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  • Improving the $$\frac{1}{3} - \frac{2}{3}$$ Conjecture for width Two Posets
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Ashwin Sah

    Extending results of Linial (1984) and Aigner (1985), we prove a uniform lower bound on the balance constant of a poset P of width 2. This constant is defined as δ(P) = max(x,y)∈P2 min{ℙ(x ≺ y), ℙ(y ≺ x)}, where ℙ(x≺y) is the probability x is less than y in a uniformly random linear extension of P. In particular, we show that if P is a width 2 poset that cannot be formed from the singleton poset and

    更新日期:2020-12-01
  • A New Family of Triangulations of ℝ P d Lorenzo Venturello, Hailun Zheng
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Lorenzo Venturello, Hailun Zheng

    We construct a family of PL triangulations of the d-dimensional real projective space ℝPd on \(\Theta \left( {{{\left( {\frac{{1 + \sqrt 5 }}{2}} \right)}^{d + 1}}} \right)\) vertices for every d s-> 1. This improves a construction due to Kühnel on 2d+1 -1 vertices.

    更新日期:2020-12-01
  • Cycle Lengths in Expanding Graphs Limor Friedman, Michael Krivelevich
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Limor Friedman, Michael Krivelevich

    For a positive constant α a graph G on n vertices is called an α-expander if every vertex set U of size at most n/2 has an external neighborhood whose size is at least α|U|. We study cycle lengths in expanding graphs. We first prove that cycle lengths in α-expanders are well distributed. Specifically, we show that for every 0 < α ⪯ 1 there exist positive constants n0, C and A = O(1/α) such that for

    更新日期:2020-12-01
  • The VC-Dimension of K -Vertex D -Polytopes
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Andrey Kupavskii

    In this short note, we show that the VC-dimension of the class of k-vertex polytopes in Rd is at most 8d2k log2k, answering an old question of Long and Warmuth. We also show that it is at least 1/3kd.

    更新日期:2020-12-01
  • The Edge-Erdős-Pósa Property
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Henning Bruhn, Matthias Heinlein, Felix Joos

    Robertson and Seymour proved that the family of all graphs containing a fixed graph H as a minor has the Erdős-Pósa property if and only if H is planar. We show that this is no longer true for the edge version of the Erdős-Pósa property, and indeed even fails when H is an arbitrary subcubic tree of large pathwidth or a long ladder. This answers a question of Raymond, Sau and Thilikos.

    更新日期:2020-12-01
  • A Lower Bound for Adaptively-Secure Collective Coin Flipping Protocols
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Yael Tauman Kalai, Ilan Komargodski, Ran Raz

    In 1985, Ben-Or and Linial (Advances in Computing Research 1989) introduced the collective coin flipping problem, where n parties communicate via a single broadcast channel and wish to generate a common random bit in the presence of adaptive Byzantine corruptions. In this model, the adversary can decide to corrupt a party in the course of the protocol as a function of the messages seen so far. They

    更新日期:2020-12-01
  • A Short Proof of Shih’s Isomorphism Theorem on Graphic Subspaces
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Zouhaier Ferchiou, Bertrand Guenin

    In his PhD thesis Shih characterized the relationship between two graphs, where the cycle space of the first is included in the cycle space of the second and the dimension of the cycle spaces differ by one [7]. However, this result never appeared in a refereed publication. As a consequence this theorem has not received the attention it deserves. We give a simpler and shorter proof of this result as

    更新日期:2020-12-01
  • Seymour’s Conjecture on 2-Connected Graphs of Large Pathwidth
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Tony Huynh, Gwenaël Joret, Piotr Micek, David R. Wood

    We prove a conjecture of Seymour (1993) stating that for every apex-forest H1 and out-erplanar graph H2 there is an integer p such that every 2-connected graph of pathwidth at least p contains H1 or H2 as a minor. An independent proof was recently obtained by Dang and Thomas [3].

    更新日期:2020-12-01
  • Base Partition for Mixed Families of Finitary and Cofinitary Matroids
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Joshua Erde, J. Pascal Gollin, Attila Joó, Paul Knappe, Max Pitz

    Let M = (Mi: i ∈ K) be a finite or infinite family consisting of matroids on a common ground set E each of which may be finitary or cofinitary. We prove the following Cantor-Bernstein-type result: If there is a collection of bases, one for each Mi, which covers the set E, and also a collection of bases which are pairwise disjoint, then there is a collection of bases which partition E. We also show

    更新日期:2020-12-01
  • New Results on Simplex-Clusters in Set Systems
    Combinatorica (IF 1.143) Pub Date : 2020-11-30
    Gabriel Currier

    A d-simplex is defined to be a collection A1,..., Ad+1 of subsets of size k of [n] such that the intersection of all of them is empty, but the intersection of any d of them is non-empty. Furthemore, a d-cluster is a collection of d+1 such sets with empty intersection and union of size ≤ 2k, and a d-simplex-cluster is such a collection that is both a d-simplex and a d-cluster. The Erdös-Chvátal d-simplex

    更新日期:2020-12-01
  • Detecting a Long Odd Hole
    Combinatorica (IF 1.143) Pub Date : 2020-11-26
    Maria Chudnovsky, Alex Scott, Paul Seymour

    for each integer ℓ ≥ 5, we give a polynomial-time algorithm to test whether a graph contains an induced cycle with length at least ℓ and odd.

    更新日期:2020-11-27
  • A Diophantine Ramsey Theorem Tomasz Schoen
    Combinatorica (IF 1.143) Pub Date : 2020-11-26
    Tomasz Schoen

    Let p ∈ ℤ [x] be any polynomial with p(0) =0, k ∈ ℕ and let c1, …, cs ∈ ℤ, s ⩾ k(k + 1), be non-zero integers such that \(\sum {{c_i} = 0} \). We show that for a wide class of coefficients c1, …, cs in every finite coloring ℕ = A1∪ ⃯ ∪ Ar there is a monochromatic solution to the equation \({c_1}x_1^k + \cdots + {c_s}x_s^k = {\rm{p}}\left(y \right).\).

    更新日期:2020-11-27
  • Patterns In Random Permutations
    Combinatorica (IF 1.143) Pub Date : 2020-08-10
    Chaim Even-Zohar

    Every k entries in a permutation can have one of k! different relative orders, called patterns. How many times does each pattern occur in a large random permutation of size n? The distribution of this k!-dimensional vector of pattern densities was studied by Janson, Nakamura, and Zeilberger (2015). Their analysis showed that some component of this vector is asymptotically multi-normal of order \(1/\sqrt

    更新日期:2020-08-11
  • 1-Subdivisions, the Fractional Chromatic Number and the Hall Ratio
    Combinatorica (IF 1.143) Pub Date : 2020-08-10
    Zdenĕk Dvořák, Patrice Ossona de Mendez, Hehui Wu

    The Hall ratio of a graph G is the maximum of |V(H)|/α(H) over all subgraphs H of G. It is easy to see that the Hall ratio of a graph is a lower bound for the fractional chromatic number. It has been asked whether conversely, the fractional chromatic number is upper bounded by a function of the Hall ratio. We answer this question in negative, by showing two results of independent interest regarding

    更新日期:2020-08-11
  • Impartial Digraphs
    Combinatorica (IF 1.143) Pub Date : 2020-08-10
    Yufei Zhao, Yunkun Zhou

    We prove a conjecture of Fox, Huang, and Lee that characterizes directed graphs that have constant density in all tournaments: they are disjoint unions of trees that are each constructed in a certain recursive way.

    更新日期:2020-08-11
  • Weakly Norming Graphs are Edge-Transitive
    Combinatorica (IF 1.143) Pub Date : 2020-07-31
    Alexander Sidorenko

    Let ℋ be the class of bounded measurable symmetric functions on [0, 1]2. For a function h ∈ ℋ and a graph G with vertex set [v1,⌦,vn} and edge set E(G), define$${t_G}(h) = \int \cdots \int {\prod\limits_{{\rm{\{ }}{v_i}{\rm{,}}{v_j}{\rm{\} }} \in E(G)} {h({x_i},{x_j})\;d{x_1} \cdots d{x_n}} } .$$Answering a question raised by Conlon and Lee, we prove that in order for tG(∣h∣)1/∣E(G)∣ to be a norm on

    更新日期:2020-07-31
  • Degeneracy and Colorings of Squares of Planar Graphs without 4-Cycles
    Combinatorica (IF 1.143) Pub Date : 2020-07-06
    Ilkyoo Choi, Daniel W. Cranston, Théo Pierron

    We prove several results on coloring squares of planar graphs without 4-cycles. First, we show that if G is such a graph, then G2 is (Δ(G) + 72)-degenerate. This implies an upper bound of Δ(G) ∣ 73 on the chromatic number of G2 as well as on several variants of the chromatic number such as the list-chromatic number, paint number, Alon-Tarsi number, and correspondence chromatic number. We also show

    更新日期:2020-07-06
  • Boolean Dimension and Tree-Width
    Combinatorica (IF 1.143) Pub Date : 2020-07-06
    Stefan Felsner, Tamás Mészáros, Piotr Micek

    Dimension is a key measure of complexity of partially ordered sets. Small dimension allows succinct encoding. Indeed if P has dimension d, then to know whether x ≤ y in P it is enough to check whether x ≤ y in each of the d linear extensions of a witnessing realizer. Focusing on the encoding aspect, Nešetřil and Pudlák defined a more expressive version of dimension. A poset P has Boolean dimension

    更新日期:2020-07-06
  • Expanding Polynomials: A Generalization of the Elekes-Rónyai Theorem to d Variables
    Combinatorica (IF 1.143) Pub Date : 2020-07-06
    Orit E. Raz, Zvi Shem-Tov

    We prove the following statement. Let f ∈ ℝ[x1,…,xd], for some d ≥ 3, and assume that f depends non-trivially in each of x1,…, xd. Then one of the following holds. (i) For every finite sets A1,…, Ad ⊂ℝ, each of size n, we have $$\left| {f\left( {{A_1} \times \ldots \times {A_d}} \right)} \right| = \Omega \left( {{n^{3/2}}} \right),$$ with constant of proportionality that depends on deg f. (ii) f is

    更新日期:2020-07-06
  • Expander Graphs — Both Local and Global
    Combinatorica (IF 1.143) Pub Date : 2020-07-06
    Michael Chapman; Nati Linial; Yuval Peled

    Let G = (V, E) be a finite graph. For v ∈ V we denote by Gv the subgraph of G that is induced by v’s neighbor set. We say that G is (a,b)-regular for a>b> 0 integers, if G is a-regular and Gv is b-regular for every v ∈ V. Recent advances in PCP theory call for the construction of infinitely many (a,b)-regular expander graphs G that are expanders also locally. Namely, all the graphs {Gv ∣ v ∈ V} should

    更新日期:2020-07-06
  • Note on the Number of Hinges Defined by a Point Set in ℝ 2
    Combinatorica (IF 1.143) Pub Date : 2020-05-22
    Misha Rudnev

    It is shown that the number of distinct types of three-point hinges, defined by a real plane set of n points is ≫n2 log−3n, where a hinge is identified by fixing two pairwise distances in a point triple. This is achieved via strengthening (modulo a logn factor) of the Guth- Katz estimate for the number of pairwise intersections of lines in ℝ3, arising in the context of the plane Erdős distinct distance

    更新日期:2020-05-22
  • Euler Tours in Hypergraphs
    Combinatorica (IF 1.143) Pub Date : 2020-05-22
    Stefan Clock, Felix Joos, Daniela Kühn, Deryk Osthus

    We show that a quasirandom k-uniform hypergraph G has a tight Euler tour subject to the necessary condition that k divides all vertex degrees. The case when G is complete confirms a conjecture of Chung, Diaconis and Graham from 1989 on the existence of universal cycles for the k-subsets of an n-set.

    更新日期:2020-05-22
  • On Almost k -Covers of Hypercubes
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    Alexander Clifton; Hao Huang

    In this paper, we consider the following problem: what is the minimum number of affine hyperplanes in ℝn, such that all the vertices of \(\overrightarrow 0\) are covered at least k times, and \({\left\{{0,1} \right\}^n}\backslash \left\{{\overrightarrow 0} \right\}\) is uncovered? The k = 1 case is the well-known Alon-Füredi theorem which says a minimum of n affine hyperplanes is required, which follows

    更新日期:2020-04-28
  • On Closest Pair in Euclidean Metric: Monochromatic is as Hard as Bichromatic
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    C. S. Karthik; Pasin Manurangsi

    Given a set of n points in ℝd, the (monochromatic) Closest Pair problem asks to find a pair of distinct points in the set that are closest in the ℓp-metric. Closest Pair is a fundamental problem in Computational Geometry and understanding its fine-grained complexity in the Euclidean metric when d = ω(log n) was raised as an open question in recent works (Abboud-Rubinstein-Williams [6], Williams [48]

    更新日期:2020-04-28
  • Short Directed Cycles in Bipartite Digraphs
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    Paul Seymour; Sophie Spirkl

    The Caccetta-Häggkvist conjecture implies that for every integer k ≥ 1, if G is a bipartite digraph, with n vertices in each part, and every vertex has out-degree more than n/(k+1), then G has a directed cycle of length at most 2k. If true this is best possible, and we prove this for k = 1, 2, 3, 4, 6 and all k ≥ 224,539.More generally, we conjecture that for every integer k ≥ 1, and every pair of

    更新日期:2020-04-28
  • On a Conjecture of Erdős on Locally Sparse Steiner Triple Systems
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    Stefan Glock; Daniela Kühn; Allan Lo; Deryk Osthus

    A famous theorem of Kirkman says that there exists a Steiner triple system of order n if and only if n ≡ 1,3 mod 6. In 1973, Erdős conjectured that one can find so-called ‘sparse’ Steiner triple systems. Roughly speaking, the aim is to have at most j−3 triples on every set of j points, which would be best possible. (Triple systems with this sparseness property are also referred to as having high girth

    更新日期:2020-04-28
  • The Language of Self-Avoiding Walks
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    Christian Lindorfer, Wolfgang Woess

    Let X = (VX, EX) be an infinite, locally finite, connected graph without loops or multiple edges. We consider the edges to be oriented, and EX is equipped with an involution which inverts the orientation. Each oriented edge is labelled by an element of a finite alphabet Σ. The labelling is assumed to be deterministic: edges with the same initial (resp. terminal) vertex have distinct labels. Furthermore

    更新日期:2020-04-28
  • Intersecting Restrictions in Clutters
    Combinatorica (IF 1.143) Pub Date : 2020-04-28
    Ahmad Abdi, Gérard Cornuéjols, Dabeen Lee

    A clutter is intersecting if the members do not have a common element yet every two members intersect. It has been conjectured that for clutters without an intersecting minor, total primal integrality and total dual integrality of the corresponding set covering linear system must be equivalent. In this paper, we provide a polynomial characterization of clutters without an intersecting minor. One important

    更新日期:2020-04-28
  • Generically Globally Rigid Graphs Have Generic Universally Rigid Frameworks
    Combinatorica (IF 1.143) Pub Date : 2020-03-22
    Robert Connelly; Steven J. Gortler; Louis Theran

    We show that any graph that is generically globally rigid in ℝd has a realization in ℝd that is both generic and universally rigid. This also implies that the graph also must have a realization in ℝd that is both infinitesimally rigid and universally rigid; such a realization serves as a certificate of generic global rigidity.Our approach involves an algorithm by Lovász, Saks and Schrijver that, for

    更新日期:2020-03-22
  • A Construction for Clique-Free Pseudorandom Graphs
    Combinatorica (IF 1.143) Pub Date : 2020-03-05
    Anurag Bishnoi; Ferdinand Ihringer; Valentina Pepe

    A construction of Alon and Krivelevich gives highly pseudorandom Kk-free graphs on n vertices with edge density equal to Θ(n−1=(k−2)). In this short note we improve their result by constructing an infinite family of highly pseudorandom Kk-free graphs with a higher edge density of Θ(n−1=(k−1)).

    更新日期:2020-03-05
  • New Bounds on Even Cycle Creating Hamiltonian Paths Using Expander Graphs
    Combinatorica (IF 1.143) Pub Date : 2020-03-05
    Gergely Harcos; Daniel Soltész

    We say that two graphs on the same vertex set are G-creating if their union (the union of their edges) contains G as a subgraph. Let Hn(G) be the maximum number of pairwise G-creating Hamiltonian paths of Kn. Cohen, Fachini and Körner proved$${n^{\frac{1}{2}n - o\left( n \right)}} \le {H_n}\left( {{C_4}} \right) \le {n^{\frac{3}{4}n + o\left( n \right)}}.$$In this paper we close the superexponential

    更新日期:2020-03-05
  • Simple Graph Density Inequalities with No Sum of Squares Proofs
    Combinatorica (IF 1.143) Pub Date : 2020-03-05
    Grigoriy Blekherman; Annie Raymond; Mohit Singh; Rekha R. Thomas

    Establishing inequalities among graph densities is a central pursuit in extremal combinatorics. A standard tool to certify the nonnegativity of a graph density expression is to write it as a sum of squares. In this paper, we identify a simple condition under which a graph density expression cannot be a sum of squares. Using this result, we prove that the Blakley-Roy inequality does not have a sum of

    更新日期:2020-03-05
  • An Asymptotically Tight Bound on the Number of Relevant Variables in a Bounded Degree Boolean function
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    John Chiarelli; Pooya Hatami; Michael Saks

    We prove that there is a constant C ≤ 6.614 such that every Boolean function of degree at most d (as a polynomial over ℝ) is a C·2d-junta, i.e., it depends on at most C·2d variables. This improves the d·2d-1 upper bound of Nisan and Szegedy [Computational Complexity 4 (1994)].The bound of C·2d is tight up to the constant C, since a read-once decision tree of depth d depends on all 2d - 1 variables

    更新日期:2020-03-04
  • The Satisfiability Threshold For Random Linear Equations
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Peter Ayre; Amin Coja-Oghlan; Pu Gao; Noëla Müller

    Let A be a random m × n matrix over the finite field \(\mathbb{F}_q\) with precisely k non-zero entries per row and let \(y\in\mathbb{F}_q^m\) be a random vector chosen independently of A. We identify the threshold m/n up to which the linear system Ax = y has a solution with high probability and analyse the geometry of the set of solutions. In the special case q = 2, known as the random k-XORSAT problem

    更新日期:2020-03-04
  • High Order Random Walks: Beyond Spectral Gap
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Tali Kaufman; Izhar Oppenheim

    We study high order random walks in high dimensional expanders; namely, in complexes which are local spectral expanders. Recent works have studied the spectrum of high order walks and deduced fast mixing. However, the spectral gap of high order walks is inherently small, due to natural obstructions (called coboundaries) that do not happen for walks on expander graphs.In this work we go beyond spectral

    更新日期:2020-03-04
  • Unbalancing Sets and An Almost Quadratic Lower Bound for Syntactically Multilinear Arithmetic Circuits
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Noga Alon; Mrinal Kumar; Ben Lee Volk

    We prove a lower bound of Ω(n2/log2n) on the size of any syntactically multilinear arithmetic circuit computing some explicit multilinear polynomial f(x1,...,xn). Our approach expands and improves upon a result of Raz, Shpilka and Yehudayoff ([34]), who proved a lower bound of Ω(n4/3/log2n) for the same polynomial. Our improvement follows from an asymptotically optimal lower bound for a generalized

    更新日期:2020-03-04
  • On Tight 4-Designs in Hamming Association Schemes
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Alexander L. Gavrilyuk; Sho Suda; Janoš Vidali

    We complete the classification of tight 4-designs in Hamming association schemes H(n,q), i.e., that of tight orthogonal arrays of strength 4, which had been open since a result by Noda (1979). To do so, we construct an association scheme attached to a tight 4-design in H(n,q) and analyze its triple intersection numbers to conclude the non-existence in all open cases.

    更新日期:2020-03-04
  • Cycle Traversability for Claw-Free Graphs and Polyhedral Maps
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Ervin Győri; Michael D. Plummer; Dong Ye; Xiaoya Zha

    Let G be a graph, and \(v \in V(G)\) and \(S \subseteq V(G)\setminus{v}\) of size at least k. An important result on graph connectivity due to Perfect states that, if v and S are k-linked, then a (k−1)-link between a vertex v and S can be extended to a k-link between v and S such that the endvertices of the (k−1)-link are also the endvertices of the k-link. We begin by proving a generalization of Perfect's

    更新日期:2020-03-04
  • Large Cliques in Hypergraphs with Forbidden Substructures
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Andreas F. Holmsen

    A result due to Gyárfás, Hubenko, and Solymosi (answering a question of Erdős) states that if a graph G on n vertices does not contain K2,2 as an induced subgraph yet has at least \(c\left(\begin{array}{c}n\\ 2\end{array}\right)\) edges, then G has a complete subgraph on at least \(\frac{c^2}{10}n\) vertices. In this paper we suggest a “higher-dimensional” analogue of the notion of an induced K2,2

    更新日期:2020-03-04
  • Number of 1-Factorizations of Regular High-Degree Graphs
    Combinatorica (IF 1.143) Pub Date : 2020-03-04
    Asaf Ferber; Vishesh Jain; Benny Sudakov

    A 1-factor in an n-vertex graph G is a collection of \(\frac{n}{2}\) vertex-disjoint edges and a 1-factorization of G is a partition of its edges into edge-disjoint 1-factors. Clearly, a 1-factorization of G cannot exist unless n is even and G is regular (that is, all vertices are of the same degree). The problem of finding 1-factorizations in graphs goes back to a paper of Kirkman in 1847 and has

    更新日期:2020-03-04
  • Packing Nearly Optimal Ramsey R (3, t ) Graphs
    Combinatorica (IF 1.143) Pub Date : 2020-02-04
    He Guo; Lutz Warnke

    In 1995 Kim famously proved the Ramsey bound R(3, t) ≤ ct2/logt by constructing an n-vertex graph that is triangle-free and has independence number at most \(C\,\sqrt {n\,\log \,n} \). We extend this celebrated result, which is best possible up to the value of the constants, by approximately decomposing the complete graph Kn into a packing of such nearly optimal Ramsey R(3,t) graphs.More precisely

    更新日期:2020-02-04
  • Homomorphism Thresholds for Odd Cycles
    Combinatorica (IF 1.143) Pub Date : 2020-02-03
    Oliver Ebsen; Mathias Schacht

    The interplay of minimum degree conditions and structural properties of large graphs with forbidden subgraphs is a central topic in extremal graph theory. For a given graph F we define the homomorphism threshold as the infimum over all α ∈ [0,1] such that every n-vertex F-free graph G with minimum degree at least αn has a homomorphic image H of bounded order (i.e. independent of n), which is F-free

    更新日期:2020-02-03
  • Stability Results on the Circumference of a Graph
    Combinatorica (IF 1.143) Pub Date : 2020-01-20
    Jie Ma; Bo Ning

    In this paper, we extend and refine previous Turán-type results on graphs with a given circumference. Let Wn,k,c be the graph obtained from a clique Kc−k+1 by adding n − (c − k + 1) isolated vertices each joined to the same k vertices of the clique, and let f(n,k,c) = e(Wn,k,c). Improving a celebrated theorem of Erdős and Gallai [8], Kopylov [18] proved that for c \max \left\{ {f(n,3,c),f\left( {n

    更新日期:2020-01-20
  • Dense Induced Bipartite Subgraphs in Triangle-Free Graphs
    Combinatorica (IF 1.143) Pub Date : 2020-01-16
    Matthew Kwan; Shoham Letzter; Benny Sudakov; Tuan Tran

    The problem of finding dense induced bipartite subgraphs in H-free graphs has a long history, and was posed 30 years ago by Erdős, Faudree, Pach and Spencer. In this paper, we obtain several results in this direction. First we prove that any H-free graph with minimum degree at least d contains an induced bipartite subgraph of minimum degree at least cH log d/log log d, thus nearly confirming one and

    更新日期:2020-01-16
  • Highly-Arc-Transitive and Descendant-Homogeneous Digraphs with Finite Out-Valency
    Combinatorica (IF 1.143) Pub Date : 2019-12-10
    Daniela A. Amato

    We investigate infinite highly-arc-transitive digraphs with two additional properties, property Z and descendant-homogeneity. We show that if D is a highly-arc-transitive descendant-homogeneous digraph with property Z and F is the subdigraph spanned by the descendant sets of a line in D, then F is a locally finite 2-ended digraph with property Z. If, moreover, D has prime out-valency, then there is

    更新日期:2019-12-10
  • Vertex-Flames in Countable Rooted Digraphs Preserving an Erdős-Menger Separation for Each Vertex
    Combinatorica (IF 1.143) Pub Date : 2019-12-03
    Attila Joó

    It follows from a theorem of Lovász that if D is a finite digraph with r ∈ V(D), then there is a spanning subdigraph E of D such that for every vertex v ≠ r the following quantities are equal: the local connectivity from r to v in D, the local connectivity from r to v in E and the indegree of v in E.In infinite combinatorics cardinality is often an overly rough measure to obtain deep results and it

    更新日期:2019-12-03
  • Incompatible Intersection Properties
    Combinatorica (IF 1.143) Pub Date : 2019-12-03
    Peter Frankl; Andrey Kupavskii

    Let F ⊂ 2 [n] be a family in which any three sets have non-empty intersection and any two sets have at least 32 elements in common. The nearly best possible bound F ≤ 2n−2 is proved. We believe that 32 can be replaced by 3 and provide a simple-looking conjecture that would imply this.

    更新日期:2019-12-03
  • Hyper-Atoms Applied to the Critical Pair Theory
    Combinatorica (IF 1.143) Pub Date : 2019-11-09
    Yahya O. Hamidoune

    The isoperimetric method is often useful for proving results regarding sumsets. Here, we introduce the notion of a hyper-atom into the method, which overcomes a previous weakness when dealing with atoms that are cosets. To show the utility of this new object, we give a new isoperimetric proof of the cornerstone of classical critical pair theory: The Kemperman Structure Theorem, proved in its so-called

    更新日期:2019-11-09
  • Rainbow Fractional Matchings
    Combinatorica (IF 1.143) Pub Date : 2019-10-29
    Ron Aharoni; Ron Holzman; Zilin Jiang

    We prove that any family E1,..., E┌rn┐ of (not necessarily distinct) sets of edges in an r-uniform hypergraph, each having a fractional matching of size n, has a rainbow fractional matching of size n (that is, a set of edges from distinct Ei’s which supports such a fractional matching). When the hypergraph is r-partite and n is an integer, the number of sets needed goes down from rn to rn−r+1. The

    更新日期:2019-10-29
  • The Colouring Number of Infinite Graphs
    Combinatorica (IF 1.143) Pub Date : 2019-10-29
    Nathan Bowler; Johannes Carmesin; Péter Komjáth; Christian Reiher

    We show that, given an infinite cardinal μ, a graph has colouring number at most μ if and only if it contains neither of two types of subgraph. We also show that every graph with infinite colouring number has a well-ordering of its vertices that simultaneously witnesses its colouring number and its cardinality.

    更新日期:2019-10-29
  • Counterexample to an Extension of the Hanani-Tutte Theorem on the Surface of Genus 4
    Combinatorica (IF 1.143) Pub Date : 2019-10-29
    Radoslav Fulek; Jan Kynčl

    We find a graph of genus 5 and its drawing on the orientable surface of genus 4 with every pair of independent edges crossing an even number of times. This shows that the strong Hanani–Tutte theorem cannot be extended to the orientable surface of genus 4. As a base step in the construction we use a counterexample to an extension of the unified Hanani–Tutte theorem on the torus.

    更新日期:2019-10-29
  • Submodular Minimization Under Congruency Constraints
    Combinatorica (IF 1.143) Pub Date : 2019-10-29
    Martin Nägele; Benny Sudakov; Rico Zenklusen

    Submodular function minimization (SFM) is a fundamental and efficiently solvable problem in combinatorial optimization with a multitude of applications in various fields. Surprisingly, there is only very little known about constraint types under which SFM remains efficiently solvable. The arguably most relevant non-trivial constraint class for which polynomial SFM algorithms are known are parity constraints

    更新日期:2019-10-29
  • Waring’s Theorem for Binary Powers
    Combinatorica (IF 1.143) Pub Date : 2019-10-29
    Daniel M. Kane; Carlo Sanna; Jeffrey Shallit

    A natural number is a binary k’ th power if its binary representation consists of k consecutive identical blocks. We prove, using tools from combinatorics, linear algebra, and number theory, an analogue of Waring’s theorem for sums of binary k’th powers. More precisely, we show that for each integer k> 2, there exists an effectively computable natural number n such that every sufficiently large multiple

    更新日期:2019-10-29
  • Exponentially Many Nowhere-Zero ℤ 3 -, ℤ 4 -, and ℤ 6 -Flows
    Combinatorica (IF 1.143) Pub Date : 2019-10-28
    Zdeněk Dvořák; Bojan Mohar; Robert Šámal

    We prove that, in several settings, a graph has exponentially many nowhere-zero flows. These results may be seen as a counting alternative to the well-known proofs of existence of ℤ3-, ℤ4-, and ℤ6-flows. In the dual setting, proving exponential number of 3-colorings of planar triangle-free graphs is a related open question due to Thomassen.

    更新日期:2019-10-28
  • Girth Six Cubic Graphs Have Petersen Minors
    Combinatorica (IF 1.143) Pub Date : 2019-10-28
    Neil Robertson; P. D. Seymour; Robin Thomas

    We prove that every 3-regular graph with no circuit of length less than six has a subgraph isomorphic to a subdivision of the Petersen graph.

    更新日期:2019-10-28
  • Clustered Colouring in Minor-Closed Classes
    Combinatorica (IF 1.143) Pub Date : 2019-10-28
    Sergey Norin; Alex Scott; Paul Seymour; David R. Wood

    The clustered chromatic number of a class of graphs is the minimum integer k such that for some integer c every graph in the class is k-colourable with monochromatic components of size at most c. We prove that for every graph H, the clustered chromatic number of the class of H-minor-free graphs is tied to the tree-depth of H. In particular, if H is connected with tree-depth t, then every H-minor-free

    更新日期:2019-10-28
  • The Planar Cayley Graphs are Effectively Enumerable I: Consistently Planar Graphs
    Combinatorica (IF 1.143) Pub Date : 2019-10-02
    Agelos Georgakopoulos; Matthias Hamann

    We obtain an effective enumeration of the family of finitely generated groups admitting a faithful, properly discontinuous action on some 2-manifold contained in the sphere. This is achieved by introducing a type of group presentation capturing exactly these groups.Extending this in a companion paper, we find group presentations capturing the planar finitely generated Cayley graphs. Thus we obtain

    更新日期:2019-10-02
  • Induced Subgraphs of Graphs With Large Chromatic Number. X. Holes of Specific Residue
    Combinatorica (IF 1.143) Pub Date : 2019-10-02
    Alex Scott; Paul Seymour

    A large body of research in graph theory concerns the induced subgraphs of graphs with large chromatic number, and especially which induced cycles must occur. In this paper, we unify and substantially extend results from a number of previous papers, showing that, for every positive integer k, every graph with large chromatic number contains either a large complete subgraph or induced cycles of all

    更新日期:2019-10-02
  • Turán’s Theorem for the Fano Plane
    Combinatorica (IF 1.143) Pub Date : 2019-10-02
    Louis Bellmann; Christian Reiher

    Confirming a conjecture of Vera T. Sós in a very strong sense, we give a complete solution to Turán's hypergraph problem for the Fano plane. That is we prove for n≥8 that among all 3-uniform hypergraphs on n vertices not containing the Fano plane there is indeed exactly one whose number of edges is maximal, namely the balanced, complete, bipartite hypergraph. Moreover, for n = 7 there is exactly one

    更新日期:2019-10-02
  • A Turán Theorem for Extensions Via an Erdős-Ko-Rado Theorem for Lagrangians
    Combinatorica (IF 1.143) Pub Date : 2019-10-02
    Adam Bene Watts; Sergey Norin; Liana Yepremyan

    The extension of an r-uniform hypergraph G is obtained from it by adding for every pair of vertices of G, which is not covered by an edge in G, an extra edge containing this pair and (r−2) new vertices. In this paper we determine the Turán number of the extension of an r-graph consisting of two vertex-disjoint edges, settling a conjecture of Hefetz and Keevash, who previously determined this Turán

    更新日期:2019-10-02
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