On the chromatic number of a simplicial complex

单纯球面的横贯与着色

Transversals and colorings of simplicial spheres

本研究受点集环绕性质启发，通过两种无穷构造给出单纯d-球面最大横贯比的下界，并小幅改进了高维单纯球面面超图色数的已有上界。

关于2-复形的边色数

On the edge-chromatic number of 2-complexes

本文提出将四色定理从二维推广至三维的开放问题，并证明嵌入给定3-流形的每个2-复形着色充分且必要需12种颜色，同时指出需12色的2-复形示例非单纯复形。

Dold's Theorem from Viewpoint of Strong Compatibility Graphs

Let $G$ be a non-trivial finite group. The well-known Dold's theorem states that: There is no continuous $G$-equivariant map from an $n$-connected simplicial $G$-complex to a free simplicial $G$-complex of dimension at most $n$. In this paper, we give a new generalization of Dold's theorem, by replacing "dimension at most $n$" with a sharper combinatorial parameter. Indeed, this parameter is the chromatic number of a new family of graphs, called strong compatibility graphs, associated to the target space. Moreover, in a series of examples, we will see that one can hope to infer much more infor…

Box complexes, neighborhood complexes, and the chromatic number

Lovasz's striking proof of Kneser's conjecture from 1978 using the Borsuk--Ulam theorem provides a lower bound on the chromatic number of a graph. We introduce the shore subdivision of simplicial complexes and use it to show an upper bound to this topological lower bound and to construct a strong Z_2-deformation retraction from the box complex (in the version introduced by Matousek and Ziegler) to the Lovasz complex. In the process, we analyze and clarify the combinatorics of the complexes involved and link their structure via several ``intermediate'' complexes.

色数、布施特伯数和比尔球的弦性

Chromatic numbers, Buchstaber numbers and chordality of Bier spheres

该研究探讨了比尔球的色数、布施特伯数及弦性分类，通过分析f向量得出一般公式，并揭示了弦性比尔球的结构特性。

The Theta Number of Simplicial Complexes

We introduce a generalization of the celebrated Lovász theta number of a graph to simplicial complexes of arbitrary dimension. Our generalization takes advantage of real simplicial cohomology theory, in particular combinatorial Laplacians, and provides a semidefinite programming upper bound of the independence number of a simplicial complex. We consider properties of the graph theta number such as the relationship to Hoffman's ratio bound and to the chromatic number and study how they extend to higher dimensions. Like in the case of graphs, the higher dimensional theta number can be extended t…

Weighted Simplicial Complexes and Weighted Analytic Torsions

A weighted simplicial complex is a simplicial complex with values (called weights) on the vertices. In this paper, we consider weighted simplicial complexes with $\mathbb{R}^2$-valued weights. We study the weighted homology and the weighted analytic torsion for such weighted simplicial complexes.

从伪分形无标度网络扩展的一类单纯复形的组合性质

Combinatorial Properties for a Class of Simplicial Complexes Extended from Pseudo-fractal Scale-free Web

本文研究伪分形无标度网络扩展的一类单纯复形的组合性质，显式确定了独立数、支配数和色数，并推导了无环定向、生成树及完美匹配等数量的闭式表达式。

Topology of the view complex

In this paper we consider a family of simplicial complexes, which we call the view complexes. Our choice of objects of study is motivated by theoretical distributed computing, since the view complex is a key simplicial construction used for protocol complexes in the snapshot computational model. We show that the view complex $\view$ can be collapsed to the well-known complex $χ(Δ^n)$, called standard chromatic subdivision of a simplex, and that $χ(Δ^n)$ is itself collapsible. Furthermore, we show that the collapses can be performed simultaneously in entire orbits of the natural symmetric group…

无洞单纯复形的无符号拉普拉斯谱半径

Signless Laplacian spectral radius of simplicial complexes without holes

研究无洞单纯复形的无符号拉普拉斯谱半径的极值问题，扩展了无环图的经典极值结果到高维单纯复形，同时建立具有指定贝蒂数的单纯复形的无符号拉普拉斯谱半径上界，并推导出超图和单纯复形的图论数上界。

Topology of Hom complexes and test graphs for bounding chromatic number

We introduce new methods for understanding the topology of $\Hom$ complexes (spaces of homomorphisms between two graphs), mostly in the context of group actions on graphs and posets. We view $\Hom(T,-)$ and $\Hom(-,G)$ as functors from graphs to posets, and introduce a functor $(-)^1$ from posets to graphs obtained by taking atoms as vertices. Our main structural results establish useful interpretations of the equivariant homotopy type of $\Hom$ complexes in terms of spaces of equivariant poset maps and $Γ$-twisted products of spaces. When $P = F(X)$ is the face poset of a simplicial complex $…

正则单纯复形的Moore界

The Moore Bound for Regular Simplicial Complexes

本文推导正则单纯复形的Moore型上界，并基于最小度给出直径的对数下界。

A short simplicial h-vector and the upper bound theorem

We verify the Upper Bound Conjecture (UBC) for a class of odd-dimensional simplicial complexes that in particular includes all Eulerian simplicial complexes with isolated singularities. The proof relies on a new invariant of simplicial complexes -- a short simplicial h-vector.

On the lattice of flats of a boolean representable simplicial complex

It is shown that the lattices of flats of boolean representable simplicial complexes are always atomistic, but semimodular if and only if the complex is a matroid. A canonical construction is introduced for arbitrary finite atomistic lattices, providing a characterization of the lattices of flats of boolean representable simplicial complexes and a decidability condition. We remark that every finite lattice occurs as the lattice of flats of some simplicial complex.

The Fundamental Group of Balanced Simplicial Complexes and Posets

We establish an upper bound on the cardinality of a minimal generating set for the fundamental group of a large family of connected, balanced simplicial complexes and, more generally, simplicial posets.

On face numbers of flag simplicial complexes

Denham, Suciu and Panov, Ray computed ranks of homotopy groups and Poincaré series of a moment-angle-complex $\mathcal Z(\mathcal K$) / Davis-Januzskiewicz space $DJ(\mathcal K)$ associated to a flag simplicial complex $\mathcal K$. In this note we revisit these results and interpret them as polynomial bounds on the face numbers of an arbitrary simplicial flag complex.

Large Simple d-Cycles in Simplicial Complexes

We show that the size of the largest simple d-cycle in a simplicial d-complex $K$ is at least a square root of $K$'s density. This generalizes a well-known classical result of Erdős and Gallai \cite{EG59} for graphs. We use methods from matroid theory applied to combinatorial simplicial complexes.

Homotopy type of the neighborhood complexes of graphs of maximal degree at most $3$ and $4$-regular circulant graphs

To estimate the lower bound for the chromatic number of a graph $G$, Lovász associated a simplicial complex $\mathcal{N}(G)$ called the neighborhood complex and relates the topological connectivity of $\mathcal{N}(G)$ to the chromatic number of $G$. More generally he proved that the chromatic number of $G$ is bounded below by the topological connectivity of $\mathcal{N}(G)$ plus $3$. In this article, we consider the graphs of maximal degree at most $3$ and $4$-regular circulant graphs. We show that each connected component of the neighborhood complexes of these graphs is homotopy equivalent ei…

Simplicial Complexes are Game Complexes

Strong placement games (SP-games) are a class of combinatorial games whose structure allows one to describe the game via simplicial complexes. A natural question is whether well-known invariants of combinatorial games, such as "game value", appear as invariants of the simplicial complexes. This paper is the first step in that direction. We show that every simplicial complex encodes a certain type of SP-game (called an "invariant SP-game") whose ruleset is independent of the board it is played on. We also show that in the class of SP-games isomorphic simplicial complexes correspond to isomorphi…

Generations of random hypergraphs and random simplicial complexes by the map algebra

We consider the random hypergraph on a finite vertex set by choosing each set of vertices as an hyperedge independently at random. We express the probability distributions of the (lower-)associated simplicial complex and the (lower-)associated independence hypergraph of the random hypergraph in terms of the probability distributions of certain random simplicial complex and certain random independence hypergraph of Erdös-Rényi type. We construct a graded structure of the map algebra explicitly and give algorithms to generate random hypergraphs and random simplicial complexes.

Relating CAT(0) cubical complexes and flag simplicial complexes

Given a finite CAT(0) cubical complex, we define a flag simplicial complex associated to it, called the crossing complex. We show that the crossing complex holds much of the combinatorial information of the original cubical complex: for example, hyperplanes in the cubical complex correspond to vertex links in the crossing complex, and the crossing complex is balanced if and only if the cubical complex is cubically balanced. The most significant result is that the sets of $f$-vectors of CAT(0) cubical complexes and flag simplicial complexes are equal, up to an invertible linear transformation.

覆盖单纯复形上拉普拉斯算子的谱

The spectra of Laplace operators on covering simplicial complexes

本文通过将覆盖单纯复形的拉普拉斯算子分解为直和，证明其谱包含基复形的谱，并把Bilu-Linial的图谱结果推广到单纯复形，同时给出上同调维数不等式。

Pure $\mathcal{O}$-sequences arising from $2$-dimensional PS ear-decomposable simplicial complexes

We show that the $h$-vector of a $2$-dimensional PS ear-decomposable simplicial complex is a pure $\mathcal{O}$-sequence. This provides a strengthening of Stanley's conjecture for matroid $h$-vectors in rank $3$. Our approach modifies the approach of combinatorial shifting for arbitrary simplicial complexes to the setting of $2$-dimensional PS ear-decomposable complexes, which allows us to greedily construct a corresponding pure multicomplex.

具有固定面数的纯单纯复形与团复形

Pure Simplicial and Clique Complexes with a Fixed Number of Facets

本文研究纯单纯复形与团复形的结构和计数问题，给出仅依赖面列表的团复形判定充要条件，证明一类纯团复形可由面邻接矩阵唯一确定，并计数固定面数的纯单纯复形及给出纯团复形数量的上界。

Collapsibility of simplicial complexes of hypergraphs

Let $\mathcal{H}$ be a hypergraph of rank $r$. We show that the simplicial complex whose simplices are the hypergraphs $\mathcal{F}\subset\mathcal{H}$ with covering number at most $p$ is $\left(\binom{r+p}{r}-1\right)$-collapsible, and the simplicial complex whose simplices are the pairwise intersecting hypergraphs $\mathcal{F}\subset\mathcal{H}$ is $\frac{1}{2}\binom{2r}{r}$-collapsible.

Finding or counting all shellings of a simplicial complex

The shellability status of previously investigated simplicial complexes with up to 24 facets is settled. In case of shellability the exact number of shellings is determined. Our algorithm merely relies on the facets, and not on additional information such as the face-numbers.

高维单纯复形的Cheeger型不等式

Cheeger type inequalities for high dimensional simplicial complexes

本文从组合学角度研究高维单纯复形上的离散Cheeger型不等式，将图论中的经典Cheeger不等式推广到高维情形。

A note on the real part of complex chromatic roots

A {\em chromatic root} is a root of the chromatic polynomial of a graph. While the real chromatic roots have been extensively studied and well understood, little is known about the {\em real parts} of chromatic roots. It is not difficult to see that the largest real chromatic root of a graph with $n$ vertices is $n-1$, and indeed, it is known that the largest real chromatic root of a graph is at most the tree-width of the graph. Analogous to these facts, it was conjectured in [8] that the real parts of chromatic roots are also bounded above by both $n-1$ and the tree-width of the graph. In thi…

布尔可表示单纯复形中的若干主题

Topics in Boolean Representable Simplicial Complexes

本文研究了布尔可表示单纯复形（BRSC）理论，探讨了其上的各种算子，并考察了更高维度的可壳性，同时提出了若干新猜想。

Intersection patterns of finite sets and of convex sets

The main result is a common generalization of results on lower bounds for the chromatic number of r-uniform hypergraphs and some of the major theorems in Tverberg-type theory, which is concerned with the intersection pattern of faces in a simplicial complex when continuously mapped to Euclidean space. As an application we get a simple proof of a generalization of a result of Kriz for certain parameters. This specializes to a short and simple proof of Kneser's conjecture. Moreover, combining this result with recent work of Mabillard and Wagner we show that the existence of certain equivariant m…

Mixing in high-dimensional expanders

We prove a generalization of the Expander Mixing Lemma for arbitrary (finite) simplicial complexes. The original lemma states that concentration of the Laplace spectrum of a graph implies combinatorial expansion (which is also referred to as mixing, or quasi-randomness). Recently, an analogue of this Lemma was proved for simplicial complexes of arbitrary dimension, provided that the skeleton of the complex is complete. More precisely, it was shown that a concentrated spectrum of the simplicial Hodge Laplacian implies a similar type of expansion as in graphs. In this paper we remove the assumpt…

虚拟Cohen-Macaulay单纯复形的条件

Conditions for Virtually Cohen--Macaulay Simplicial Complexes

本文给出单纯复形成为虚拟Cohen-Macaulay单纯复形的充分条件，引入虚拟可剥单纯复形这一可剥复形的推广，并证明其具有虚拟Cohen-Macaulay性质，推广了可剥复形是Cohen-Macaulay的结论。

Simplicial cycles and the computation of simplicial trees

We generalize the concept of a cycle from graphs to simplicial complexes. We show that a simplicial cycle is either a sequence of facets connected in the shape of a circle, or is a cone over such a structure. We show that a simplicial tree is a connected cycle-free simplicial complex, and use this characterization to produce an algorithm that checks in polynomial time whether a simplicial complex is a tree. We also present an efficient algorithm for checking whether a simplicial complex is grafted, and therefore Cohen-Macaulay.

On the maximum number of non attacking rooks on a high-dimensional simplicial chessboard

The simplicial rook graph ${\rm \mathcal{SR}}(m,n)$ is the graph whose vertices are vectors in $ \mathbb{N}^m$ such that for each vector the summation of its coordinates is $n$ and two vertices are adjacent if their corresponding vectors differ in exactly two coordinates. Martin and Wagner (Graphs Combin. (2015) 31:1589--1611) asked about the independence number of ${\rm \mathcal{SR}}(m,n)$ that is the maximum number of non attacking rooks which can be placed on a $(m-1)$-dimensional simplicial chessboard of side length $n+1$. In this work, we solve this problem and show that $α({\rm \mathcal{…

Composition of simplicial complexes, polytopes and multigraded Betti numbers

For a simplicial complex K on m vertices and simplicial complexes K1,...,Km a composed simplicial complex K(K1,...,Km) is introduced. This construction generalizes an iterated simplicial wedge construction studied by A. Bahri, M. Bendersky, F. R. Cohen and S. Gitler and allows to describe the combinatorics of generalized joins of polytopes P(P1,...,Pm) defined by G. Agnarsson. The composition defines a structure of an operad on a set of finite simplicial complexes. We prove the following: (1) a composed complex K(K1,...,Km) is a simplicial sphere iff K is a simplicial sphere and Ki are the bou…

The Rado Simplicial Complex

A Rado simplicial complex X is a generalisation of the well-known Rado graph. X is a countable simplicial complex which contains any countable simplicial complex as its induced subcomplex. The Rado simplicial complex is highly symmetric, it is homogeneous: any isomorphism between finite induced subcomplexes can be extended to an isomorphism of the whole complex. We show that the Rado complex X is unique up to isomorphism and suggest several explicit constructions. We also show that a random simplicial complex on countably many vertices is a Rado complex with probability 1. The geometric realis…

Necessary Conditions for Geometric Realizability of Simplicial Complexes

We associate with any simplicial complex $\K$ and any integer $m$ a system of linear equations and inequalities. If $\K$ has a simplicial embedding in $\R^m$ then the system has an integer solution. This result extends the work of I. Novik (2000).

Linear Residuals and Gallai-Simplicial Complexes

In this paper, we give a new algebraic criterion for the {\em shellability} of (non-pure) simplicial complex $Δ$ over $[n]$, shellable in the sense of Björner and Wachs \cite{BW}. We show that the spanning simplicial complex of doubly uni-cyclic graph is non-pure shellable. Moreover, we introduce the concept of Gallai-simplicial complex $Δ_Γ(G)$ of a finite simple graph $G$. We applied the obtained criterion to discuss the shellability of Gallai simplicial complexes associated to various classes of graphs..

正交表示、向量色数与扩展复杂度

Orthonormal representations, vector chromatic number, and extension complexity

本研究通过构建Alon和Szegedy近正交向量的二分推广，得到多个涉及Lovász theta函数、向量色数等的极值问题强界，并推导了两个具独立研究价值的向量色数通用下界。

Lucas sequences in t-uniform simplicial complexes

We introduce t-uniform simplicial complexes and we show that the lengths of spheres in such complexes are the terms of certain Lucas sequences. We find optimal constants for the linear isoperimetric inequality in the hyperbolic case.

The facet ideal of a simplicial complex

To a simplicial complex, we associate a square-free monomial ideal in the polynomial ring generated by its vertex set over a field. We study algebraic properties of this ideal via combinatorial properties of the simplicial complex. By generalizing the notion of a tree from graphs to simplicial complexes, we show that ideals associated to trees satisfy sliding depth condition, and therefore have normal and Cohen-Macaulay Rees rings. We also discuss connections with the theory of Stanley-Reisner rings.

Dirac's theorem on simplicial matroids

We introduce the notion of k-hyperclique complexes, i.e., the largest simplicial complexes on the set [n] with a fixed k-skeleton. These simplicial complexes are a higher-dimensional analogue of clique (or flag) complexes (case k=2) and they are a rich new class of simplicial complexes. We show that Dirac's theorem on chordal graphs has a higher-dimensional analogue in which graphs and clique complexes get replaced, respectively, by simplicial matroids and k-hyperclique complexes. We prove also a higher-dimensional analogue of Stanley's reformulation of Dirac's theorem on chordal graphs.

On simplicial and cubical complexes with short links

We consider closed simplicial and cubical $n$-complexes in terms of link of their $(n-2)$-faces. Especially, we consider the case, when this link has size 3 or 4, i.e., every $(n-2)$-face is contained in 3 or 4 $n$-faces. Such simplicial complexes with {\em short} (i.e. of length 3 or 4) links are completely classified by their {\em characteristic partition}. We consider also embedding into hypercubes of the skeletons of simplicial and cubical complexes.

On algebraic and combinatorial properties of weighted simplicial complexes

Weighted simplicial complexes (WSCs) are powerful tools for describing weighted cloud data or networks with weighted nodes. In this paper, we propose a novel approach to study WSCs via the concept of polarization. Polarization of a WSC allows one to construct a new (unweighted) simplicial complex which coincides with an object called the mixed wreath product. This new construction preserves several properties and invariants of the underlying simplicial complex of a WSC. Our main focus is to analyze WSCs through their underlying simplicial complexes and mixed wreath products. Combinatorially, w…

单纯复形的双倍体

The double of a simplicial complex

本研究提出单纯复形的双倍化与r倍化概念，证明其连通性结果，并建立该概念与同调稳定性中快r倍稳定化的关联。

关于具有极值总Betti数和总双阶Betti数的单纯复形

On Simplicial Complexes with Extremal Total Betti number and Total Bigraded Betti Number

本文研究了在每一维给定顶点数的情况下，确定哪些单纯复形的总Betti数和总双阶Betti数达到最大或最小值。

单纯复形的线图

Line graphs of simplicial complexes

本文研究纯单纯复形的线图，证明可基于其线图的组合结构计算面理想的第二阶分次Betti数，并刻画了线图为完全（二部）图的纯单纯复形及线图应满足的条件。

单纯复形与单纯集大小的拓扑下界

Topological lower bounds on the sizes of simplicial complexes and simplicial sets

本文证明满足特定拓扑条件的 n 维空间（如 n 维环面）在任何三角剖分或可缩面单纯集表示中至少具有 2^n 个 n 维面，给出了单纯结构大小的拓扑下界。

On Codimension one Embedding of Simplicial Complexes

We study $d$-dimensional simplicial complexes that are PL embeddable in $\mathbb{R}^{d+1}$. It is shown that such a complex must satisfy a certain homological condition. The existence of this obstruction allows us to provide a systematic approach to deriving upper bounds for the number of top-dimensional faces of such complexes, particularly in low dimensions.

Colorings of simplicial complexes and vector bundles over Davis-Januszkiewicz spaces

We show that coloring properties of a simplicial complex K are reflected by splitting properties of a bundle over the associated Davis-Januszkiewicz space whose Chern classes are given by the elementary symmetric polynomials in the generators of the Stanley-Reisner algebra of K.

A simplicial complex is uniquely determined by its set of discrete Morse functions

We prove that a connected simplicial complex is uniquely determined by its complex of discrete Morse functions. This settles a question raised by Chari and Joswig. In the 1-dimensional case, this implies that the complex of rooted forests of a connected graph G completely determines G.

$k$-shellable simplicial complexes and graphs

In this paper we show that a $k$-shellable simplicial complex is the expansion of a shellable complex. We prove that the face ring of a pure $k$-shellable simplicial complex satisfies the Stanley conjecture. In this way, by applying expansion functor to the face ring of a given pure shellable complex, we construct a large class of rings satisfying the Stanley conjecture. Also, by presenting some characterizations of $k$-shellable graphs, we extend some results due to Castrillón-Cruz, Cruz-Estrada and Van Tuyl-Villareal.

Enriched homology and cohomology modules of simplicial complexes

For a simplicial complex X on {1,2, ..., n} we define enriched homology and cohomology modules. They are graded modules over k[x_1, ..., x_n] whose ranks are equal to the dimensions of the reduced homology and cohomology groups. We characterize Cohen-Macaulay, l-Cohen-Macaulay, Buchsbaum, and Gorenstein* complexes X, and also orientable manifolds in terms of the enriched modules. We introduce the notion of girth for simplicial complexes and make a conjecture relating the girth to invariants of the simplicial complex. We also put strong vanishing conditions on the enriched homology modules and …

Erratum: Limit theorems for Betti numbers of random simplicial complexes

We correct the proofs of the main theorems in our paper "Limit theorems for Betti numbers of random simplicial complexes".