A note on vertex orders for stability number

We investigate vertex orders that can be used to obtain maximum stable sets by a simple greedy algorithm in polynomial time in some classes of graphs. We characterize a class of graphs for which the stability number can be obtained by a simple greedy algorithm. This class properly contains previously known classes of graphs for which the stability number can be computed in polynomial time. © 1999 John Wiley & Sons, Inc. J Graph Theory 30: 113–120, 1999     

The integral stable allocation problem on graphs

As a generalisation of the stable matching problem Baïou and Balinski (2002) [1] defined the stable allocation problem for bipartite graphs, where both the edges and the vertices may have capacities. They constructed a so-called inductive algorithm, that always finds a stable allocation in strongly polynomial time. Here, we generalise their algorithm for non-bipartite graphs with integral capacities. We show that the algorithm does not remain polynomial, although we also present a scaling technique that makes the algorithm weakly polynomial.     

Coloring Fuzzy Circular Interval Graphs

Computing the weighted coloring number of graphs is a classical topic in combinatorics and graph theory. Recently these problems have again attracted a lot of attention for the class of quasi-line graphs and more specifically fuzzy circular interval graphs.The problem is NP-complete for quasi-line graphs. For the subclass of fuzzy circular interval graphs however, one can compute the weighted coloring number in polynomial time using recent results of Chudnovsky and Ovetsky and of King and Reed. Whether one could actually compute an optimal weighted coloring of a fuzzy circular interval graph in polynomial time however was still open.We provide a combinatorial algorithm that computes weighted colorings and the weighted coloring number for fuzzy circular interval graphs efficiently. The algorithm reduces the problem to the case of circular interval graphs, then making use of an algorithm by Gijswijt to compute integer decompositions.     

A generalization of König-Egervary graphs and heuristics for the maximum independent set problem with improved approximation ratios

We first study a generalization of the König-Egervary graphs, the class of the κ-KE graphs, and propose an exact polynomial time algorithm solving maximum independent set problem in this class. Next, we show how this result can be efficiently used to devise polynomial time approximation algorithms with improved approximation ratios for the maximum independent set problem in general graphs.     

On the colored Tutte polynomial of a graph of bounded treewidth

We observe that a formula given by Negami [Polynomial invariants of graphs, Trans. Amer. Math. Soc. 299 (1987) 601-622] for the Tutte polynomial of a k-sum of two graphs generalizes to a colored Tutte polynomial. Consequently, an algorithm of Andrzejak [An algorithm for the Tutte polynomials of graphs of bounded treewidth, Discrete Math. 190 (1998) 39-54] may be directly adapted to compute the colored Tutte polynomial of a graph of bounded treewidth in polynomial time. This result has also been proven by Makowsky [Colored Tutte polynomials and Kauffman brackets for graphs of bounded tree width, Discrete Appl. Math. 145 (2005) 276-290], using a different algorithm based on logical techniques.     

Polarity of chordal graphs

Polar graphs are a common generalization of bipartite, cobipartite, and split graphs. They are defined by the existence of a certain partition of vertices, which is NP-complete to decide for general graphs. It has been recently proved that for cographs, the existence of such a partition can be characterized by finitely many forbidden subgraphs, and hence tested in polynomial time. In this paper we address the question of polarity of chordal graphs, arguing that this is in essence a question of colourability, and hence chordal graphs are a natural restriction. We observe that there is no finite forbidden subgraph characterization of polarity in chordal graphs; nevertheless we present a polynomial time algorithm for polarity of chordal graphs. We focus on a special case of polarity (called monopolarity) which turns out to be the central concept for our algorithms. For the case of monopolar graphs, we illustrate the structure of all minimal obstructions; it turns out that they can all be described by a certain graph grammar, permitting our monopolarity algorithm to be cast as a certifying algorithm.     

On the recognition of fuzzy circular interval graphs

Fuzzy circular interval graphs are a generalization of proper circular arc graphs and have been recently introduced by Chudnovsky and Seymour as a fundamental subclass of claw-free graphs. In this paper, we provide a polynomial time algorithm for recognizing such graphs, and more importantly for building a suitable model for these graphs.     

On the Polarity and Monopolarity of Graphs

Polarity and monopolarity are properties of graphs defined in terms of the existence of certain vertex partitions; graphs with polarity and monopolarity are respectively called polar and monopolar graphs. These two properties commonly generalize bipartite and split graphs, but are hard to recognize in general. In this article we identify two classes of graphs, triangle‐free graphs and claw‐free graphs, restricting to which provide novel impact on the complexity of the recognition problems. More precisely, we prove that recognizing polarity or monopolarity remains NP‐complete for triangle‐free graphs. We also show that for claw‐free graphs the former is NP‐complete and the latter is polynomial time solvable. This is in sharp contrast to a recent result that both polarity and monopolarity can be recognized in linear time for line graphs. Our proofs for the NP‐completeness are simple reductions. The polynomial time algorithm for recognizing the monopolarity of claw‐free graphs uses a subroutine similar to the well‐known breadth‐first search algorithm and is based on a new structural characterization of monopolar claw‐free graphs, a generalization of one for monopolar line graphs obtained earlier.     

Generating unlabeled connected cubic planar graphs uniformly at random

We present an expected polynomial time algorithm to generate an unlabeled connected cubic planar graph uniformly at random. We first consider rooted connected cubic planar graphs, i.e., we count connected cubic planar graphs up to isomorphisms that fix a certain directed edge. Based on decompositions along the connectivity structure, we derive recurrence formulas for the exact number of rooted cubic planar graphs. This leads to rooted 3‐connected cubic planar graphs, which have a unique embedding on the sphere. Special care has to be taken for rooted graphs that have a sense‐reversing automorphism. Therefore we introduce the concept of colored networks, which stand in bijective correspondence to rooted 3‐connected cubic planar graphs with given symmetries. Colored networks can again be decomposed along the connectivity structure. For rooted 3‐connected cubic planar graphs embedded in the plane, we switch to the dual and count rooted triangulations. Since all these numbers can be evaluated in polynomial time using dynamic programming, rooted connected cubic planar graphs can be generated uniformly at random in polynomial time by inverting the decomposition along the connectivity structure. To generate connected cubic planar graphs without a root uniformly at random, we apply rejection sampling and obtain an expected polynomial time algorithm. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008     

Edge coloring nearly bipartite graphs

We give a simple polynomial time algorithm to compute the chromatic index of graphs which can be made bipartite by deleting a vertex. An analysis of this algorithm shows that for such graphs, the chromatic index is the roundup of the fractional chromatic index.     

An algorithm for finding hamilton paths and cycles in random graphs

This paper describes a polynomial time algorithm HAM that searches for Hamilton cycles in undirected graphs. On a random graph its asymptotic probability of success is that of the existence of such a cycle. If all graphs withn vertices are considered equally likely, then using dynamic programming on failure leads to an algorithm with polynomial expected time. The algorithm HAM is also used to solve the symmetric bottleneck travelling salesman problem with probability tending to 1, asn tends to ∞. Various modifications of HAM are shown to solve several Hamilton path problems. Supported by NSF Grant MCS 810 4854.     

Recognizing bull-free perfect graphs

A bull is the graph obtained from a triangle by adding two pendant vertices adjacent to distinct vertices of the triangle. Chvátal and Sbihi showed that the strong perfect graph conjecture holds for Bull-free graphs. We give a polynomial time recognition algorithm for Bull-free perfect graphs.     

On the complexity of recognizing perfectly orderable graphs

The question whether a polynomial time recognition algorithm for the class of perfectly orderable graphs exists was posed by Chvátal in 1981 when he introduced the notion of perfect orders. Since then several classes of perfectly orderable graphs have been identified. In this note we prove that recognizing perfectly orderable graphs is NP-complete.     

Which line-graphs are perfectly orderable?

We characterize (by forbidden induced subgraphs) those line-graphs that are perfectly orderable. Implicit in our presentation is a polynomial, time algorithm for recognizing these graphs.     

A polynomial time algorithm to compute the Abelian kernel of a finite monoid

The time complexity of the best previously known algorithm to compute the Abelian kernel of a finite monoid with a fixed number of generators is exponential. In this paper we use results on subgroups of the free Abelian group and constructions on labeled graphs to develop a polynomial time algorithm for this problem.     

Weakly bipartite graphs and the Max-cut problem

A new class of graphs, called weakly bipartite graphs, is introduced. A graph is called weakly bipartite if its bipartite subgraph polytope coincides with a certain polyhedron related to odd cycle constraints. The class of weakly bipartite graphs contains for instance the class of bipartite graphs and the class of planar graphs. It is shown that the max-cut problem can be solved in polynomial time for weakly bipartite graphs. The polynomical algorithm presented is based on the ellipsoid method and an algorithm that computes a shortest path of even length.     

Recognizing sign solvable graphs

A characterization of sign solvable graphs is presented together with a polynomial algorithm for recognizing such graphs.     

A Polynomial Time Algorithm to Compute the Abelian Kernel of a Finite Monoid

Abstract. The time complexity of the best previously known algorithm to compute the Abelian kernel of a finite monoid with a fixed number of generators is exponential. In this paper we use results on subgroups of the free Abelian group and constructions on labeled graphs to develop a polynomial time algorithm for this problem.     

Optimization via enumeration: a new algorithm for the Max Cut Problem

We present a polynomial time algorithm to find the maximum weight of an edge-cut in graphs embeddable on an arbitrary orientable surface, with integral weights bounded in the absolute value by a polynomial of the size of the graph.</ The algorithm has been implemented for toroidal grids using modular arithmetics and the generalized nested dissection method. The applications in statistical physics are discussed. Received: June 1999 / Accepted: December 2000?Published online March 22, 2001