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Sharp bounds for decomposing graphs into edges and triangles 将图分解为边和三角形的锐利边界
Pub Date : 2019-07-26 DOI: 10.1017/S0963548320000358
Adam Blumenthal, Bernard Lidick'y, Yanitsa Pehova, Florian Pfender, O. Pikhurko, Jan Volec
Abstract For a real constant α, let $pi _3^alpha (G)$ be the minimum of twice the number of K2’s plus α times the number of K3’s over all edge decompositions of G into copies of K2 and K3, where Kr denotes the complete graph on r vertices. Let $pi _3^alpha (n)$ be the maximum of $pi _3^alpha (G)$ over all graphs G with n vertices. The extremal function $pi _3^3(n)$ was first studied by Győri and Tuza (Studia Sci. Math. Hungar. 22 (1987) 315–320). In recent progress on this problem, Král’, Lidický, Martins and Pehova (Combin. Probab. Comput. 28 (2019) 465–472) proved via flag algebras that$pi _3^3(n) le (1/2 + o(1)){n^2}$. We extend their result by determining the exact value of $pi _3^alpha (n)$ and the set of extremal graphs for all α and sufficiently large n. In particular, we show for α = 3 that Kn and the complete bipartite graph ${K_{lfloor n/2 rfloor,lceil n/2 rceil }}$ are the only possible extremal examples for large n.
对于一个实常数α,设$pi _3^alpha (G)$为K2的数目的两倍加上α乘以K3的数目在G分解成K2和K3副本的所有边上的最小值,其中Kr表示r个顶点上的完整图。设$pi _3^alpha (n)$为$pi _3^alpha (G)$在所有图G上n个顶点的最大值。末梢功能$pi _3^3(n)$最早由Győri和Tuza (Studia Sci)研究。数学。匈牙利。22(1987)315-320)。在这个问题的最新进展中,Král ', Lidický,马丁斯和佩霍瓦(联合)。可能吧。Comput. 28(2019) 465-472)通过标志代数证明了$pi _3^3(n) le (1/2 + o(1)){n^2}$。我们通过确定$pi _3^alpha (n)$的精确值和对于所有α和足够大的n的极值图集来扩展他们的结果。特别是,我们证明了对于α = 3, Kn和完全二部图${K_{lfloor n/2 rfloor,lceil n/2 rceil }}$是大n的唯一可能的极值例子。
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引用次数: 3
Pseudorandom hypergraph matchings 伪随机超图匹配
Pub Date : 2019-07-23 DOI: 10.1017/S0963548320000280
S. Ehard, Stefan Glock, Felix Joos
Abstract A celebrated theorem of Pippenger states that any almost regular hypergraph with small codegrees has an almost perfect matching. We show that one can find such an almost perfect matching which is ‘pseudorandom’, meaning that, for instance, the matching contains as many edges from a given set of edges as predicted by a heuristic argument.
Pippenger的一个著名定理指出,任何具有小余度的几乎正则超图都具有几乎完美匹配。我们证明了人们可以找到这样一个几乎完美的匹配,它是“伪随机”的,这意味着,例如,匹配包含了由启发式论证预测的给定边缘集合中的许多边。
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引用次数: 18
Erratum to ‘On Percolation and the Bunkbed Conjecture’ 《论渗流与双层猜想》的勘误
Pub Date : 2019-07-22 DOI: 10.1017/S0963548319000038
Svante Linusson
Abstract There was an incorrect argument in the proof of the main theorem in ‘On percolation and the bunkbed conjecture’, in Combin. Probab. Comput. (2011) 20 103–117 doi: 10.1017/S0963548309990666. I thus no longer claim to have a proof for the bunkbed conjecture for outerplanar graphs.
摘要:在《论渗流与铺层猜想》中的一个主要定理的证明中,有一个错误的论证。Probab。第一版。(2011) 20 103-117 doi: 10.1017/S0963548309990666。因此,我不再声称对外平面图的层状猜想有证明。
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引用次数: 3
An approximate version of Jackson’s conjecture 杰克逊猜想的近似版本
Pub Date : 2019-07-19 DOI: 10.1017/S0963548320000152
Anita Liebenau, Yanitsa Pehova
Abstract A diregular bipartite tournament is a balanced complete bipartite graph whose edges are oriented so that every vertex has the same in- and out-degree. In 1981 Jackson showed that a diregular bipartite tournament contains a Hamilton cycle, and conjectured that in fact its edge set can be partitioned into Hamilton cycles. We prove an approximate version of this conjecture: for every ε > 0 there exists n0 such that every diregular bipartite tournament on 2n ≥ n0 vertices contains a collection of (1/2–ε)n cycles of length at least (2–ε)n. Increasing the degree by a small proportion allows us to prove the existence of many Hamilton cycles: for every c > 1/2 and ε > 0 there exists n0 such that every cn-regular bipartite digraph on 2n ≥ n0 vertices contains (1−ε)cn edge-disjoint Hamilton cycles.
非正则二部比武是一种平衡的完全二部图,它的边是定向的,使得每个顶点都有相同的进出度。1981年,Jackson证明了一个不规则二部环包含一个Hamilton环,并推测其边集实际上可以划分为Hamilton环。我们证明了这个猜想的一个近似版本:对于每一个ε > 0,存在n个≥n个顶点上的每一个不规则二部竞赛包含长度至少为(2 -ε)n的(1/2 -ε)n个环的集合。将度增加一个小比例,我们可以证明许多Hamilton环的存在性:对于每个c > 1/2且ε > 0,存在不存在这样的条件,即在2n≥n0个顶点上的每个n正则二部有向图包含(1−ε)cn个边不相交的Hamilton环。
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引用次数: 0
Expected Maximum Block Size in Critical Random Graphs 临界随机图中期望的最大块大小
Pub Date : 2019-07-01 DOI: 10.1017/S0963548319000154
Vonjy Rasendrahasina, Andry Rasoanaivo, V. Ravelomanana
Abstract Let G(n,M) be a uniform random graph with n vertices and M edges. Let ${wp_{n,m}}$ be the maximum block size of G(n,M), that is, the maximum size of its maximal 2-connected induced subgraphs. We determine the expectation of ${wp_{n,m}}$ near the critical point M = n/2. When n − 2M ≫ n2/3, we find a constant c1 such that $$c_1 = lim_{n rightarrow infty} left({1 - frac{2M}{n}} right) ,E({wp_{n,m}}).$$ Inside the window of transition of G(n,M) with M = (n/2)(1 + λn−1/3), where λ is any real number, we find an exact analytic expression for $$c_2(lambda) = lim_{n rightarrow infty} frac{E{left({wp_{n,{{(n/2)}({1+lambda n^{-1/3}})}}}right)}}{n^{1/3}}.$$ This study relies on the symbolic method and analytic tools from generating function theory, which enable us to describe the evolution of $n^{-1/3},E{left({wp_{n,{{(n/2)}({1+lambda n^{-1/3}})}}}right)}$ as a function of λ.
设G(n,M)是一个有n个顶点和M条边的均匀随机图。设${wp_{n,m}}$为G(n,M)的最大块大小,即其最大2连通诱导子图的最大大小。我们确定${wp_{n,m}}$在临界点M = n/2附近的期望。当n−2M > n2/3时,我们找到了一个常数c1,使得$$c_1 = lim_{n rightarrow infty} left({1 - frac{2M}{n}} right) ,E({wp_{n,m}}).$$在M = (n/2)(1 + λn−1/3)的G(n,M)的跃迁窗口内,当λ为任意实数时,我们找到了$$c_2(lambda) = lim_{n rightarrow infty} frac{E{left({wp_{n,{{(n/2)}({1+lambda n^{-1/3}})}}}right)}}{n^{1/3}}.$$的精确解析表达式。本研究依靠生成函数理论中的符号方法和解析工具,使我们能够将$n^{-1/3},E{left({wp_{n,{{(n/2)}({1+lambda n^{-1/3}})}}}right)}$的演化描述为λ的函数。
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引用次数: 0
On Erdős–Ko–Rado for random hypergraphs I 在Erdős-Ko-Rado上随机超图I
Pub Date : 2019-06-25 DOI: 10.1017/S0963548319000117
Arran Hamm, J. Kahn
Abstract A family of sets is intersecting if no two of its members are disjoint, and has the Erdős–Ko–Rado property (or is EKR) if each of its largest intersecting subfamilies has non-empty intersection. Denote by ${{cal H}_k}(n,p)$ the random family in which each k-subset of {1, …, n} is present with probability p, independent of other choices. A question first studied by Balogh, Bohman and Mubayi asks: begin{equation} {rm{For what }}p = p(n,k){rm{is}}{{cal H}_k}(n,p){rm{likely to be EKR}}? end{equation} Here, for fixed c < 1/4, and $k lt sqrt {cnlog n} $ we give a precise answer to this question, characterizing those sequences p = p(n, k) for which begin{equation} {mathbb{P}}({{cal H}_k}(n,p){rm{is EKR}}{kern 1pt} ) to 1{rm{as }}n to infty . end{equation}
摘要:如果集合族中没有两个成员不相交,则该集合族是相交的,并且如果其最大的相交子族中的每一个都有非空相交,则该集合族具有Erdős-Ko-Rado性质(或EKR)。用${{cal H}_k}(n,p)$表示随机族,其中每个{1,…,n}的k子集以p的概率存在,独立于其他选择。Balogh, Bohman和Mubayi首先研究了一个问题:begin{equation} {rm{For what }}p = p(n,k){rm{is}}{{cal H}_k}(n,p){rm{likely to be EKR}}? end{equation}在这里,对于固定c < 1/4,和$k lt sqrt {cnlog n} $,我们给出了这个问题的精确答案,表征了那些序列p = p(n, k) begin{equation} {mathbb{P}}({{cal H}_k}(n,p){rm{is EKR}}{kern 1pt} ) to 1{rm{as }}n to infty . end{equation}
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引用次数: 4
Logarithmic Sobolev inequalities in discrete product spaces 离散积空间中的对数Sobolev不等式
Pub Date : 2019-06-13 DOI: 10.1017/S0963548319000099
K. Marton
Abstract The aim of this paper is to prove an inequality between relative entropy and the sum of average conditional relative entropies of the following form: for a fixed probability measure q on , ( is a finite set), and any probability measure on , (*) $$D(p||q){rm{le}}C cdot sumlimits_{i = 1}^n {{rm{mathbb{E}}}_p D(p_i ( cdot |Y_1 ,{rm{ }}...,{rm{ }}Y_{i - 1} ,{rm{ }}Y_{i + 1} ,...,{rm{ }}Y_n )||q_i ( cdot |Y_1 ,{rm{ }}...,{rm{ }}Y_{i - 1} ,{rm{ }}Y_{i + 1} ,{rm{ }}...,{rm{ }}Y_n )),} $$ where pi(· |y1, …, yi−1, yi+1, …, yn) and qi(· |x1, …, xi−1, xi+1, …, xn) denote the local specifications for p resp. q, that is, the conditional distributions of the ith coordinate, given the other coordinates. The constant C depends on (the local specifications of) q. The inequality (*) ismeaningful in product spaces, in both the discrete and the continuous case, and can be used to prove a logarithmic Sobolev inequality for q, provided uniform logarithmic Sobolev inequalities are available for qi(· |x1, …, xi−1, xi+1, …, xn), for all fixed i and fixed (x1, …, xi−1, xi+1, …, xn). Inequality (*) directly implies that the Gibbs sampler associated with q is a contraction for relative entropy. In this paper we derive inequality (*), and thereby a logarithmic Sobolev inequality, in discrete product spaces, by proving inequalities for an appropriate Wasserstein-like distance.
摘要本文的目的是证明相对熵与平均条件相对熵之和之间的不等式,其形式如下:对于一个固定的概率测度q,(是一个有限集合),和任意一个概率测度(*)$$D(p||q){rm{le}}C cdot sumlimits_{i = 1}^n {{rm{mathbb{E}}}_p D(p_i ( cdot |Y_1 ,{rm{ }}...,{rm{ }}Y_{i - 1} ,{rm{ }}Y_{i + 1} ,...,{rm{ }}Y_n )||q_i ( cdot |Y_1 ,{rm{ }}...,{rm{ }}Y_{i - 1} ,{rm{ }}Y_{i + 1} ,{rm{ }}...,{rm{ }}Y_n )),} $$,其中pi(·|y1,…,yi−1,yi+1,…,yn)和qi(·|x1,…,xi−1,xi+1,…,xn)表示p的局部规范。Q,也就是第i个坐标的条件分布,给定其他坐标。常数C依赖于q的(局部规范)。不等式(*)在离散和连续情况下的积空间中都是有意义的,并且可以用来证明q的一个对数Sobolev不等式,前提是qi(·|x1,…,xi−1,xi+1,…,xn)对于所有固定i和固定(x1,…,xi−1,xi+1,…,xn)具有一致的对数Sobolev不等式。不等式(*)直接表示与q相关的吉布斯采样器是相对熵的收缩。本文通过证明一个适当的类wasserstein距离的不等式,推导出离散积空间中的不等式(*),从而得到一个对数Sobolev不等式。
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引用次数: 22
A near-exponential improvement of a bound of Erdős and Lovász on maximal intersecting families 极大相交族上Erdős和Lovász界的近指数改进
Pub Date : 2019-06-04 DOI: 10.1017/S0963548319000142
P. Frankl
Abstract Let m(k) denote the maximum number of edges in a non-extendable, intersecting k-graph. Erdős and Lovász proved that m(k) ≤ kk. For k ≥ 625 we prove m(k) < kk・e−k1/4/6.
设m(k)表示不可扩展的相交k图的最大边数。Erdős和Lovász证明了m(k)≤kk。对于k≥625,证明m(k) < kk·e−k1/4/6。
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引用次数: 6
A quantitative Lovász criterion for Property B 性质B的定量Lovász标准
Pub Date : 2019-03-12 DOI: 10.1017/S0963548320000334
Asaf Ferber, A. Shapira
Abstract A well-known observation of Lovász is that if a hypergraph is not 2-colourable, then at least one pair of its edges intersect at a single vertex. In this short paper we consider the quantitative version of Lovász’s criterion. That is, we ask how many pairs of edges intersecting at a single vertex should belong to a non-2-colourable n-uniform hypergraph. Our main result is an exact answer to this question, which further characterizes all the extremal hypergraphs. The proof combines Bollobás’s two families theorem with Pluhar’s randomized colouring algorithm.
Lovász的一个著名的观察是,如果一个超图不是2色的,那么它的至少一对边相交于一个顶点。在这篇短文中,我们考虑Lovász准则的定量版本。也就是说,我们问有多少对相交于单个顶点的边应该属于一个非2色n均匀超图。我们的主要结果是这个问题的精确答案,它进一步表征了所有极值超图。这个证明结合了Bollobás的两族定理和Pluhar的随机着色算法。
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引用次数: 1
Dirac’s theorem for random regular graphs 随机正则图的狄拉克定理
Pub Date : 2019-03-12 DOI: 10.1017/S0963548320000346
Padraig Condon, Alberto Espuny Díaz, António Girão, Daniela Kühn, Deryk Osthus
Abstract We prove a ‘resilience’ version of Dirac’s theorem in the setting of random regular graphs. More precisely, we show that whenever d is sufficiently large compared to $epsilon > 0$ , a.a.s. the following holds. Let $G'$ be any subgraph of the random n-vertex d-regular graph $G_{n,d}$ with minimum degree at least $$(1/2 + epsilon )d$$ . Then $G'$ is Hamiltonian. This proves a conjecture of Ben-Shimon, Krivelevich and Sudakov. Our result is best possible: firstly the condition that d is large cannot be omitted, and secondly the minimum degree bound cannot be improved.
摘要在随机正则图的集合中证明了狄拉克定理的一个“弹性”版本。更准确地说,我们表明,当d与$epsilon > 0$相比足够大时,也就是说,以下成立。设$G'$为随机n顶点d正则图$G_{n,d}$最小度至少为$$(1/2 + epsilon )d$$的任意子图。那么$G'$就是汉密尔顿函数。这证明了Ben-Shimon, Krivelevich和Sudakov的一个猜想。我们的结果是最好的:首先d很大的条件不能省略,其次最小度界不能改进。
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引用次数: 4
期刊
Combinatorics, Probability and Computing
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