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For questions about simplicial sets, simplicial (co)algebras and simplicial objects in other categories; geometric realization, Dold-Kan correspondence, simplicial resolutions etc.

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
2 votes
1 answer
600 views

Finding automorphism groups of simplicial complexes

Question: Given a finite simplicial complex $K$, what general techniques allow one to efficiently compute (a presentation of) the group $\text{Aut}(K)$ of $K$'s automorphisms? Since this is str …
Vidit Nanda's user avatar
  • 15.5k
13 votes
Accepted

Testing simplicial complexes for shellability

Since there were no answers for a few months, I asked this question to my colleague and triangulation expert Frank Lutz. Since his response was wonderful and exhaustive, I am reproducing it here for t …
Vidit Nanda's user avatar
  • 15.5k
25 votes
5 answers
3k views

Testing simplicial complexes for shellability

Question Are there efficient algorithms to check if a finite simplicial complex defined in terms of its maximal facets is shellable? By efficient here I am willing to consider anything with smaller …
Vidit Nanda's user avatar
  • 15.5k
10 votes
1 answer
1k views

Can we invert barycentric subdivision?

With apologies to fellow algebraic topologists, I confess that I have no idea how to answer this innocent-looking question: (1) Let's say we know that a finite simplicial complex $S$ is the baryce …
Vidit Nanda's user avatar
  • 15.5k
4 votes
3 answers
843 views

Constructing a simplicial set homology-equivalent to a given CW complex

I would like to compute the homology of certain low dimensional CW complexes and I am hoping to take advantage of software that handles simplicial sets as input. Thus, I would like to convert a CW com …
Vidit Nanda's user avatar
  • 15.5k
24 votes
4 answers
2k views

How many simplicial complexes on n vertices up to homotopy equivalence?

Fix a number $n$, and define $\gamma(n)$ to be the number of simplicial complexes on $n$ unlabeled vertices up to homotopy equivalence. It is unlikely that an explicit formula exists, but what is know …
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
5 votes
0 answers
230 views

Adding morphisms to a category without changing homotopy type

I have a really tame category $C$: there are only finitely many objects $C_0$, each hom-set $C(x,y)$ for $x,y \in C_0$ has at most one element, and (aside from identity morphisms) if $C(x,y)$ is non-e …
Vidit Nanda's user avatar
  • 15.5k
3 votes
0 answers
174 views

Recognizing Simplicial (Quasi)Fibrations

Let's say we are given two finite simplicial complexes, which I will suggestively call $E$ and $B$. We'd like an algorithm for the following decision problem: Does there exist a simplicial map $p: …
Vidit Nanda's user avatar
  • 15.5k
5 votes
Accepted

Persistent homology of Markovian dynamical systems

The answer to the question as stated seems to be "no". Consider a three-element Markov partition $\mathcal{M} = \{A, B, C\}$ with directed edges $(A,B)$, $(B,C)$ and $(C,A)$. There is an obvious peri …
Vidit Nanda's user avatar
  • 15.5k
6 votes
0 answers
172 views

Uniformly sampling from the set of all simplicial maps

Let $K$ and $L$ be finite simplicial complexes that remain fixed throughout. How does one efficiently sample (according to the uniform distribution) elements from the finite set of simplicial map …
Vidit Nanda's user avatar
  • 15.5k
8 votes

Is there a discrete Cerf theory?

I realize that I am several months late to the Cerf theory party, but this paper of Chari and Joswig might be of interest to the original poster and certainly deserves a mention in the context of this …
Vidit Nanda's user avatar
  • 15.5k
5 votes
Accepted

Algebraic Morse theory

It's always nice to see people working on discrete Morse theory. Answer 1 It is an "if and only if". Meaning: the partial order $\prec$ is defined by $\alpha \prec \gamma$ if and only if $\gamma$ pr …
Vidit Nanda's user avatar
  • 15.5k
8 votes
Accepted

Combinatorial distance between simplicial complexes

This got too long for a comment, so I am placing it here. I don't think there is a theory already out there, but that should not be too surprising. After all, the "combinatorial distance" between a s …
Vidit Nanda's user avatar
  • 15.5k