Greed is good: Approximating independent sets in sparse and bounded-degree graphs

Theminimum-degree greedy algorithm, or Greedy for short, is a simple and well-studied method for finding independent sets in graphs. We show that it achieves a performance ratio of (Δ+2)/3 for approximating independent sets in graphs with degree bounded by Δ. The analysis yields a precise characterization of the size of the independent sets found by the algorithm as a function of the independence number, as well as a generalization of Turán’s bound. We also analyze the algorithm when run in combination with a known preprocessing technique, and obtain an improved performance ratio on graphs with average degree , improving on the previous best of Hochbaum. Finally, we present an efficient parallel and distributed algorithm attaining the performance guarantees of Greedy. Gordon Gekko [29]. A preliminary version of this paper appeared at the 26th ACM Symposium on Theory of Computing, 1994. This work was done while both authors were at the Japan Advanced Institute of Science and Technology, Hokuriku.     

Finding hidden independent sets in interval graphs

We design efficient competitive algorithms for discovering hidden information using few queries. Specifically, consider a game in a given set of intervals (and their implied interval graph G) in which our goal is to discover an (unknown) independent set X by making the fewest queries of the form “Is point p covered by an interval in X?” Our interest in this problem stems from two applications: experimental gene discovery with PCR technology and the game of Battleship (in a 1-dimensional setting). We provide adaptive algorithms for both the verification scenario (given an independent set, is it X?) and the discovery scenario (find X without any information). Under some assumptions, these algorithms use an asymptotically optimal number of queries in every instance.     

On handshakes in random graphs

In [Y. Métivier, N. Saheb, A. Zemmari, Analysis of a randomized rendezvous algorithm, Inform. and Comput. 184 (2003) 109-128], the authors introduce and analyze a randomized procedure to implement handshakes in graphs. In this paper, we investigate the same problem in random graphs. We prove results on the probability of success and we study the distribution of the random variable which counts the number of handshakes in the graph.     

A note on maximum independent sets in rectangle intersection graphs

Finding the maximum independent set in the intersection graph of n axis-parallel rectangles is NP-hard. We re-examine two known approximation results for this problem. For the case of rectangles of unit height, Agarwal, van Kreveld and Suri [Comput. Geom. Theory Appl. 11 (1998) 209-218] gave a (1+1/k)-factor algorithm with an O(nlogn+n^2k−1) time bound for any integer constant k?1; we describe a similar algorithm running in only O(nlogn+nΔ^k−1) time, where Δ?n denotes the maximum number of rectangles a point can be in. For the general case, Berman, DasGupta, Muthukrishnan and Ramaswami [J. Algorithms 41 (2001) 443-470] gave a ⌈log_kn⌉-factor algorithm with an O(n^k+1) time bound for any integer constant k?2; we describe similar algorithms running in O(nlogn+nΔ^k−2) and n^O(k/logk) time.     

Counting maximal independent sets in directed path graphs

The problem of counting maximal independent sets is #P-complete for chordal graphs but solvable in polynomial time for its subclass of interval graphs. This work improves upon both of these results by showing that the problem remains #P-complete when restricted to directed path graphs but that a further restriction to rooted directed path graphs admits a polynomial time solution.     

On the independence number and Hamiltonicity of uniform random intersection graphs

In the uniform random intersection graphs model, denoted by G_n,m,λ, to each vertex v we assign exactly λ randomly chosen labels of some label set M of m labels and we connect every pair of vertices that has at least one label in common. In this model, we estimate the independence number α(G_n,m,λ), for the wide range m=⌊n^α⌋,α<1 and λ=O(m^1/4). We also prove the Hamiltonicity of this model by an interesting combinatorial construction. Finally, we give a brief note concerning the independence number of G_n,m,p random intersection graphs, in which each vertex chooses labels with probability p.     

Feedback vertex sets in star graphs

In a graph G=(V,E), a subset F⊂V(G) is a feedback vertex set of G if the subgraph induced by V(G)?F is acyclic. In this paper, we propose an algorithm for finding a small feedback vertex set of a star graph. Indeed, our algorithm can derive an upper bound to the size of the feedback vertex set for star graphs. Also by applying the properties of regular graphs, a lower bound can easily be achieved for star graphs.     

The distant-2 chromatic number of random proximity and random geometric graphs

We are interested in finding bounds for the distant-2 chromatic number of geometric graphs drawn from different models. We consider two undirected models of random graphs: random geometric graphs and random proximity graphs for which sharp connectivity thresholds have been shown. We are interested in a.a.s. connected graphs close just above the connectivity threshold. For such subfamilies of random graphs we show that the distant-2-chromatic number is Θ(logn) with high probability. The result on random geometric graphs is extended to the random sector graphs defined in [J. Díaz, J. Petit, M. Serna. A random graph model for optical networks of sensors, IEEE Transactions on Mobile Computing 2 (2003) 143-154].     

Approximating maximum weight K-colorable subgraphs in chordal graphs

We present a 2-approximation algorithm for the problem of finding the maximum weight K-colorable subgraph in a given chordal graph with node weights. The running time of the algorithm is O(K(n+m)), where n and m are the number of vertices and edges in the given graph.     

On disjoint maximal independent sets in graphs

An old problem in graph theory is to characterize the graphs that admit two disjoint maximal independent sets.     

Efficient dominating sets in circulant graphs with domination number prime

Cayley graphs of finite cyclic group ${\mathbb{Z}}_n$ are called circulant graphs and denoted by $\mathrm{Cay}({\mathbb{Z}}_n,S)$. For $\mathrm{Cay}({\mathbb{Z}}_n,S)$ with $\frac{n}{|S|+1}$ prime, we give a necessary and sufficient condition for the existence of efficient dominating sets and characterize completely all its efficient dominating sets.     

Steiner trees in uniformly quasi-bipartite graphs

The area of approximation algorithms for the Steiner tree problem in graphs has seen continuous progress over the last years. Currently the best approximation algorithm has a performance ratio of 1.550. This is still far away from 1.0074, the largest known lower bound on the achievable performance ratio. As all instances resulting from known lower bound reductions are uniformly quasi-bipartite, it is interesting whether this special case can be approximated better than the general case. We present an approximation algorithm with performance ratio 73/60<1.217 for the uniformly quasi-bipartite case. This improves on the previously known ratio of 1.279 of Robins and Zelikovsky. We use a new method of analysis that combines ideas from the greedy algorithm for set cover with a matroid-style exchange argument to model the connectivity constraint. As a consequence, we are able to provide a tight instance.     

Function-described graphs for modelling objects represented by sets of attributed graphs

We present in this article the model function-described graph (FDG), which is a type of compact representation of a set of attributed graphs (AGs) that borrow from random graphs the capability of probabilistic modelling of structural and attribute information. We define the FDGs, their features and two distance measures between AGs (unclassified patterns) and FDGs (models or classes) and we also explain an efficient matching algorithm. Two applications of FDGs are presented: in the former, FDGs are used for modelling and matching 3D-objects described by multiple views, whereas in the latter, they are used for representing and recognising human faces, described also by several views.     

Combinatorial algorithms for feedback problems in directed graphs

Given a weighted directed graph G=(V,A), the minimum feedback arc set problem consists of finding a minimum weight set of arcs A′⊆A such that the directed graph (V,A?A′) is acyclic. Similarly, the minimum feedback vertex set problem consists of finding a minimum weight set of vertices containing at least one vertex for each directed cycle. Both problems are NP-complete. We present simple combinatorial algorithms for these problems that achieve an approximation ratio bounded by the length, in terms of number of arcs, of a longest simple cycle of the digraph.     

An efficient parallel algorithm for computing a large independent set in a planar graph

Let (G) denote the independence number of a graphG, that is the maximum number of pairwise independent vertices inG. We present a parallel algorithm that computes in a planar graphG = (V, E), an independent set such that ¦I¦ (G)/2. The algorithm runs in timeOlog² n) and requires a linear number of processors. This is achieved by denning a new set of reductions that can be executed locally and simultaneously; furthermore, it is shown that a constant fraction of the vertices in the graph are reducible. This is the best known approximation scheme when the number of processors available is linear; parallel implementation of known sequential algorithms requires many more processors.Joseph Naor was supported by Contract ONR N00014-88-K-0166. Most of this work was done while he was a post-doctoral fellow at the Department of Computer Science, University of Southern California, Los Angeles, CA 90089-0782, USA.     

On the random generation of monotone data sets

Many of the state-of-the-art classification algorithms for data with linearly ordered attribute domains and a linearly ordered label set insist on the monotonicity of the induced classification rule. Training and evaluation of such algorithms requires the availability of sufficiently general monotone data sets. In this short contribution we introduce an algorithm that allows for the (almost) uniform random generation of monotone data sets based on the Markov Chain Monte Carlo method.     

Bounds on the bisection width for random d -regular graphs

In this paper we provide an explicit way to compute asymptotically almost sure upper bounds on the bisection width of random d$d$-regular graphs, for any value of d$d$. The upper bounds are obtained from the analysis of the performance of a randomized greedy algorithm to find bisections of d$d$-regular graphs. We provide bounds for 5≤d≤12$5\le d\le 12$. We also give empirical values of the size of the bisection found by the algorithm for some small values of d$d$ and compare them with numerical approximations of our theoretical bounds. Our analysis also gives asymptotic lower bounds for the size of the maximum bisection.