq k$ is present iff all of its $(k-1)$-dimensional faces are present in $X$. A standard exa …
Vidit Nanda's user avatar
  • 15.5k
13 votes
1 answer
469 views

When does localization preserve homotopy type of classifying spaces?

Let $\mathcal{C}$ be a small category and $\Sigma$ a collection of morphisms in $\mathcal{C}$. Denote by $F_\Sigma:\mathcal{C} \to \mathcal{C}[\Sigma^{-1}]$ the usual quotient functor from $\mathcal{C …
Vidit Nanda's user avatar
  • 15.5k
10 votes
Accepted

Persistent homology over the integers

As mentioned in Carlsson and Zomorodian's paper (to which you have linked), the problem of computing persistence barcodes with coefficients in a ring $R$ relies essentially on classifying graded modul …
Vidit Nanda's user avatar
  • 15.5k
10 votes
1 answer
283 views

Localizing 2-categories about a single morphism

This question is a straight-up reference request, but of course I will be grateful for an answer in the event that no references are readily available. Consider a strict $2$-category $\mathbf{C}$ and …
Vidit Nanda's user avatar
  • 15.5k
8 votes
0 answers
369 views

Is there a 2-categorical, equivariant version of Quillen's Theorem A?

Quillen's Theorem A says that a functor $F:C \to D$ (between 1-categories) induces a homotopy equivalence of classifying spaces $BC \simeq BD$ if for every object $d$ in $D$ the fiber category $F/d$ h …
Vidit Nanda's user avatar
  • 15.5k
8 votes

How to visualize the Microsupport of a Sheaf?

I'd been hoping for months that someone would come along and answer this question: every time I encounter the definition of microsupport, my brain responds with a flash of anger followed by a protract …
Vidit Nanda's user avatar
  • 15.5k
7 votes
2 answers
1k views

What's an initial object in a poset-enriched category?

I have a functor $F:C \to D$ between poset-enriched categories, and I'd like to show that the induced map on classifying spaces is a homotopy-equivalence. To this end, I am trying to establish the pre …
Vidit Nanda's user avatar
  • 15.5k
7 votes
Accepted

Homotopy theory of acyclic categories

Here is a cool new (and very readable) preprint which uses the second barycentric subdivision (as discussed in Zhen Lin, Fernando Muro and Peter May's comments) to construct a cofibrantly generated mo …
Vidit Nanda's user avatar
  • 15.5k
6 votes
Accepted

The nerve of categories preserves weak equivalence?

Such questions are typically framed in terms of Classifying Spaces, but the answer is yes. It follows from, for instance, from Proposition 2.1 in Graeme Segal's article Classifying spaces and spectra …
Vidit Nanda's user avatar
  • 15.5k
5 votes
Accepted

Slice-category-like terminology question

I think what you have defined is just called the category of endomorphisms in $\mathcal{C}$. See for instance Marian Mrozek's 1992 paper Normal functors and retractors in categories of endomorphisms f …
Vidit Nanda's user avatar
  • 15.5k
5 votes
2 answers
436 views

How does one Segal-subdivide a 2-category?

Let $\mathcal{C}$ be a small category. Then, its Segal subdivision $\text{sd }\mathcal{C}$ is a new category whose objects are morphisms of $\mathcal{C}$, and a morphism from $f:x \to y$ to $g: w \to …
Vidit Nanda's user avatar
  • 15.5k

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