arXiv subjects

Ryan R. Martin

Publications and source records attributed to Ryan R. Martin.

At least 73 records · Page 4Linked to original sources

arXiv2016

A version of Szemerédi's regularity lemma for multicolored graphs and directed graphs that is suitable for induced graphs

In this manuscript we develop a version of Szemerédi's regularity lemma that is suitable for analyzing multicolorings of complete graphs and directed graphs. In this, we follow the proof of Alon, Fischer, Krivelevich and M. Szegedy [Combinatorica, 20(4) (2000), 451--476] who prove a similar result for graphs. The purpose is to extend classical results on dense hereditary properties, such as the speed of the property or edit distance, to the above-mentioned combinatorial objects.

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arXiv2016

Edit distance and its computation

In this paper, we provide a method for determining the asymptotic value of the maximum edit distance from a given hereditary property. This method permits the edit distance to be computed without using Szemerédi's Regularity Lemma directly. Using this new method, we are able to compute the edit distance from hereditary properties for which it was previously unknown. For some graphs $H$, the edit distance from ${\rm Forb}(H)$ is computed, where ${\rm forb}(H)$ is the class of graphs which contain no induced copy of graph $H$. Those graphs for which we determine the edit distance asymptotically are $H=K_a+E_b$, an $a$-clique with $b$ isolated vertices, and $H=K_{3,3}$, a complete bipartite graph. We also provide a graph, the first such construction, for which the edit distance cannot be determined just by considering partitions of the vertex set into cliques and cocliques. In the process, we develop weighted generalizations of Turán's theorem, which may be of independent interest.

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arXiv2016

On the editing distance of graphs

An edge-operation on a graph $G$ is defined to be either the deletion of an existing edge or the addition of a nonexisting edge. Given a family of graphs $\mathcal{G}$, the editing distance from $G$ to $\mathcal{G}$ is the smallest number of edge-operations needed to modify $G$ into a graph from $\mathcal{G}$. In this paper, we fix a graph $H$ and consider ${\rm Forb}(n,H)$, the set of all graphs on $n$ vertices that have no induced copy of $H$. We provide bounds for the maximum over all $n$-vertex graphs $G$ of the editing distance from $G$ to ${\rm Forb}(n,H)$, using an invariant we call the {\it binary chromatic number} of the graph $H$. We give asymptotically tight bounds for that distance when $H$ is self-complementary and exact results for several small graphs $H$.

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arXiv2016

The diameter game

A large class of Positional Games are defined on the complete graph on $n$ vertices. The players, Maker and Breaker, take the edges of the graph in turns, and Maker wins iff his subgraph has a given -- usually monotone -- property. Here we introduce the $d$-diameter game, which means that Maker wins iff the diameter of his subgraph is at most $d$. We investigate the biased version of the game; i.e., when the players may take more than one, and not necessarily the same number of edges, in a turn. Our main result is that we proved that the $2$-diameter game has the following surprising property: Breaker wins the game in which each player chooses one edge per turn, but Maker wins as long as he is permitted to choose $2$ edges in each turn whereas Breaker can choose as many as $(1/9)n^{1/8}/(\ln n)^{3/8}$. In addition, we investigate $d$-diameter games for $d\ge 3$. The diameter games are strongly related to the degree games. Thus, we also provide a generalization of the fair degree game for the biased case.

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arXiv2016

$Q_2$-free families in the Boolean lattice

For a family $\mathcal{F}$ of subsets of [n]=\{1, 2, ..., n} ordered by inclusion, and a partially ordered set P, we say that $\mathcal{F}$ is P-free if it does not contain a subposet isomorphic to P. Let $ex(n, P)$ be the largest size of a P-free family of subsets of [n]. Let $Q_2$ be the poset with distinct elements a, b, c, d, a<b, c<d; i.e., the 2-dimensional Boolean lattice. We show that $2N -o(N) \leq ex(n, Q_2)\leq 2.283261N +o(N), $ where $N = \binom{n}{\lfloor n/2 \rfloor}$. We also prove that the largest $Q_2$-free family of subsets of [n] having at most three different sizes has at most 2.20711N members.

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arXiv2016

On the computation of edit distance functions

The edit distance between two graphs on the same labeled vertex set is the size of the symmetric difference of the edge sets. The edit distance function of the hereditary property, $\mathcal{H}$, is a function of $p\in[0,1]$ and is the limit of the maximum normalized distance between a graph of density $p$ and $\mathcal{H}$. This paper uses the symmetrization method of Sidorenko in order to compute the edit distance function of various hereditary properties. For any graph $H$, ${\rm Forb}(H)$ denotes the property of not having an induced copy of $H$. We compute the edit distance function for ${\rm Forb}(H)$, where $H$ is any split graph, and the graph $H_9$, a graph first used to describe the difficulties in computing the edit distance function.

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arXiv2016

Expected values of parameters associated with the minimum rank of a graph

We investigate the expected value of various graph parameters associated with the minimum rank of a graph, including minimum rank/maximum nullity and related Colin de Verdière-type parameters. Let $G(v,p)$ denote the usual Erdős-Rényi random graph on $v$ vertices with edge probability $p$. We obtain bounds for the expected value of the random variables ${\rm mr}(G(v,p))$, ${\rm M}(G(v,p))$, $ν(G(v,p))$ and $ξ(G(v,p))$, which yield bounds on the average values of these parameters over all labeled graphs of order $v$.

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arXiv2016

Tripartite Version of the Corrádi-Hajnal Theorem

Let $G$ be a tripartite graph with $N$ vertices in each vertex class. If each vertex is adjacent to at least $(2/3)N$ vertices in each of the other classes, then either $G$ contains a subgraph that consists of $N$ vertex-disjoint triangles or $G$ is a specific graph in which each vertex is adjacent to exactly $(2/3)N$ vertices in each of the other classes.

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arXiv2016

On Avoider-Enforcer games

In the Avoider-Enforcer game on the complete graph $K_n$, the players (Avoider and Enforcer) each take an edge in turn. Given a graph property $\mathcal{P}$, Enforcer wins the game if Avoider's graph has the property $\mathcal{P}$. An important parameter is $τ_E({\cal P})$, the smallest integer $t$ such that Enforcer can win the game against any opponent in $t$ rounds. In this paper, let $\mathcal{F}$ be an arbitrary family of graphs and $\mathcal{P}$ be the property that a member of $\mathcal{F}$ is a subgraph or is an induced subgraph. We determine the asymptotic value of $τ_E(\mathcal{P})$ when $\mathcal{F}$ contains no bipartite graph and establish that $τ_E(\mathcal{P})=o(n^2)$ if $\mathcal{F}$ contains a bipartite graph. The proof uses the game of JumbleG and the Szemerédi Regularity Lemma.

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arXiv2016

Multicolor and directed edit distance

The editing of a combinatorial object is the alteration of some of its elements such that the resulting object satisfies a certain fixed property. The edit problem for graphs, when the edges are added or deleted, was first studied independently by the authors and Kézdy [J. Graph Theory (2008), 58(2), 123--138] and by Alon and Stav [Random Structures Algorithms (2008), 33(1), 87--104]. In this paper, a generalization of graph editing is considered for multicolorings of the complete graph as well as for directed graphs. Specifically, the number of edge-recolorings sufficient to be performed on any edge-colored complete graph to satisfy a given hereditary property is investigated. The theory for computing the edit distance is extended using random structures and so-called types or colored homomorphisms of graphs.

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arXiv2016

On the complexity of Chooser-Picker positional games

Two new versions of the so-called Maker-Breaker Positional Games are defined by József Beck in [{\em Combinatorica} {\bf 22}(2) (2002) 169--216]. He defines two players, Picker and Chooser. In each round, Picker takes a pair of elements not already selected and Chooser keeps one and returns the other to Picker. In the Picker-Chooser version Picker plays as Maker and Chooser plays as Breaker, while the roles are swapped in the Chooser-Picker version. The outcome of these games is sometimes very similar to that of the traditional Maker-Breaker games. Here we show that both Picker-Chooser and Chooser-Picker games are NP-hard, which gives support to the paradigm that the games behave similarly while being quite different in definition. We also investigate the pairing strategies for Maker-Breaker games, and apply these results to the game called "Snaky."

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arXiv2016

Regular colorings and factors of regular graphs

An $(r-1,1)$-coloring of an $r$-regular graph $G$ is an edge coloring such that each vertex is incident to $r-1$ edges of one color and $1$ edge of a different color. In this paper, we completely characterize all $4$-regular pseudographs (graphs that may contain parallel edges and loops) which do not have a $(3,1)$-coloring. An $\{r-1,1\}$-factor of an $r$-regular graph is a spanning subgraph in which each vertex has degree either $r-1$ or $1$. We prove various conditions that that must hold for any vertex-minimal $5$-regular pseudographs without $(4,1)$-colorings or without $\{4,1\}$-factors. Finally, for each $r\geq 6$ we construct graphs that are not $(r-1,1)$-colorable and, more generally, are not $(r-t,t)$-colorable for small $t$.

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arXiv2016

Rainbow arithmetic progressions

In this paper, we investigate the anti-Ramsey (more precisely, anti-van der Waerden) properties of arithmetic progressions. For positive integers $n$ and $k$, the expression $aw([n],k)$ denotes the smallest number of colors with which the integers $\{1,\ldots,n\}$ can be colored and still guarantee there is a rainbow arithmetic progression of length $k$. We establish that $aw([n],3)=Θ(\log n)$ and $aw([n],k)=n^{1-o(1)}$ for $k\geq 4$. For positive integers $n$ and $k$, the expression $aw(Z_n,k)$ denotes the smallest number of colors with which elements of the cyclic group of order $n$ can be colored and still guarantee there is a rainbow arithmetic progression of length $k$. In this setting, arithmetic progressions can "wrap around," and $aw(Z_n,3)$ behaves quite differently from $aw([n],3)$, depending on the divisibility of $n$. As shown in [Jungić et al., \textit{Combin. Probab. Comput.}, 2003], $aw(Z_{2^m},3) = 3$ for any positive integer $m$. We establish that $aw(Z_n,3)$ can be computed from knowledge of $aw(Z_p,3)$ for all of the prime factors $p$ of $n$. However, for $k\geq 4$, the behavior is similar to the previous case, that is, $aw(Z_n,k)=n^{1-o(1)}$.

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arXiv2015

On the Edit Distance of Powers of Cycles

The edit distance between two graphs on the same labeled vertex set is defined to be the size of the symmetric difference of the edge sets. The edit distance function of a hereditary property $\mathcal{H}$ is a function of $p\in [0,1]$ that measures, in the limit, the maximum normalized edit distance between a graph of density $p$ and $\mathcal{H}$. In this paper, we address the edit distance function for $\mbox{Forb}(H)$, where $H=C_h^t$, the $t^{\rm th}$ power of the cycle of length $h$. For $h\geq 2t(t+1)+1$ and $h$ not divisible by $t+1$, we determine the function for all values of $p$. For $h\geq 2t(t+1)+1$ and $h$ divisible by $t+1$, the function is obtained for all but small values of $p$. We also obtain some results for smaller values of $h$.

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arXiv2015

An improved bound on the diamond-free poset problem

In the theory of partially-ordered sets, the two-dimensional Boolean lattice is known as the diamond. In this paper, we show that, if $\mathcal{F}$ is a family in the $n$-dimensional Boolean lattice that has no diamond as a subposet, then $|\mathcal{F}|\leq 2.206653{n\choose \lfloor n/2\rfloor}$, improving a bound by the authors and Michael Young.

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arXiv2014

On the edit distance from $K_{2,t}$-free graphs (Extended Version)

The edit distance between two graphs on the same vertex set is defined to be the size of the symmetric difference of their edge sets. The edit distance function of a hereditary property, $\mathcal{H}$, is a function of $p$, and measures, asymptotically, the furthest graph of edge density $p$ from $\mathcal{H}$ under this metric. In this paper, we address the hereditary property $\forb(K_{2,t})$, the property of having no induced copy of the complete bipartite graph with 2 vertices in one class and $t$ in the other. Employing an assortment of techniques and colored regularity graph constructions, we are able to determine the edit distance function over the entire domain $p\in [0,1]$ when $t=3,4$ and extend the interval over which the edit distance function for $\forb(K_{2,t})$ is known for all values of $t$, determining its maximum value for all odd $t$. We also prove that the function for odd $t$ has a nontrivial interval on which it achieves its maximum. These are the only known principal hereditary properties for which this occurs. In the process of studying this class of functions, we encounter some surprising connections to extremal graph theory problems, such as strongly regular graphs and the problem of Zarankiewicz. This is an extended version of a paper with the same name now published in the Journal of Graph Theory \cite{jgt_version}. In particular, this version contains Appendix A, which has tables and graphs pertaining to the hereditary property $\forb(K_{2,t})$ for small $t$, and Appendix B, which has the proofs of Lemma 31, Proposition 32, Proposition 33, and Lemma 34.

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arXiv2013

On the approximate shape of degree sequences that are not potentially $H$-graphic

A sequence of nonnegative integers $π$ is {\it graphic} if it is the degree sequence of some graph $G$. In this case we say that $G$ is a \textit{realization} of $π$, and we write $π=π(G)$. A graphic sequence $π$ is {\it potentially $H$-graphic} if there is a realization of $π$ that contains $H$ as a subgraph. Given nonincreasing graphic sequences $π_1=(d_1,\ldots,d_n)$ and $π_2 = (s_1,\ldots,s_n)$, we say that $π_1$ {\it majorizes} $π_2$ if $d_i \geq s_i$ for all $i$, $1 \leq i \leq n$. In 1970, Erdős showed that for any $K_{r+1}$-free graph $H$, there exists an $r$-partite graph $G$ such that $π(G)$ majorizes $π(H)$. In 2005, Pikhurko and Taraz generalized this notion and showed that for any graph $F$ with chromatic number $r+1$, the degree sequence of an $F$-free graph is, in an appropriate sense, nearly majorized by the degree sequence of an $r$-partite graph. In this paper, we give similar results for degree sequences that are not potentially $H$-graphic. In particular, there is a graphic sequence $π^*(H)$ such that if $π$ is a graphic sequence that is not potentially $H$-graphic, then $π$ is close to being majorized by $π^*(H)$. Similar to the role played by complete multipartite graphs in the traditional extremal setting, the sequence $π^*(H)$ asymptotically gives the maximum possible sum of a graphic sequence $π$ that is not potentially $H$-graphic.

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arXiv2013

Twins in graphs

A basic pigeonhole principle insures an existence of two objects of the same type if the number of objects is larger than the number of types. Can such a principle be extended to a more complex combinatorial structure? Here, we address such a question for graphs. We call two disjoint subsets $A, B$ of vertices \emph{\textbf{twins}} if they have the same cardinality and induce subgraphs of the same size. Let $t(G)$ be the largest $k$ such that $G$ has twins on $k$ vertices each. We provide the bounds on $t(G)$ in terms of the number of edges and vertices using discrepancy results for induced subgraphs. In addition, we give conditions under which $t(G)= |V(G)|/2$ and show that if $G$ is a forest then $t(G) \geq |V(G)|/2 - 1$.

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