Questions tagged [simplicial-complexes]
The simplicial-complexes tag has no usage guidance.
265
questions
5
votes
1
answer
226
views
Is the smooth singular simplicial set of a smooth manifold a Kan complex?
It is classical that the singular simplicial set of a topological space is a Kan complex. This is elementary and already due to presumably Kan.
Q: Is the smooth singular simplicial set of a smooth ...
- 479
1
vote
1
answer
100
views
Annulus theorem for pseudomanifolds
Lets say I take an arbitrary closed and smooth $d$-manifolds $\mathcal{M}$. Now, it is a well-known fact that whenever I take two (sufficiently nice embedded) closed $d$-balls $B_{1}$ and $B_{2}$ in $\...
- 1,147
3
votes
2
answers
156
views
The exactness of the associated chain complex of a simplicial free abelian group over a finite set and the normalization theorem
Update:
Now I know why my method fails. But I still wanna know how to work out the original question, that is to show the exactness of the chain complex $C_*(X)$ except for two positions $n=0,N-1$.
...
- 389
2
votes
0
answers
204
views
Do the polyhedral homologies of a polyhedron coincide with the polyhedral homologies of its subdivision?
Definition. A convex polytope is a compact finite intersection of hyperplanes in $\mathbb{R}^n$
Definition. The polycomplex is the following data set:
a set of convex polytopes, closed under ...
- 6,484
1
vote
2
answers
216
views
Is the poset $\mathrm{Idl}_{\neq \emptyset, P}(P)$ of nonempty, proper ideals in a finite connected poset $P$ (empty or) weakly contractible?
$\DeclareMathOperator\Idl{Idl}$Let $P$ be a finite, connected poset with at least two elements, and let $\Idl_{\neq \emptyset, P}(P)$ be the set of downward closed sets $S \subset P$ such that $S \neq ...
- 61.6k
6
votes
2
answers
451
views
An equality for the reduced homology related to the comparability graph of a poset
$\DeclareMathOperator\width{width}$Let $P$ be a finite poset with $n$ elements (we can assume that $P$ is connected and has width at most $n-2$). The comparability graph $G_P=(V,E)$ associated to $P$ ...
- 26.1k
0
votes
0
answers
259
views
Another definition of singular homology
The singular homology is defined via standard simplex. Now if I propose another definition of singular homology groups, based on arbitrary simplex, as follows:
Let $X$ be a topological space. A $n$-...
- 771
1
vote
0
answers
148
views
Lifting theorem for finite spaces: replacing perfect normality by normality
In the Lifting theorem for finite spaces (Thm. 3.5, Eric Wofsey, quoted below),
can one relax the condition "$A$ is a closed subset of a perfectly normal $X$" to
"$A\to X$ has the right ...
- 317
2
votes
1
answer
551
views
The nerve of the Ising category
A semi-simplicial complex (see Moore's 1958 paper: Semi-simplicial complexes and Postnikov systems, available at http://www-math.mit.edu/~hrm/kansem/moore-semi-simplicial-complexes.pdf, cf MathSciNet) ...
- 10.3k
3
votes
1
answer
243
views
Unimodality of $f$-vectors of $0/1$-polytopes
It is known that the face vectors (aka $f$-vectors) of general polytopes need not be unimodal. This even fails for simple or simplicial polytopes, as was shown first by Björner.
My question is if ...
- 1,879
3
votes
0
answers
146
views
Which Stiefel-Whitney numbers can be extended to manifolds with boundaries?
The Stiefel-Whitney numbers are classical topological manifold invariants obtained by integrating some local quantity (a cup product of Stiefel-Whitney classes) over the manifold. Which Stiefel-...
- 2,901
6
votes
0
answers
121
views
Hamiltonicity for triangulations of the 3-sphere
A classical theorem of Whitney states that the 1-skeleton of every triangulation of the 2-sphere $\mathbb{S}^2$ has a Hamilton cycle as long as each of its 3-cycles bounds a triangle.
I'm wondering if ...
- 1,854
3
votes
1
answer
421
views
Euler characteristic of pseudomanifolds with boundary
It is a well-known fact that for every compact oriented odd-dimensional manifold $\mathcal{M}$ with boundary it holds that
$$\chi(\mathcal{M})=\frac{1}{2}\chi(\partial\mathcal{M}).$$
In particular, if ...
- 1,147
6
votes
1
answer
553
views
Properties a triangulation must have in order to describe a manifold
I am mainly interested in the $3$-dimensional case. It is a well-known fact, following from the work of E. E. Moise and R. H. Bing in the 1950s, that every $3$-dimensional topological manifold (with ...
- 1,147
1
vote
0
answers
138
views
Simplicial realization of the circle action on the free loop space
Given a simply connected topological space $X$, it is well known that its free loop space $LX$ has cohomology being the Hochschild homology of the singular cochains [1]:
$$HH_\bullet(S^\star X) \simeq ...
- 5,038
0
votes
1
answer
232
views
Simplex invariants?
Let $k=s$ be a positive definite symmetric function and a simililarity over the natural numbers such that $k(a,a) = 1 $ for all natural numbers $a$.
A similarity $s:X\times X \rightarrow \mathbb{R}$ ...
- 2,462
4
votes
1
answer
330
views
An acyclic simplicial complex where all links are generalised homology spheres
We say that a simplicial complex $K$ is acyclic if it's integral reduced simplicial homology groups are trivial in all dimensions.
For a vertex ${v} \in K$, we define the link
$$lk(v) :=\{\sigma \in K ...
- 198
4
votes
1
answer
258
views
"Permutation matrix" but non-zero entries are replaced by $e^{ix}$
Birkhoff–von Neumann theorem states that a polytope formed by a set of doubly-stochastic matrices has extreme points that are permutation matrices.
I am wondering if there is a similar theorem for a ...
- 389
3
votes
1
answer
400
views
Is every map from a simplicial complex to a sphere homotopic to a simplicial map to the boundary of a k-simplex?
Suppose that $X$ is a simplicial complex, and $f:X \rightarrow S^k$ a continuous map to a sphere. Is $f$ always homotopic to a simplicial map to the boundary of a $(n+1)$-simplex, $\partial \Delta^k$?
...
- 269
7
votes
1
answer
315
views
For which pairs $k$ and $n$, $n\mid{{n-2} \choose {k}}$
The question
This question that arose in a discussion with Ron Adin is quite simple:
For which pairs $k$ and $n$ does $n$ divide ${{n-2} \choose {k}}$?
Simple observations
It is easy to see that ...
- 24.2k
5
votes
0
answers
299
views
Self-dual simplicial complexes
A simplicial complex $K$ on a vertex set $[m] = \{1,...,m \}$ - where here we say vertex set to mean an ambient set of vertices on which $K$ is defined - is self-dual if it is equal to its Alexander ...
- 198
6
votes
1
answer
284
views
Irreducible representations of the symmetric group on homology of simplicial complex
I am following Wall's paper A note on symmetry of singularities and I have some questions regarding representation theory and the homology of some objects:
Consider an action of $\Sigma_k$ on a finite ...
- 258
2
votes
0
answers
162
views
Enumeration and encoding of simplicial complexes
I'd like to know how to enumerate and encode all (abstract) simplicial complexes of a given kind.
To start as simple as possible, consider the familiy $\mathcal{S}_n^{d}$ of simplicial complexes which ...
- 9,628
7
votes
1
answer
231
views
Are there simplicial spheres with "non-geometric symmetries"?
Let $\Delta$ be a simplicial sphere, that is, a finite (abstract) simplicial complex whose canonical geometric realization $|\Delta|$ is homeomorphic to a sphere $\mathbf S^d\subset\Bbb R^{d+1}$.
...
- 12.6k
8
votes
1
answer
1k
views
What is a subdivision of an abstract simplicial complex?
I am looking for the definition of the subdivision of a simplicial complex.
When the complex is defined in a geometric way, then the definition is pretty simple :
the complex σ(C) is a subdivision of ...
- 81
3
votes
0
answers
94
views
When is it possible to extend several linear orders defined "locally" into a single linear order defined "globally"?
This is a somewhat fuzzy question, so I will try my best to give a formulation which includes everything relevant while excluding everything else. I would like to find out if anyone else has studied ...
- 183
6
votes
0
answers
84
views
Can a spherical simplicial complex have more than one "central" inversion?
Let $\Delta$ be a finite connected simplicial complex. Call a simplicial map $\phi:\Delta\to\Delta$ an inversion if
$\phi$ is an involution, that is $\phi\circ\phi=\mathrm{id}$, and
$\phi$ is not ...
- 12.6k
4
votes
0
answers
130
views
"Baues poset" of shellings of simplicial polytope?
Let me start with some background I want to use as analogy.
Consider a (convex) polytope $P$ and its set of triangulations. Among all the triangulations, a well behaved subset are the regular ones: ...
- 22.9k
1
vote
0
answers
67
views
Generalization of Moore graphs -- not geometries
I was thinking about the following problem: Suppose you have a $k$-uniform hyper graph (simplicial complex of dimension $k$) with a complete $(k-1)$-skeleton, and some form of regularity (e.g. every ...
- 525
8
votes
2
answers
199
views
Constructing a $0/1$ polytope from an abstract simplicial complex
Let us fix $\Delta$ a finite simplicial complex, and label the vertices of $\Delta$ as $\{1,2,\ldots,n\}$. For each $F\in \Delta$ let us consider the point in $\mathbb{R}^n$ given by:
$$e_F := \sum_{i\...
- 1,879
3
votes
2
answers
272
views
Inequality of $h$-vectors of shellable simplicial complexes
I've been studying the article of Bjorner entitled "Homology and shellability of matroid complexes". At a certain point he states an exercise that says:
Let $\Delta$ be a shellable ...
- 1,879
3
votes
3
answers
381
views
Are there characteristic-dependent Betti numbers in characteristic not equal to two?
Let $\Delta$ be a finite simplicial complex on $n$ vertices. Let $S = \mathbb{k}[\mathbf{x}]$ be a polynomial ring over a field $\mathbb{k}$ in $n$ variables and $I$ be the Stanley-Reisner ideal of $\...
- 974
3
votes
0
answers
184
views
Skeleton of $\mathcal{G}$-simplicial complex
I'm trying to prove a result with simplicial complex endowed with an action of a groupoid. Let start with a formal definition :
$\textbf{Definition.}$ [$\mathcal{G}$-simplicial complex] Let $\mathcal{...
- 53
4
votes
1
answer
409
views
What is the significance of ear decompositions for non-graphic matroids?
On Wikipedia there is subsection in the article on ear decompositions of graphs titled "Matroids":
Now as defined above, the circuits of a matroid can not always be listed to satisfy the ...
- 2,506
8
votes
0
answers
163
views
Is there a combinatorial representation of general topological manifolds similar to triangulations?
Piece-wise linear manifolds are combinatorially represented by simplicial complexes modulo Pachner moves. However, for dimensions greater than $3$, the notions of piece-wise linear and topological ...
- 2,901
5
votes
2
answers
389
views
Does the union of the subcomplexes of $X$ that contain a given subcomplex and whose inclusion in $X$ is trivial on $\pi_1$, have trivial $\pi_1$?
Let $X$ be a simplicial complex and let $A \subset X$ be a contractible subcomplex on the same set of vertices as $X$. Is it true that the union $$\bigcup C$$ taken over all complexes $A \subset C \...
- 153
6
votes
1
answer
186
views
Existence of equivariant triangulations
I'm looking for a compatibility result which links two types of structures that could be imposed on a topological space $X$:
Call $X$ triangulable if there exists a finite simplicial complex $K$ ...
- 15.5k
11
votes
1
answer
237
views
A class of simplicial complexes defined by arithmetic properties
The purpose of my question is to ask about properties in a certain class of 3-dimensional (and other odd dimensional) simplicial complexes. I will first describe the construction in 3 dimension and ...
- 24.2k
3
votes
0
answers
144
views
On a generalization of the Borsuk-Ulam theorem / Tucker's lemma for a map from simplex to its boundary
The Borsuk-Ulam theorem is equivalent to $S^{n-1}$ not being a retract of $B^n$. <totally wrong! or else 2+2=4 is equivalent to the Poincare conjecture/thm>
How shall i prove the following ...
3
votes
0
answers
146
views
F-vectors of simplicial complex and f-vectors of non-faces of simplicial complex
Is there any result which gives us a relation between f-vector of simplicial complex and f-vector of nonfaces of a simplicial complex?
Thank you
- 51
7
votes
1
answer
404
views
Visualize (co)sketeton of a simplicial set (geometrical intuition)
I want to understand if there is an intuition approchable with
most possible 'elementary geometrical' knowledge for
$n$-(co)skeleta of simplicial sets?
Formally sketleton & coskeleton functions ...
- 6,000
2
votes
0
answers
228
views
Combinatorial condition for orientability on simplicial complexes
Let $K$ be a simplicial complex whose geometric realization is a topological or smooth manifold.
Is it possible to restate the condition of orientability of $M$ exclusively in (combinatorial) terms of ...
- 909
2
votes
3
answers
466
views
Alternating sum over collections of sets
Let $\mathbf{P}$ be a collection of subsets of a finite set $X$. Let $\mathscr{S}$ be the set of all subsets $\mathbf{S}\subset \mathbf{P}$ such that $\bigcup_{S\in \mathbf{S}} S = X$. Can one give a ...
- 19.4k
3
votes
0
answers
140
views
Acyclic extensions of acyclic simplicial complexes
Say an abstract simplicial complex $X$ is acyclic if its reduced integral simplicial homology groups $\tilde{\mathrm{H}}^{\Delta}_p(X)$ vanish for all $p\geq 0$. Is it the case that, for any $n>0$, ...
- 61
9
votes
2
answers
401
views
Alternating sum over collections closed under containment
Let $\mathscr{C}$ be a collection of subsets of a finite set $P$. Assume $\mathscr{C}$ is closed under containment: if $S\subset P$ is in $\mathscr{C}$, then every set $S'\subset P$ containing $S$ is ...
- 19.4k
6
votes
1
answer
113
views
Subdivision of closed homology manifold reference request
I am interested in the barycentric subdivision of closed homology manifolds.
Definition A (finite) simplicial complex $K$ is a closed homology manifold of dimension $n$ if for every $k$-simplex, its ...
- 909
8
votes
0
answers
142
views
Small flag triangulations
In geometric group theory and low-dimensional topology, asking for a triangulation of a specific space to be flag can often be somewhat more cumbersome than just turning a CW-complex into a simplicial ...
- 101
11
votes
1
answer
447
views
How much smaller is the Čech complex than the Vietoris-Rips complex?
The Čech complex
is a subcomplex of the
Vietoris-Rips complex.
The V-R complex
includes as a simplex a set of points with pairwise
distances at most $\epsilon$,
whereas the Č complex
includes as a ...
- 149k
2
votes
0
answers
79
views
Graphs with vanishing homology and behaviour of the suspension of that graphs
Let $G$ be a simple graph. The clique complex, $\Delta(G)$ of the graph $G$ is the simplicial complex given by the collection of all complete subgraph of $G.$ Now we define the graph homology $H^{Gr}_\...
- 745
2
votes
1
answer
348
views
Alexander duality for Homology sphere which is the Geometric realization of a finite simplicial complex
The Alexander duality Theorem is usually stated for a triangulable pair $(\mathbb S^n, Y)$ where $Y$ is a subset of the standard sphere $\mathbb S^n$. My question is: Does the duality also hold if we ...
- 280