Minimum cycle cover and Chinese postman problems on mixed graphs with bounded tree-width

Let M=(V,E,A) be a mixed graph with vertex set V, edge set E and arc set A. A cycle cover of M is a family C={C₁,…,C_k} of cycles of M such that each edge/arc of M belongs to at least one cycle in C. The weight of C is . The minimum cycle cover problem is the following: given a strongly connected mixed graph M without bridges, find a cycle cover of M with weight as small as possible. The Chinese postman problem is: given a strongly connected mixed graph M, find a minimum length closed walk using all edges and arcs of M. These problems are NP-hard. We show that they can be solved in polynomial time if M has bounded tree-width.     

Coloring some classes of mixed graphs

We consider the coloring problem for mixed graphs, that is, for graphs containing edges and arcs. A mixed coloring c is a coloring such that for every edge [x_i,x_j], c(x_i)≠c(x_j) and for every arc (x_p,x_q), c(x_p)<c(x_q). We will analyse the complexity status of this problem for some special classes of graphs.     

The maximum integer multiterminal flow problem in directed graphs

Given an arc-capacitated digraph and k terminal vertices, the directed maximum integer multiterminal flow problem is to route the maximum number of flow units between the terminals. We introduce a new parameter k_L?k for this problem and study its complexity with respect to k_L.     

On extremal k-outconnected graphs

Let G be a minimally k-connected graph with n nodes and m edges. Mader proved that if n?3k-2 then m?k(n-k), and for n?3k-1 an equality is possible if, and only if, G is the complete bipartite graph K_k,n-k. Cai proved that if n?3k-2 then m?⌊(n+k)²/8⌋, and listed the cases when this bound is tight.In this paper we prove a more general theorem, which implies similar results for minimally k-outconnected graphs; a graph is called k-outconnected from r if it contains k internally disjoint paths from r to every other node.     

A characterization of maximal non-k-factor-critical graphs

A graph G of order p is k-factor-critical,where p and k are positive integers with the same parity, if the deletion of any set of k vertices results in a graph with a perfect matching. G is called maximal non-k-factor-critical if G is not k-factor-critical but G+e is k-factor-critical for every missing edge e∉E(G). A connected graph G with a perfect matching on 2n vertices is k-extendable, for 1?k?n-1, if for every matching M of size k in G there is a perfect matching in G containing all edges of M. G is called maximal non-k-extendable if G is not k-extendable but G+e is k-extendable for every missing edge e∉E(G) . A connected bipartite graph G with a bipartitioning set (X,Y) such that |X|=|Y|=n is maximal non-k-extendable bipartite if G is not k-extendable but G+xy is k-extendable for any edge xy∉E(G) with x∈X and y∈Y. A complete characterization of maximal non-k-factor-critical graphs, maximal non-k-extendable graphs and maximal non-k-extendable bipartite graphs is given.     

Covering 2‐connected 3‐regular graphs with disjoint paths

A path cover of a graph is a set of disjoint paths so that every vertex in the graph is contained in one of the paths. The path cover number of graph G is the cardinality of a path cover with the minimum number of paths. Reed in 1996 conjectured that a 2‐connected 3‐regular graph has path cover number at most . In this article, we confirm this conjecture.     

The fine structure of octahedron-free graphs

This paper is one of a series of papers in which, for a family L of graphs, we describe the typical structure of graphs not containing any L∈L. In this paper, we prove sharp results about the case L={O₆}, where O₆ is the graph with 6 vertices and 12 edges, given by the edges of an octahedron. Among others, we prove the following results.(a) The vertex set of almost every O₆-free graph can be partitioned into two classes of almost equal sizes, U₁ and U₂, where the graph spanned by U₁ is a C₄-free and that by U₂ is P₃-free.(b) Similar assertions hold when L is the family of all graphs with 6 vertices and 12 edges.(c) If H is a graph with a color-critical edge and χ(H)=p+1, then almost every sH-free graph becomes p-chromatic after the deletion of some s−1 vertices, where sH is the graph formed by s vertex disjoint copies of H.These results are natural extensions of theorems of classical extremal graph theory. To show that results like those above do not hold in great generality, we provide examples for which the analogs of our results do not hold.     

Finding thet-join structure of graphs

t-joins are generalizations of postman tours, matchings, and paths;t-cuts contain planar multicommodity flows as a special case. In this paper we present a polynomial time combinatorial algorithm that determines a minimumt-join and a maximum packing oft-cuts and that ends up with a Gallai-Edmonds type structural decompostion of (G, t) pairs, independent of the running of the algorithm. It only uses simple combinatorial steps such as the symmetric difference of two sets of edges and does not use any shrinking operations.     

Bandwidth theorem for random graphs

A graph G is said to have bandwidth at most b, if there exists a labeling of the vertices by 1,2,…,n, so that |i−j|?b whenever {i,j} is an edge of G. Recently, Böttcher, Schacht, and Taraz verified a conjecture of Bollobás and Komlós which says that for every positive r, Δ, γ, there exists β such that if H is an n-vertex r-chromatic graph with maximum degree at most Δ which has bandwidth at most βn, then any graph G on n vertices with minimum degree at least (1−1/r+γ)n contains a copy of H for large enough n. In this paper, we extend this theorem to dense random graphs. For bipartite H, this answers an open question of Böttcher, Kohayakawa, and Taraz. It appears that for non-bipartite H the direct extension is not possible, and one needs in addition that some vertices of H have independent neighborhoods. We also obtain an asymptotically tight bound for the maximum number of vertex disjoint copies of a fixed r-chromatic graph H₀ which one can find in a spanning subgraph of G(n,p) with minimum degree (1−1/r+γ)np.     

On (a, b)-consecutive petersen graphs

The generalized Petersen graphsP(n,k), n≥3, 1≤k<n/2, consist of an outern-cyclex _o x ₁ x ₂...x _n−1 , a set ofn spokesx _i y _i (0≤i≤n−1), andn inner edgesy _i y _i +k with indices taken modulon. This paper deals with (a,b)-consecutive labelings of generalized Petersen graphP(n,k).     

Isometric path numbers of graphs

An isometric path between two vertices in a graph G is a shortest path joining them. The isometric path number of G, denoted by ip(G), is the minimum number of isometric paths needed to cover all vertices of G. In this paper, we determine exact values of isometric path numbers of complete r-partite graphs and Cartesian products of 2 or 3 complete graphs.     

Expander graphs based on GRH with an application to elliptic curve cryptography

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We present a construction of expander graphs obtained from Cayley graphs of narrow ray class groups, whose eigenvalue bounds follow from the Generalized Riemann Hypothesis. Our result implies that the Cayley graph of ^∗(Z/qZ) with respect to small prime generators is an expander. As another application, we show that the graph of small prime degree isogenies between ordinary elliptic curves achieves nonnegligible eigenvalue separation, and explain the relationship between the expansion properties of these graphs and the security of the elliptic curve discrete logarithm problem.

Video

For a video summary of this paper, please visit http://www.youtube.com/watch?v=7jwxmKWWsyM.     

Structural Remarks on Bipartite Graphs with Unique f-Factors

In this note we will derive some structural results for a bipartite graph G with a unique f-factor. Two necessary conditions will be that G is saturated, meaning that the addition of any edge leads to a second f-factor, and that f_A, f_B≥1. Here f_A and f_B are defined as the minimum of f over the vertices in the two partite sets A and B of G, respectively. Our main result states that G has at least f_A + f_B vertices for which d_G (v) = f(v) holds.     

Some observations on maximum weight stable sets in certain P5-free graphs

The maximum weight stable set problem (MWS) is the weighted version of the maximum stable set problem (MS), which is NP-hard. The class of P₅-free graphs – i.e., graphs with no induced path of five vertices – is the unique minimal class, defined by forbidding a single connected subgraph, for which the computational complexity of MS is an open question. At the same time, it is known that MS can be efficiently solved for (_P5,F)$(P_5\text{,}F)$-free graphs, where F is any graph of five vertices different to a C₅. In this paper we introduce some observations on P₅-free graphs, and apply them to introduce certain subclasses of such graphs for which one can efficiently solve MWS. That extends or improves some known results, and implies – together with other known results – that MWS can be efficiently solved for (_P5,F)$(P_5\text{,}F)$-free graphs where F is any graph of five vertices different to a C₅.     

Covering graphs with matchings of fixed size

Let m be a positive integer and let G be a graph. We consider the question: can the edge set E(G) of G be expressed as the union of a set M of matchings of G each of which has size exactly m? If this happens, we say that G is [m]-coverable and we call M an [m]-covering of G. It is interesting to consider minimum[m]-coverings, i.e. [m]-coverings containing as few matchings as possible. Such [m]-coverings will be called excessive[m]-factorizations. The number of matchings in an excessive [m]-factorization is a graph parameter which will be called the excessive[m]-index and denoted by . In this paper we begin the study of this new parameter as well as of a number of other related graph parameters.     

Finding monotone paths in edge-ordered graphs

An edge-ordering of a graph G=(V,E) is a one-to-one function f from E to a subset of the set of positive integers. A path P in G is called an f-ascent if f increases along the edge sequence of P. The heighth(f) of f is the maximum length of an f-ascent in G.In this paper we deal with computational problems concerning finding ascents in graphs. We prove that for a given edge-ordering f of a graph G the problem of determining the value of h(f) is NP-hard. In particular, the problem of deciding whether there is an f-ascent containing all the vertices of G is NP-complete. We also study several variants of this problem, discuss randomized and deterministic approaches and provide an algorithm for the finding of ascents of order at least k in graphs of order n in running time O(4^kn^O(1)).     

The d-precoloring problem for k-degenerate graphs

In this paper we deal with the d-PRECOLORING EXTENSION (d-PREXT) problem in various classes of graphs. The d-PREXT problem is the special case of PRECOLORING EXTENSION problem where, for a fixed constant d, input instances are restricted to contain at most d precolored vertices for every available color. The goal is to decide if there exists an extension of given precoloring using only available colors or to find it.We present a linear time algorithm for both, the decision and the search version of d-PREXT, in the following cases: (i) restricted to the class of k-degenerate graphs (hence also planar graphs) and with sufficiently large set S of available colors, and (ii) restricted to the class of partial k-trees (without any size restriction on S). We also study the following problem related to d-PREXT: given an instance of the d-PREXT problem which is extendable by colors of S, what is the minimum number of colors of S sufficient to use for precolorless vertices over all such extensions? We establish lower and upper bounds on this value for k-degenerate graphs and its various subclasses (e.g., planar graphs, outerplanar graphs) and prove tight results for the class of trees.