Browsing by Author "Thilikos Touloupas, Dimitrios"

1.5Approximation for treewidth of graphs excluding a graph with one crossing as a minor
Demaine, Erik D.; Hajiaghayi, Mohammad Taghi; Thilikos Touloupas, Dimitrios (200205)
External research report
Open AccessWe give polynomialtime constantfactor approximation algorithms for the treewidth and branchwidth of any Hminorfree graph for a given graph H with crossing number at most 1. The approximation factors are 1.5 for ... 
A Constructive linear time algorithm for small cutwidth
Thilikos Touloupas, Dimitrios; Serna Iglesias, María José; Bodlaender, Hans L. (200009)
External research report
Open AccessThe cutwidth of a graph G is defined to be the smallest integer k such that the vertices of G can be arranged in a linear layout [v(1),...,v(n)] in such a way that for every i=1,...,n1, there are at most k edges with the ... 
A list of parameterized problems in bioinformatics
Félix Ávila, Liliana; García Chacón, Alina; Serna Iglesias, María José; Thilikos Touloupas, Dimitrios (200605)
External research report
Open AccessIn this report we present a list of problems that originated in bionformatics. Our aim is to collect information on such problems that have been analyzed from the point of view of Parameterized Complexity. For every problem ... 
A note on the subgraphs of the (2×\infty)grid
Díaz Cort, Josep; Kaminski, Marcin; Thilikos Touloupas, Dimitrios (201002)
Article
Restricted access  publisher's policyWe give a lineartime algorithm checking whether a graph is a subgraph of the (2×k)grid for some value of k. Our algorithm is based on a detailed characterization of the structure of such graphs. 
A Polynomial time algorithm for the cutwidth of bounded degree graphs with small treewidth
Thilikos Touloupas, Dimitrios; Serna Iglesias, María José; Bodlaender, Hans L. (200102)
External research report
Open AccessThe cutwidth of a graph G is defined to be the smallest integer k such that the vertices of G can be arranged in a vertex ordering [v(1),...,v(n)] in a way that, for every i=1,...,n1, there are at most k ... 
A Simple and fast approach for solving problems on planar graphs
Fomin, Fedor V.; Thilikos Touloupas, Dimitrios (200312)
External research report
Open AccessIt is well known that the celebrated LiptonTarjan planar separation theorem, in a combination with a divideandconquer strategy leads to many complexity results for planar graph problems. For example, by using this ... 
Bidimensional parameters and local treewidth
Demaine, Erik D.; Fomin, Fedor V.; Hajiaghayi, Mohammad Taghi; Thilikos Touloupas, Dimitrios (200310)
External research report
Open AccessFor several graph theoretic parameters such as vertex cover and dominating set, it is known that if their values are bounded by k then the treewidth of the graph is bounded by some function of k. This fact is used as the ... 
Complexity issues on bounded restrictive Hcoloring
Serna Iglesias, María José; Díaz Cort, Josep; Thilikos Touloupas, Dimitrios (200404)
External research report
Open AccessWe consider the restriction of the Hcoloring problem where we are looking for homomorphisms were the number of preimages of certain vertices of H is bounded by some fixed constant. We consider the decision and the ... 
Connected and internal graph searching
Barrière Figueroa, Eulalia; Fraigniaud, Pierre; Santoro, Nicola; Thilikos Touloupas, Dimitrios (200207)
External research report
Open AccessThis paper is concerned with the graph searching game. The search number es(G) of a graph G is the smallest number of searchers required to clear G. A search strategy is monotone (m) if no recontamination ever occurs. It ... 
Connected graph searching
Barrière Figueroa, Eulalia; Flocchini, Paola; Fomin, Fedor V.; Fraigniaud, Pierre; Nisse, Nicolas; Santoro, Nicola; Thilikos Touloupas, Dimitrios (20100926)
External research report
Open AccessIn graph searching game the opponents are a set of searchers and a fugitive in a graph. The searchers try to capture the fugitive by applying some sequence moves that include placement, removal, or sliding of a searcher ... 
Constructive linear time algorithms for branchwidth
Thilikos Touloupas, Dimitrios; Bodlaender, Hans L. (200010)
External research report
Open AccessWe prove that, for any fixed k, one can construct a linear time algorithm that checks if a graph has branchwidth at most k and, if so, outputs a branch decomposition of minimum width. 
Counting Hcolorings of partial ktrees
Díaz Cort, Josep; Serna Iglesias, María José; Thilikos Touloupas, Dimitrios (200011)
External research report
Open AccessThe problem of counting all Hcolorings of a graph G of n vertices is considered. While the problem is, in general #Pcomplete, we give linear time algorithms that solve the main variants of this problem when the input ... 
Counting list hcolorings and variants
Díaz Cort, Josep; Serna Iglesias, María José; Thilikos Touloupas, Dimitrios (200105)
External research report
Open AccessWe study the counting versions of several variants of the Hcoloring problem. The complexity of the Hcoloring problem is described by a dichotomy theorem that classifies the problem according to the the topology of H ... 
Derivation of algorithms for cutwidth and related graph layout problems
Bodlaender, Hans; Michael, Fellows; Thilikos Touloupas, Dimitrios (200207)
External research report
Open AccessIn this paper, we investigate algorithms for some related graph parameters. Each of these asks for a linear ordering of the vertices of the graph (or can be formulated as such), and there are constructive linear time ... 
Dominating sets and local treewidth
Thilikos Touloupas, Dimitrios; Fomin, Fedor V. (200309)
External research report
Open AccessIt is known that the treewidth of a planar graph with a dominating set of size d is O(sqrt{d}) and this fact is used as the basis for several fixed parameter algorithms on planar graphs. An interesting question ... 
Dominating sets in planar graphs: branchwidth and exponential speedup
Fomin, Fedor V.; Thilikos Touloupas, Dimitrios (200212)
External research report
Open AccessGraph minors theory, developed by Robertson & Seymour, provides a list of powerful theoretical results and tools. However, the wide spread opinion in Graph Algorithms community about this theory is that it is mainly of ... 
Efficient algorithms for parameterized Hcolorings
Díaz Cort, Josep; Serna Iglesias, María José; Thilikos Touloupas, Dimitrios (200512)
External research report
Open AccessWe study the fixed parameter tractability of the restrictive $H$coloring and the restrictive list $H$coloring problems, introduced in~cite{DST01b}. The parameterizations are defined by fixing the number of preimages of ... 
Exponential speedup of fixed parameter algorithms on K_{3,3}minorfree or K_{5}minorfree graphs
Hajiaghayi, Mohammad Taghi; Demaine, Erik D.; Thilikos Touloupas, Dimitrios (200203)
External research report
Open AccessWe present a fixed parameter algorithm that constructively solves the kdominating set problem on graphs excluding one of the K_{5} or K_3,3 as a minor in time O(3^{6sqrt{34 k}}n^{O(1)}). In fact, we present our ... 
Fast approximation schemes for K_{3,3}minorfree or K_{5}minorfree graphs
Hajiaghayi, Mohammad Taghi; Nishimura, Naomi; Ragde, Prabhakar; Thilikos Touloupas, Dimitrios (200205)
External research report
Open AccessAs the class of graphs of bounded treewidth is of limited size, we need to solve NPhard problems for wider classes of graphs than this class. Eppstein introduced a new concept which can be considered as a generalization ... 
Fast fixedparameter tractable algorithms for (nontrivial) generalizations of vertex cover
Nishimura, Naomi; Ragde, Prabhakar; Thilikos Touloupas, Dimitrios (200109)
External research report
Open AccessOur goal in this paper is the development of fast algorithms for recognizing general classes of graphs. We seek algorithms whose complexity can be expressed as a linear function of the graph size plus an exponential function ...