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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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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: …
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 …
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 …
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 …
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 …
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 …