bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deferred On-Line Bipartite Matching
Authors: Kozik Jakub , Matecki Grzegorz 
Published: January 2020

We present a new model for the problem of on-line matching on bipartite graphs. Suppose that one part of a graph is given, but the vertices of the other part are presented in an on-line fashion. In the classical version, each incoming vertex is either irrevocably matched to a vertex from the other part or stays unmatched forever. In our version, an algorithm is allowed to match the new vertex to a group of elements (possibly empty). Later on, the algorithm can decide to remove some vertices from the group and assign them to another (just presented) vertex, with the restriction that each element belongs to at most one group. We present an optimal (deterministic) algorithm for this problem and prove that its competitive ratio equals $1-\pi/\cosh(\frac{\sqrt{3}}{2}\pi)\approx 0.588$.
Combinatorics Meets Potential Theory
Authors: D'Arco Philippe , Lacivita Valentina , Mustapha Sami 
Published: January 2020

Using potential theoretic techniques, we show how it is possible to determine the dominant asymptotics for the number of walks of length $n$, restricted to the positive quadrant and taking unit steps in a balanced set $\Gamma$. The approach is illustrated through an example of inhomogeneous space walk. This walk takes its steps in $\{ \leftarrow, \uparrow, \rightarrow, \downarrow \}$ or $\{ \swarrow, \leftarrow, \nwarrow, \uparrow,\nearrow, \rightarrow, \searrow, \downarrow \}$, depending on the parity of the coordinates of its positions. The exponential growth of our model is $(4\phi)^n$, where $\phi= \frac{1+\sqrt 5}{2}$denotes the Golden ratio, while the subexponential growth is like $1/n$.As an application of our approach we prove the non-D-finiteness in two dimensions of the length generating functions corresponding to nonsingular small step sets with an infinite group and zero-drift.
Preserving the Number of Cycles of Length $k$ in a Growing Uniform Permutation
Authors: Duchon Philippe , Duvignau Romaric 
Published: January 2020

The goal of this work is to describe a uniform generation tree for permutations which preserves the number of $k$-cycles between any permutation (except for a small unavoidable subset of optimal size) of the tree and its direct children. Moreover, the tree we describe has the property that if the number of $k$-cycles does not change during any $k$ consecutive levels, then any further random descent will always yield permutations with that same number of $k$-cycles. This specific additional property yields interesting applications for exact sampling. We describe a new random generation algorithm for permutations with a fixed number of $k$-cycles in $n+\mathcal{O}(1)$ expected calls to a random integer sampler. Another application is a combinatorial algorithm for exact sampling from the Poisson distribution with parameter $1/k$.
Counting Trees in Graphs
Authors: Verstraete Jacques , Mubayi Dhruv 
Published: January 2020

Erdős and Simonovits proved that the number of paths of length $t$ in an $n$-vertex graph of average degree $d$ is at least $(1 - \delta) nd(d - 1) \cdots (d - t + 1)$, where $\delta = (\log d)^{-1/2 + o(1)}$ as $d \rightarrow \infty$. In this paper, we strengthen and generalize this result as follows. Let $T$ be a tree with $t$ edges. We prove that for any $n$-vertex graph $G$ of average degree $d$ and minimum degree greater than $t$, the number of labelled copies of $T$ in $G$ is at least \[(1 - \varepsilon) n d(d - 1) \cdots (d - t + 1)\] where $\varepsilon = O(d^{-2})$ as $d \rightarrow \infty$. This bound is tight except for the term $1 - \varepsilon$, as shown by a disjoint union of cliques. Our proof is obtained by first showing a lower bound that is a convex function of the degree sequence of $G$, and this answers a question of Dellamonica et. al.
Anti-van der Waerden Numbers of 3-Term Arithmetic Progression
Authors: Berikkyzy Zhanar , Shulte Alex , Young Michael 
Published: January 2020

The anti-van der Waerden number, denoted by $aw([n],k)$, is the smallest $r$ such that every exact $r$-coloring of $[n]$ contains a rainbow $k$-term arithmetic progression. Butler et. al. showed that $\lceil \log_3 n \rceil + 2 \le aw([n],3) \le \lceil \log_2 n \rceil + 1$, and conjectured that there exists a constant $C$ such that $aw([n],3) \le \lceil \log_3 n \rceil + C$. In this paper, we show this conjecture is true by determining $aw([n],3)$ for all $n$. We prove that for $7\cdot 3^{m-2}+1 \leq n \leq 21 \cdot 3^{m-2}$, \begin{equation*} aw([n],3)=\left\{\begin{array}{ll} m+2, & \mbox{if $n=3^m$} \\ m+3, & \mbox{otherwise}. \end{array}\right. \end{equation*}
Binary Covering Arrays on Tournaments
Authors: Maltais Elizabeth , Moura Lucia , Newman Mike 
Published: January 2020

We introduce graph-dependent covering arrays which generalize covering arrays on graphs, introduced by Meagher and Stevens (2005), and graph-dependent partition systems, studied by Gargano, Körner, and Vaccaro (1994). A covering array $\hbox{CA}(n; 2, G, H)$ (of strength 2) on column graph $G$ and alphabet graph $H$ is an $n\times |V(G)|$ array with symbols $V(H)$ such that for every arc $ij \in E(G)$ and for every arc $ab\in E(H)$, there exists a row $\vec{r} = (r_{1},\dots, r_{|V(G)|})$ such that $(r_{i}, r_{j}) = (a,b)$. We prove bounds on $n$ when $G$ is a tournament graph and $E(H)$ consists of the edge $(0,1)$, which corresponds to a directed version of Sperner's 1928 theorem. For two infinite families of column graphs, transitive and so-called circular tournaments, we give constructions of covering arrays which are optimal infinitely often.
Cycles in the Graph of Overlapping Permutations Avoiding Barred Patterns
Authors: Qin Guizhi , Yan Sherry  H.F. 
Published: January 2020

As a variation of De Bruijn graphs on strings of symbols, the graph of overlapping permutations has a directed edge $\pi(1)\pi(2)\ldots \pi(n+1)$ from the standardization of $\pi(1)\pi(2)\ldots \pi(n)$ to the standardization of $\pi(2)\pi(3)\ldots \pi(n+1)$. In this paper, we consider the enumeration of $d$-cycles in the subgraph of overlapping $(231, 4\bar{1}32)$-avoiding permutations. To this end, we introduce the notions of marked Motzkin paths and marked Riordan paths, where a marked Motzkin (resp. Riordan) path is a Motzkin (resp. Riordan) path in which exactly one step before the leftmost return point is marked. We show that the number of closed walks of length $d$ in the subgraph of overlapping $(231, 4\bar{1}32)$-avoiding permutations are closely related to the number of marked Motzkin paths and that of marked Riordan paths. By establishing bijections, we get the enumerations of marked Motzkin paths and marked Riordan paths. As a corollary, we provide bijective proofs of two identities involving Catalan numbers in answer to the problem posed by Ehrenborg, Kitaev and Steingrímsson. Moreover, we get the enumerations of $(231, 4\bar{1}32)$-avoiding affine permutations and $(312, 32\bar{4}1)$-avoiding affine permutations.
Pieri Rules for Classical Groups and Equinumeration between Generalized Oscillating Tableaux and Semistandard Tableaux
Authors: Okada Soichi 
Published: January 2020

We present several equinumerous results between generalized oscillating tableaux and semistandard tableaux and give a representation-theoretic proof to them. As one of the key ingredients of the proof, we provide Pieri rules for the symplectic and orthogonal groups.
Cluster Automorphisms and the Marked Exchange Graphs of Skew-Symmetrizable Cluster Algebras
Authors: Lawson John W 
Published: January 2020

Cluster automorphisms have been shown to have links to the mapping class groups of surfaces, maximal green sequences and to exchange graph automorphisms for skew-symmetric cluster algebras. In this paper we generalise these results to the skew-symmetrizable case by introducing a marking on the exchange graph. Many skew-symmetrizable matrices unfold to skew-symmetric matrices and we consider how cluster automorphisms behave under this unfolding with applications to coverings of orbifolds by surfaces.
Combinatorics of Exceptional Sequences in Type A
Authors: Garver Alexander , Igusa Kiyoshi , Matherne Jacob P. , Ostroff Jonah 
Published: January 2020

Exceptional sequences are certain sequences of quiver representations. We introduce a class of objects called strand diagrams and use these to classify exceptional sequences of representations of a quiver whose underlying graph is a type $\mathbb{A}_n$ Dynkin diagram. We also use variations of these objects to classify $c$-matrices of such quivers, to interpret exceptional sequences as linear extensions of explicitly constructed posets, and to give a simple bijection between exceptional sequences and certain saturated chains in the lattice of noncrossing partitions.
On the Minimum Length of Linear Codes over the Field of 9 Elements
Authors: Kumegawa Kazuki , Okazaki Ysukasa , Maruta Tatsuya 
Published: January 2020

We construct a lot of new $[n,4,d]_9$ codes whose lengths are close to the Griesmer bound and prove the nonexistence of some linear codes attaining the Griesmer bound using some geometric techniques through projective geometries to determine the exact value of $n_9(4,d)$ or to improve the known bound on $n_9(4,d)$ for given values of $d$, where $n_q(k,d)$ is the minimum length $n$ for which an $[n,k,d]_q$ code exists. We also give the updated table for $n_9(4,d)$ for all $d$ except some known cases.
On the Queue-Number of Graphs with Bounded Tree-Width
Authors: Wiechert Veit 
Published: January 2020

A queue layout of a graph consists of a linear order on the vertices and an assignment of the edges to queues, such that no two edges in a single queue are nested. The minimum number of queues needed in a queue layout of a graph is called its queue-number. We show that for each $k\geq0$, graphs with tree-width at most $k$ have queue-number at most $2^k-1$. This improves upon double exponential upper bounds due to Dujmović et al. and Giacomo et al. As a consequence we obtain that these graphs have track-number at most $2^{O(k^2)}$. We complement these results by a construction of $k$-trees that have queue-number at least $k+1$. Already in the case $k=2$ this is an improvement to existing results and solves a problem of Rengarajan and Veni Madhavan, namely, that the maximal queue-number of $2$-trees is equal to $3$.
Perfect Matchings of Trimmed Aztec Rectangles
Authors: Lai Tri 
Published: January 2020

We consider several new families of subgraphs of the square grid whose matchings are enumerated by powers of several small prime numbers: $2$, $3$, $5$, and $11$. Our graphs are obtained by trimming two opposite corners of an Aztec rectangle. The result yields a proof of a conjecture posed by Ciucu. In addition, we reveal a hidden connection between our graphs and the hexagonal dungeons introduced by Blum.
New Combinatorial Formulas for Cluster Monomials of Type $A$ Quivers
Authors: Lee Kyungyong , Li Li , Nguyen Ba 
Published: January 2020

Lots of research focuses on the combinatorics behind various bases of cluster algebras. This paper studies the natural basis of a type $A$ cluster algebra, which consists of all cluster monomials. We introduce a new kind of combinatorial formula for the cluster monomials in terms of the so-called globally compatible collections. We give bijective proofs of these formulas by comparing with the well-known combinatorial models of the $T$-paths and of the perfect matchings in a snake diagram. For cluster variables of a type $A$ cluster algebra, we give a bijection that relates our new formula with the theta functions constructed by Gross, Hacking, Keel and Kontsevich.
Countable Menger's Theorem with Finitary Matroid Constraints on the Ingoing Edges
Authors: Joó Attila 
Published: January 2020

We present a strengthening of the countable Menger's theorem of R. Aharoni. Let $ D=(V,A) $ be a countable digraph with $ s\neq t\in V $ and let $\mathcal{M}=\bigoplus_{v\in V}\mathcal{M}_v $ be a matroid on $ A $ where $ \mathcal{M}_v $ is a finitary matroid on the ingoing edges of $ v $. We show that there is a system of edge-disjoint $ s \rightarrow t $ paths $ \mathcal{P} $ such that the united edge set of these paths is $ \mathcal{M} $-independent, and there is a $ C \subseteq A $ consisting of one edge from each element of $\mathcal{P} $ for which $ \mathsf{span}_{\mathcal{M}}(C) $ covers all the $ s\rightarrow t $ paths in $ D $.
Avoiding Fractional Powers over the Natural Numbers
Authors: Pudwell Lara , Rowland Eric 
Published: January 2020

We study the lexicographically least infinite $a/b$-power-free word on the alphabet of non-negative integers. Frequently this word is a fixed point of a uniform morphism, or closely related to one. For example, the lexicographically least $7/4$-power-free word is a fixed point of a $50847$-uniform morphism. We identify the structure of the lexicographically least $a/b$-power-free word for three infinite families of rationals $a/b$ as well many "sporadic" rationals that do not seem to belong to general families. To accomplish this, we develop an automated procedure for proving $a/b$-power-freeness for morphisms of a certain form, both for explicit and symbolic rational numbers $a/b$. Finally, we establish a connection to words on a finite alphabet. Namely, the lexicographically least $27/23$-power-free word is in fact a word on the finite alphabet $\{0, 1, 2\}$, and its sequence of letters is $353$-automatic.
On Repetition Thresholds of Caterpillars and Trees of Bounded Degree
Authors: Lužar Borut , Ochem Pascal , Pinlou Alexandre 
Published: January 2020

The repetition threshold is the smallest real number $\alpha$ such that there exists an infinite word over a $k$-letter alphabet that avoids repetition of exponent strictly greater than $\alpha$. This notion can be generalized to graph classes. In this paper, we completely determine the repetition thresholds for caterpillars and caterpillars of maximum degree $3$. Additionally, we present bounds for the repetition thresholds of trees with bounded maximum degrees.
Identities in Plactic, Hypoplactic, Sylvester, Baxter, and Related Monoids
Authors: Cain Alan J. , Malheiro António 
Published: January 2020

This paper considers whether non-trivial identities are satisfied by certain `plactic-like' monoids that, like the plactic monoid, are closely connected to combinatorics. New results show that the hypoplactic, sylvester, Baxter, stalactic, and taiga monoids satisfy identities, and indeed give shortest identities satisfied by these monoids. The existing state of knowledge is discussed for the plactic monoid and left and right patience sorting monoids.
The $Z$-Polynomial of a Matroid
Authors: Proudfoot Nicholas , Xu Yuan , Young Ben 
Published: January 2020

We introduce the $Z$-polynomial of a matroid, which we define in terms of the Kazhdan-Lusztig polynomial. We then exploit a symmetry of the $Z$-polynomial to derive a new recursion for Kazhdan-Lusztig coefficients. We solve this recursion, obtaining a closed formula for Kazhdan-Lusztig coefficients as alternating sums of multi-indexed Whitney numbers. For realizable matroids, we give a cohomological interpretation of the $Z$-polynomial in which the symmetry is a manifestation of Poincaré duality.
A New Approach for Examining $q$-Steiner Systems
Authors: Etzion Tuvi 
Published: January 2020

One of the most intriguing problems in $q$-analogs of designs and codes is the existence question of an infinite family of $q$-analog of Steiner systems (spreads not included) in general, and the existence question for the $q$-analog of the Fano plane in particular.We exhibit a completely new method to attack this problem. In the process we define a new family of designs whose existence is implied by the existence of $q$-Steiner systems, but could exist even if the related $q$-Steiner systems do not exist.The method is based on a possible system obtained by puncturing all the subspaces of the $q$-Steiner system several times. We define the punctured system as a new type of design and enumerate the number of subspaces of various types that it might have. It will be evident that its existence does not imply the existence of the related $q$-Steiner system. On the other hand, this type of design demonstrates how close can we get to the related $q$-Steiner system.Necessary conditions for the existence of such designs are presented. These necessary conditions will be also necessary conditions for the existence of the related $q$-Steiner system. Trivial and nontrivial direct constructions and a nontrivial recursive construction for such designs are given. Some of the designs have a symmetric structure, which is uniform in the dimensions of the existing subspaces in the system. Most constructions are based on this uniform structure of the design or its punctured designs.
Proper Distinguishing Colorings with Few Colors for Graphs with Girth at Least 5
Authors: Cranston Daniel W. 
Published: January 2020

The distinguishing chromatic number, $\chi_D(G)$, of a graph $G$ is the smallest number of colors in a proper coloring, $\varphi$, of $G$, such that the only automorphism of $G$ that preserves all colors of $\varphi$ is the identity map. Collins and Trenk conjectured that if $G$ is connected with girth at least 5 and $G\ne C_6$, then $\chi_D(G)\leqslant \Delta+1$. We prove this conjecture.
Monochromatic Cycle Partitions of $2$-Coloured Graphs with Minimum Degree $3n/4$
Authors: Letzter Shoham 
Published: January 2020

Balogh, Barát, Gerbner, Gyárfás, and Sárközy made the following conjecture. Let $G$ be a graph on $n$ vertices with minimum degree at least $3n/4$. Then for every $2$-edge-colouring of $G$, the vertex set $V(G)$ may be partitioned into two vertex-disjoint cycles, one of each colour.
We prove this conjecture for large $n$, improving approximate results by the aforementioned authors and by DeBiasio and Nelsen.
On the Maximum Degree of Path-Pairable Planar Graphs
Authors: Girão António , Mészáros Gábor , Popielarz Kamil , Snyder Richard 
Published: January 2020

A graph is path-pairable if for any pairing of its vertices there exist edge-disjoint paths joining the vertices in each pair. We investigate the behaviour of the maximum degree in path-pairable planar graphs. We show that any $n$-vertex path-pairable planar graph must contain a vertex of degree linear in $n$. Our result generalizes to graphs embeddable on a surface of finite genus.
A Splitter Theorem for 3-Connected 2-Polymatroids
Authors: Oxley James , Semple Charles , Whittle Geoff 
Published: January 2020

Seymour's Splitter Theorem is a basic inductive tool for dealing with $3$-connected matroids. This paper proves a generalization of that theorem for the class of $2$-polymatroids. Such structures include matroids, and they model both sets of points and lines in a projective space and sets of edges in a graph. A series compression in such a structure is an analogue of contracting an edge of a graph that is in a series pair. A $2$-polymatroid $N$ is an s-minor of a $2$-polymatroid $M$ if $N$ can be obtained from $M$ by a sequence of contractions, series compressions, and dual-contractions, where the last are modified deletions. The main result proves that if $M$ and $N$ are $3$-connected $2$-polymatroids such that $N$ is an s-minor of $M$, then $M$ has a $3$-connected s-minor $M'$ that has an s-minor isomorphic to $N$ and has $|E(M)| - 1$ elements unless $M$ is a whirl or the cycle matroid of a wheel. In the exceptional case, such an $M'$ can be found with $|E(M)| - 2$ elements.
Proof of Chapoton's Conjecture on Newton Polygons of $q$-Ehrhart Polynomials
Authors: Kim Jang Soo , Song U-Keun 
Published: January 2020

Recently, Chapoton found a $q$-analog of Ehrhart polynomials, which are polynomials in $x$ whose coefficients are rational functions in $q$. Chapoton conjectured the shape of the Newton polygon of the numerator of the $q$-Ehrhart polynomial of an order polytope. In this paper, we prove Chapoton's conjecture.
The cd-Indices of Intervals in the Uncrossing Partial Order on Matchings
Authors: Kim Younghwan 
Published: January 2020

We study flag enumeration in intervals in the uncrossing partial order on matchings. We produce a recursion for the cd-indices of intervals in the uncrossing poset $P_n$. We explicitly describe the matchings by constructing an order-reversing bijection. We obtain a recursion for the ab-indices of intervals in the poset $\hat{P}_n$, the poset $P_n$ with a unique minimum $\hat0$ adjoined.
On Graph-Theoretical Invariants of Combinatorial Manifolds
Authors: Cavicchioli Alberto , Spaggiari Fulvia 
Published: January 2020

The goal of this paper is to give some theorems which relate to the problem of classifying combinatorial (resp. smooth) closed manifolds up to piecewise-linear (PL) homeomorphism. For this, we use the combinatorial approach to the topology of PL manifolds by means of a special kind of edge-colored graphs, called crystallizations. Within this representation theory, Bracho and Montejano introduced in 1987 a nonnegative numerical invariant, called the reduced complexity, for any closed $n$-dimensional PL manifold. Here we consider this invariant, and extend in this context the concept of average order first introduced by Luo and Stong in 1993, and successively investigated by Tamura in 1996 and 1998. Then we obtain some classification results for closed connected smooth low-dimensional manifolds according to reduced complexity and average order. Finally, we answer to a question posed by Trout in 2013.
Quasirandomness in Hypergraphs
Authors: Aigner-Horev Elad , Conlon David , Hàn Hiệp , Person Yury , Schacht Mathias 
Published: January 2020

An $n$-vertex graph $G$ of edge density $p$ is considered to be quasirandom if it shares several important properties with the random graph $G(n,p)$. A well-known theorem of Chung, Graham and Wilson states that many such `typical' properties are asymptotically equivalent and, thus, a graph $G$ possessing one such property automatically satisfies the others.In recent years, work in this area has focused on uncovering more quasirandom graph properties and on extending the known results to other discrete structures. In the context of hypergraphs, however, one may consider several different notions of quasirandomness. A complete description of these notions has been provided recently by Towsner, who proved several central equivalences using an analytic framework. We give short and purely combinatorial proofs of the main equivalences in Towsner's result.
$\lambda$-Euler's Difference Table for Colored Permutations
Authors: Han Bin 
Published: January 2020

Motivated by the $\lambda$-Euler's difference table of Eriksen et al. and colored Euler's difference table of Faliharimalala and Zeng, we study the $\lambda$-analogue of colored Euler's difference table and generalize their results. We generalize the number of permutations with $k$-excedances studied by Liese and Remmel in colored permutations. We also extend Wang et al.'s recent results about $r$-derangements by relating with the sequences arising from the difference table.
On Generalized Choice and Coloring Numbers
Authors: Dvořák Zdenēk , Pekárek Jakub , Sereni Jean-Sébastien 
Published: January 2020

A well-known result of Alon shows that the coloring number of a graph is bounded by a function of its choosability. We explore this relationship in a more general setting with relaxed assumptions on color classes, encoded by a graph parameter.
Using Symbolic Computation to Prove Nonexistence of Distance-Regular Graphs
Authors: Vidali Janoš 
Published: January 2020

A package for the Sage computer algebra system is developed for checking feasibility of a given intersection array for a distance-regular graph. We use this tool to show that there is no distance-regular graph with intersection array$$\{(2r+1)(4r+1)(4t-1), 8r(4rt-r+2t), (r+t)(4r+1); 1, (r+t)(4r+1), 4r(2r+1)(4t-1)\} (r, t \geq 1),$$$\{135,\! 128,\! 16; 1,\! 16,\! 120\}$, $\{234,\! 165,\! 12; 1,\! 30,\! 198\}$ or $\{55,\! 54,\! 50,\! 35,\! 10; 1,\! 5,\! 20,\! 45,\! 55\}$. In all cases, the proofs rely on equality in the Krein condition, from which triple intersection numbers are determined. Further combinatorial arguments are then used to derive nonexistence.