Abstract: Let HPn,m,k be drawn uniformly from all m-edge, k-uniform, k-partite hypergraphs where each part of the partition is a disjoint copy of [n]. We let HPn,m,k(κ) be an edge colored version, where we color each edge randomly from one of κ colors. We show that if κ=n and m=Knlogn where K is sufficiently large then w.h.p. there is a rainbow colored perfect matching. I.e. a perfect matching in which every edge has a different color. We also show that if n is even and m=Knlogn where K is sufficiently large then w.h.p. there is a rainbow colored Hamilton cycle in Gn,m(n). Here Gn,m(n) denotes a random edge coloring of Gn,m with n colors. When n is odd, our proof requires m=ω(nlogn) for there to be a rainbow Hamilton cycle. © 2015 Wiley Periodicals, Inc. Random Struct. Alg., 48, 503–523, 2016

Abstract: Finding a proper coloring of a t-colorable graph G with t colors is a classic NP-hard problem when t ≥ 3. In this work, we investigate the approximate coloring problem in which the objective is to find a proper c-coloring of G where c ≥ t. We show that for all t ≥ 3, it is NP-hard to find a c-coloring when c ≤ 2t - 2. In the regime where t is small, this improves, via a unified approach, the previously best known hardness result of c ≤ max{2t - 5, t + 2⌊t/3⌋ - 1} [9, 21, 13]. For example, we show that 6-coloring a 4-colorable graph is NP-hard, improving on the NP-hardness of 5-coloring a 4-colorable graph. We also generalize this to related problems on the strong coloring of hypergraphs. A k-uniform hypergraph H is t-strong colorable (where t ≥ k) if there is a t-coloring of the vertices such that no two vertices in each hyperedge of H have the same color. We show that if t = ⌈3k/2⌉, then it is NP-hard to find a 2-coloring of the vertices of H such that no hyperedge is monochromatic. We conjecture that a similar hardness holds for t = k + 1. We establish the NP-hardness of these problems by reducing from the hardness of the Label Cover problem, via a "dictatorship test" gadget graph. By combinatorially classifying all possible colorings of this graph, we can infer labels to provide to the label cover problem. This approach generalizes the "weak polymorphism" framework of [3], though interestingly our results are "PCP-free" in that they do not require any approximation gap in the starting Label Cover instance.

Abstract: The strong chromatic index of a multigraph is the minimum k such that the edge set can be k -colored requiring that each color class induces a matching. We verify a conjecture of Faudree, Gyarfas, Schelp and Tuza, showing that every planar multigraph with maximum degree at most 3 has strong chromatic index at most 9, which is sharp.

Abstract: A graph G is -colorable if can be partitioned into two sets and so that the maximum degree of is at most j and of is at most k. While the problem of verifying whether a graph is 0, 0-colorable is easy, the similar problem with in place of 0, 0 is NP-complete for all nonnegative j and k with . Let denote the supremum of all x such that for some constant every graph G with girth g and for every is -colorable. It was proved recently that . In a companion paper, we find the exact value . In this article, we show that increasing g from 5 further on does not increase much. Our constructions show that for every g, . We also find exact values of for all g and all .

Abstract: We present a local algorithm producing an independent set of expected size $0.44533n$ on large-girth 3-regular graphs and $0.40407n$ on large-girth 4-regular graphs. We also construct a cut (or bisection or bipartite subgraph) with $1.34105n$ edges on large-girth 3-regular graphs. These decrease the gaps between the best known upper and lower bounds from $0.0178$ to $0.01$, from $0.0242$ to $0.0123$ and from $0.0724$ to $0.0616$, respectively. We are using local algorithms, therefore, the method also provides upper bounds for the fractional coloring numbers of $1 / 0.44533 \approx 2.24554$ and $1 / 0.40407 \approx 2.4748$ and fractional edge coloring number $1.5 / 1.34105 \approx 1.1185$. Our algorithms are applications of the technique introduced by Hoppen and Wormald.

Abstract: We study choosability with separation which is a constrained version of list coloring of graphs. A k,d-list assignment L of a graph G is a function that assigns to each vertex v a list Lv of at least k colors and for any adjacent pair xy, the lists Lx and Ly share at most d colors. A graph G is k,d-choosable if there exists an L-coloring of G for every k,d-list assignment L. This concept is also known as choosability with separation. We prove that planar graphs without 4-cycles are 3, 1-choosable and that planar graphs without 5- and 6-cycles are 3, 1-choosable. In addition, we give an alternative and slightly stronger proof that triangle-free planar graphs are 3, 1-choosable.

Abstract: We give a linear-time algorithm to decide 3-colorability of a triangle-free graph embedded in a fixed surface, and a quadratic-time algorithm to output a 3-coloring in the affirmative case. The algorithms also allow to prescribe the coloring of a bounded number of vertices.

Abstract: Graph coloring is a fundamental NP-hard problem in graph theory. It has a wide range of real applications, such as Operations Research, Communication Network, Computational Biology and Compiler Optimization. Notable efforts have been spent on designing its approximation algorithms. Halldrsson proposed the algorithm (denoted as SampleIS) with the current best known approximation ratio. However, its time complexity is O(|G|3), where |G| is the number of vertices of a graph G. It is clear that SampleIS is not practical for large graphs. In this paper, we propose a practical vertex-cut based coloring technique (VColor) for coloring large graphs. First, we partition G into k connected components (CCs) of a small size s by removing a vertex-cut component (VCC). For each CC, we apply our novel coloring algorithm, based on maximal independent set enumeration. The approximation ratio and the time complexity for coloring the k CCs are log s + 1 and O(ks23s/3), respectively, whereas those of SampleIS are ks(log log ks)2/ log3 ks and O(k3s3). For the VCC, we simply apply SampleIS. To combine the colorings of the CCs and the VCC, we propose a maximum matching based algorithm. Second, in the context of a database of graphs, users may color many graphs. We propose an optimization technique, inspired by multi-query optimization, for coloring a set of graphs. We design a VP hierarchy (VPH) to represent the common subgraphs as the common CCs. Third, we propose techniques for determining the optimal values of the parameters of VColor. Our extensive experimental evaluation on real-world graphs confirms the efficiency and/or effectiveness of our proposed techniques. In particular, VColor is more than 500 times faster than SampleIS, and the number of colors used are comparable on real graphs Yeast and LS.

Abstract: Abstract A unique-maximum k-coloring with respect to faces of a plane graph G is a coloring with colors 1, . . . , k so that, for each face of G, the maximum color occurs exactly once on the vertices of α. We prove that any plane graph is unique-maximum 3-colorable and has a proper unique-maximum coloring with 6 colors.

Abstract: We consider coloring problems in the distributed message-passing setting. The previously-known deterministic algorithms for edge-coloring employed at least (2Delta - 1) colors, even though any graph admits an edge-coloring with Delta + 1 colors [V64]. Moreover, the previously-known deterministic algorithms that employed at most O(Delta) colors required superlogarithmic time [B15,BE10,BE11,FHK15]. In the current paper we devise deterministic edge-coloring algorithms that employ only Delta + o(Delta) colors, for a very wide family of graphs. Specifically, as long as the arboricity is a = O(Delta^{1 - \epsilon}), for a constant epsilon > 0, our algorithm computes such a coloring within {polylogarithmic} deterministic time. We also devise significantly improved deterministic edge-coloring algorithms for {general graphs} for a very wide range of parameters. Specifically, for any value $\chi$ in the range [4Delta, 2^{o(log Delta)} \cdot Delta], our \chi-edge-coloring algorithm has smaller running time than the best previously-known \chi-edge-coloring algorithms. Our algorithms are actually much more general, since edge-coloring is equivalent to {vertex-coloring of line graphs.} Our method is applicable to vertex-coloring of the family of graphs with {bounded diversity} that contains line graphs, line graphs of hypergraphs, and many other graphs. Our results are obtained using a novel technique that connects vertices or edges in a certain way that reduces clique size. The resulting structures, which we call {connectors}, can be colored more efficiently than the original graph. Moreover, the color classes constitute simpler subgraphs that can be colored even more efficiently using appropriate connectors. Hence, we recurse until we obtain sufficiently simple structures that are colored directly. We introduce several types of connectors that are useful for various scenarios.

Abstract: An edge coloring of a graph is said to be an r-local coloring if the edges incident to any vertex are colored with at most r colors. Generalizing a result of Bessy and Thomasse, we prove that the vertex set of any 2-locally colored complete graph may be partitioned into two disjoint monochromatic cycles of different colors. Moreover, for any natural number r, we show that the vertex set of any r-locally colored complete graph may be partitioned into Or2logr disjoint monochromatic cycles. This generalizes a result of Erdi¾?s, Gyarfas, and Pyber.

Abstract: We show that depth first search can be used to give a proper coloring of connected signed graphs G using at most $$\Delta (G)$$ colors, provided G is different from a balanced complete graph, a balanced cycle of odd length, and an unbalanced cycle of even length, thus giving a new, short proof to the generalization of Brooks’ theorem to signed graphs, first proved by Macajova, Raspaud, and Skoviera.

Abstract: In 2004, Karonski, ?uczak and Thomason posed the conjecture that every graph admits a vertex-coloring edge 3-weighting using colors 1, 2, 3. In this paper, we prove that this conjecture is true if the vertex-coloring is relaxed to be a tree-coloring. Furthermore, we verify that some classes of graphs permit tree-coloring edge 2-weightings.

Abstract: The geometric thickness \(\bar\theta\)(G) of a graph G is the smallest integer t such that there exist a straight-line drawing Γ of G and a partition of its straight-line edges into t subsets, where each subset induces a planar drawing in Γ. Over a decade ago, Hutchinson, Shermer, and Vince proved that any n-vertex graph with geometric thickness two can have at most 6n − 18 edges, and for every n ≥ 8 they constructed a geometric thickness two graph with 6n − 20 edges. In this paper, we construct geometric thickness two graphs with 6n − 19 edges for every n ≥ 9, which improves the previously known 6n − 20 lower bound. We then construct a thickness two graph with 10 vertices that has geometric thickness three, and prove that the problem of recognizing geometric thickness two graphs is NP-hard, answering two questions posed by Dillencourt, Eppstein and Hirschberg. Finally, we prove the NP-hardness of coloring graphs of geometric thickness t with 4t − 1 colors, which strengthens a result of McGrae and Zito, when t = 2.

Abstract: Let G be a Class 1 graph with maximum degree 4 and let be an integer. We show that any proper t-edge coloring of G can be transformed to any proper 4-edge coloring of G using only transformations on 2-colored subgraphs (so-called interchanges). This settles the smallest previously unsolved case of a well-known problem of Vizing on interchanges, posed in 1965. Using our result we give an affirmative answer to a question of Mohar for two classes of graphs: we show that all proper 5-edge colorings of a Class 1 graph with maximum degree 4 are Kempe equivalent, that is, can be transformed to each other by interchanges, and that all proper 7-edge colorings of a Class 2 graph with maximum degree 5 are Kempe equivalent.

Abstract: In a proper vertex coloring of a graph a subgraph is colorful if its vertices are colored with different colors. It is well-known (see for example in Gy a rf a s (1980)) that in every proper coloring of a $k$-chromatic graph there is a colorful path $P_k$ on $k$ vertices. The first author proved in 1987 that $k$-chromatic and triangle-free graphs have a path $P_k$ which is an induced subgraph . N.R. Aravind conjectured that these results can be put together: in every proper coloring of a $k$-chromatic triangle-free graph, there is an induced colorful $P_k$. Here we prove the following weaker result providing some evidence towards this conjecture: For a suitable function $f(k)$, in any proper coloring of an $f(k)$-chromatic graph of girth at least five, there is an induced colorful path on $k$ vertices.

Abstract: In recent years there has been increased interest in extremal problems for "counting" parameters of graphs. For example, the Kahn-Zhao theorem gives an upper bound on the number of independent sets in a d-regular graph. In the same spirit, the Upper Matching Conjecture claims an upper bound on the number of k-matchings in a d-regular graph. Here we consider both matchings and matchings of fixed sizes in graphs with a given number of vertices and edges. We prove that the graph with the fewest matchings is either the lex or the colex graph. Similarly, for fixed k, the graph with the fewest k-matchings is either the lex or the colex graph. To prove these results we first prove that the lex bipartite graph has the fewest matchings of all sizes among bipartite graphs with fixed part sizes and a given number of edges.

Abstract: Coloring the vertices of a graph G according to certain conditions can be considered as a random experiment and a discrete random variable (r.v.) X can be defined as the number of vertices having a particular color in the proper coloring of G and a probability mass function for this random variable can be defined accordingly. In this paper, we extend the concepts of mean and variance to a modified injective graph coloring and determine the values of these parameters for a number of standard graphs.

Abstract: A proper edge-coloring of a graph G is an assignment of colors to the edges of G such that adjacent edges receive distinct colors. A proper edge-coloring defines at each vertex the set of colors of its incident edges. Following the terminology introduced by Hor?ak, Kalinowski, Meszka and Wo?niak, we call such a set of colors the palette of the vertex. What is the minimum number of distinct palettes taken over all proper edge-colorings of G? A complete answer is known for complete graphs and cubic graphs. We study in some detail the problem for 4-regular graphs. In particular, we show that certain values of the palette index imply the existence of an even cycle decomposition of size 3 (a partition of the edge-set of a graph into 3 2-regular subgraphs whose connected components are cycles of even length). This result can be extended to 4d-regular graphs. Moreover, in studying the palette index of a 4-regular graph, the following problem arises: does there exist a 4-regular graph whose even cycle decompositions cannot have size smaller than 4?