Publications

  • [Show abstract] [Hide abstract]
    ABSTRACT: Given a fixed positive integer k ≥ 2, let G be a simple graph of order n ≥ 6k. It is proved that if the minimum degree of G is at least n/2 + 1, then for every pair of vertices x and y, there exists a Hamiltonian cycle such that the distance between x and y along that cycle is precisely k.
    Graphs and Combinatorics 01/2014; · 0.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We address an old (1977) conjecture of a subset of the authors (a variant of Ryser's conjecture): in every r-coloring of the edges of a biclique [A,B] (complete bipartite graph), the vertex set can be covered by the vertices of at most 2r-2 monochromatic connected components. We reduce this conjecture to design-like conjectures, where the monochromatic components of the color classes are bicliques [X,Y] with nonempty blocks X and Y. We prove this conjecture for r<6. We show that the width (the number of bicliques) in every color class of any spanning r-coloring is at most 2^{r-1} (and this is best possible).
    12/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We prove that any cycle
    01/2008;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An adjacent vertex distinguishing edge-coloring of a simple graph G is a proper edge-coloring of G such that no pair of adjacent vertices meets the same set of colors. The minimum number of colors χ a ' (G) required to give G an adjacent vertex distinguishing coloring is studied for graphs with no isolated edge. We prove χ a ' (G)≤5 for such graphs with maximum degree Δ(G)=3 and prove χ a ' (G)≤Δ(G)+2 for bipartite graphs. These bounds are tight. For k-chromatic graphs G without isolated edges we prove a weaker result of the form χ a ' (G)=Δ(G)+O(logk).
    SIAM Journal on Discrete Mathematics 01/2007; 21(1). · 0.66 Impact Factor
  • Source
    A. Gyárfás, J. Lehel
    [Show abstract] [Hide abstract]
    ABSTRACT: A graph coloring algorithm that immediately colors the vertices taken from a list without looking ahead or changing colors already assigned is called “on-line coloring.” The properties of on-line colorings are investigated in several classes of graphs. In many cases we find on-line colorings that use no more colors than some function of the largest clique size of the graph. We show that the first fit on-line coloring has an absolute performance ratio of two for the complement of chordal graphs. We prove an upper bound for the performance ratio of the first fit coloring on interval graphs. It is also shown that there are simple families resisting any on-line algorithm: no on-line algorithm can color all trees by a bounded number of colors.
    Journal of Graph Theory 10/2006; 12(2):217 - 227. · 0.63 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Let T n be the complete binary tree of height n considered as the Hasse-diagram of a poset with its root 1 n as the maximum element. For a tree or forest T, we count the embeddings of T into T n as posets by the functions A(n;T)=|{S⊆T n 1 n ∈S,S≅T}|, and B(n;T)=|{S⊆T n 1 n ∉S,S≅T}|. Here we summarize what we know about the ratio A(n;T)/B(n;T) in the case of T being a chain or an antichain.
    Ars Combinatoria -Waterloo then Winnipeg- 01/2006; 79. · 0.28 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A graph is Ramsey unsaturated if there exists a proper supergraph of the same order with the same Ramsey number, and Ramsey saturated otherwise. We present some conjectures and results concerning both Ramsey saturated and unsaturated graphs. In particular, we show that cycles Cn and paths Pn on n vertices are Ramsey unsaturated for all n ≥ 5. © 2005 Wiley Periodicals, Inc.
    Journal of Graph Theory 08/2005; 51(1):22 - 32. · 0.63 Impact Factor
  • G. Kubicki, J. Lehel
    Discrete Mathematics 01/2002; 249(1). · 0.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Let N(n,k) be the minimum number of pairwise edge disjoint monochromatic complete graphs Kk in any 2-coloring of the edges of a Kn. Upper and lower bounds on N(n,k) will be given for k⩾3. For k=3, exact values will be given for n⩽11, and these will be used to give a lower bound for N(n,3).
    Discrete Mathematics 03/2001; 231(s 1–3):135–141. · 0.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Distance avoiding partitions of the Euclidean space IR n are investigated in this note. We introduce the concept of combinatorial functions of distances for point congurations, and prove a measure theoretic lemma for these functions. Using the lemma we give a new proof of a result of Larman and Rogers which had only the original combinatorial proof so far. The same lemma is used to obtain new theorems in the direction that the removal of small" sets from IR n does not decrease the unit distance chromatic number, and furthermore, that the hypothetical unit distance subgraphs of IR 2 which are not 6{colorable must have large" order. 1. Introduction. A coloring of the points of a subset A IR n using k colors is called a proper k{coloring of A if every point receives one color and no two points at unit distance apart receive the same color. The unit{distance chromatic number of A IR n is the minimum integer k such that A has a proper k{coloring. In other words, the unit...
    07/2000;
  • [Show abstract] [Hide abstract]
    ABSTRACT: It will be shown that if G is a graph of order n which contains a triangle, a cycle of length n or n−1 and at least cn odd cycles of different lengths for some positive constant c, then there exists some positive constant k=k(c) such that G contains at least kn 1/6 even cycles of different lengths. Other results on the number of even cycle lengths which appear in graphs with many different odd length cycles will be given.
    Graphs and Combinatorics 01/2000; 16:399-410. · 0.35 Impact Factor
  • Source
    Combinatorica 01/1999; · 0.56 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: . We introduce the visibility number b(G) of a graph G, which is the minimum t such that G can be represented by assigning each vertex a union of at most t horizontal segments in the plane so that vertices u; v are adjacent if and only if some point assigned to u sees some point assigned to v via a vertical segment unobstructed by other assigned points. We prove the following: 1) every planar graph has visibility number at most 2, which is sharp. 2) r b(K m;n ) r + 1, where r = d(mn + 4)=(2m + 2n)e. 3) dn=6e b(K n ) dn=6e + 1. 4) When G has n vertices, b(G) dn=6e + 2. 1. INTRODUCTION Researchers in computational geometry have studied the use of graphs to model visibility relations in the plane. For example, in a polygon in the plane we say that two vertices "see" each other if the segment joining them lies inside the polygon. Letting vertices that see each other be adjacent defines the visibility graph of the polygon. Similarly, we can define a visibility graph on a set of line s...
    08/1998;
  • Jeno Lehel
    [Show abstract] [Hide abstract]
    ABSTRACT: In the area of on-line algorithms the existence of competitive algorithms is one of the most frequently studied questions. However, competitive on-line graph coloring algorithms exist only for very restricted families of graphs. We introduce a new concept, called on-line competitive algorithms. Our main problem is whether on-line competitive coloring algorithms exist for all classes of graphs. This concept can be useful if somebody must design an on-line coloring algorithm and the input graph is only known to be in a specified class of graphs. In this case the designer want to get the best algorithm but this is usually hard. An on-line competitive algorithm offers less: it comes together with a function f such that for every graph in the class the number of colors it uses can be bounded by f(Ø (G)) where Ø (G) is the minimum number of colors can be achieved at all for that graph by any on-line algorithm (that algorithm may know the graph in advance). Of course, the smaller is f ...
    12/1997;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The k-spectrum sk(G) of a graph G is the set of all positive integers that occur as the size of an induced k-vertex subgraph of G. In this paper we determine the minimum order and size of a graph G with sk (G) = {0, 1, …,(2k)} and consider the more general question of describing those sets S ⊆ {0,1, … ,(2k)} such that S = sk(G) for some graph G.
    Discrete Mathematics 04/1996; 150(s 1–3):103–113. · 0.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Let (x, y) be an edge of a graph G. Then the rotation of (x, y) about x is the operation of removing (x, y) from G and inserting (x, y′) as an edge, where y′ is a vertex of G. The rotation distance between graphs G and H is the minimum number of rotations necessary to transform G into H. Lower and upper bounds are given on the rotation distance of two graphs in terms of their greatest common subgraphs and their partial rotation link of largest cardinality. We also propose some extermal problems for the rotation distance of trees.
    Discrete Mathematics 03/1994; 126(s 1–3):121–135. · 0.58 Impact Factor
  • Discrete Applied Mathematics 07/1993; 44(1-3):191-203. · 0.72 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A hypcrgraph is irregular if no two of its vertices have t11e same degree. It is shown for all r 2 3 and n 2 r + 3, that there exist irregular r-uniform hypergraphs of order n. For r 2 6 it is proved that almost allr-uniform hypergraphs are irregular. A linear upper bound is given for the irregularity strength of hypergraphs of order nand fixed rank. Furthemwre the irregularity strength of complete and complete equipartite hypcrgraphs is determined.
    JCMCC. The Journal of Combinatorial Mathematics and Combinatorial Computing. 01/1992; 11.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: It is proved that if t is a fixed positive integer and n is sufficiently large, then each graph of order n with minimum degree n − t has an assignment of weights 1, 2 or 3 to the edges in such a way that weighted degrees of the vertices become distinct.
    Discrete Mathematics 08/1991; 91(1):45–59. · 0.58 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: There are several characterizations of graphical sequences, yet only little work on the possible structure of these graphs. We begin considering the question of the possible clique number attained by graphs with the same degree sequence. In particular, it is shown that the maximum difference between the largest and the smallest possible clique number for graphs realizing the same degree sequence of n elements is n-cn 2/3 for sufficiently large n. Additional results on sequences having a realization H with ω(H)≥k are presented.
    Journal of Graph Theory - JGT. 01/1991;

32 Followers View all