Zhu [15] introduced circular-perfect graphs as a superclass of the well-known perfect graphs and as an important χ-bound class of graphs with the smallest non-trivial χ-binding function χ(G) ≤ ω(G)+1. Perfect graphs have been recently characterized as those graphs without odd holes and odd antiholes as induced subgraphs [4]; in particular, perfect graphs are closed under complementation [7]. In...

We extend the notion of a minor from matroids to simplicial complexes. We show that the class of matroids, as well as the class of independence complexes, is characterized by a single forbidden minor. Inspired by a recent result of Aharoni and Berger, we investigate possible ways to extend the matroid intersection theorem to simplicial complexes.

Given a symmetric m by m matrix M over 0, 1, ∗, the M -partition problem asks whether or not an input graph G can be partitioned into m parts corresponding to the rows (and columns) of M so that two distinct vertices from parts i and j (possibly with i = j) are nonadjacent if M(i, j) = 0, and adjacent if M(i, j) = 1. These matrix partition problems generalize graph colourings and homomorphisms,...

Many diverse engineering problems can be modeled with solid sweeping in a conceptually simple and intuitive way, and sweeps are considered to be one of the basic representation schemes in solid modeling. However, many properties of sweeps as well as their “informational completeness” are not well understood, which is the primary reason why computational support for solid sweeping remains scarce...

Finite obstruction set characterizations for lower ideals in the minor order are guaranteed to exist by the Graph Minor Theorem. In this paper we characterize several families of graphs with small feedback sets, namely k1-Feedback Vertex Set, k2-Feedback Edge Set and (k1,k2){Feedback Vertex/Edge Set, for small integer parameters k1 and k2. Our constructive methods can compute obstruction sets f...

Efficiency of cellulase-free xylanases is one of the determining factors in paper and pulp industries. Use of microbes which can produce cellulase-free xylanases may help to overcome the current challenges in kraft pulp processing. Isolation and screening of microorganisms from local samples offers a possibility for obtaining the potential microbes for this purpose. This research was therefore ...

Each fixed graph H gives rise to a list H-colouring problem. The complexity of list H-colouring problems has recently been fully classified: if H is a bi-arc graph, the problem is polynomial-time solvable, and otherwise it is NP-complete. The proof of this fact relies on a forbidden substructure characterization of bi-arc graphs, reminiscent of the theorem of Lekkerkerker and Boland on interval...

The coloring problem is studied in the paper for graph classes defined by two small forbidden induced subgraphs. We prove some sufficient conditions for effective solvability of the problem in such classes. As their corollary we determine the computational complexity for all sets of two connected forbidden induced subgraphs with at most five vertices except 13 explicitly enumerated cases.

We characterize the class of intersection graphs of hypergraphs with a rank of at most 3 and a multiplicity of at most 2 by means of a finite list of forbidden induced subgraphs in the class of threshold graphs. 2000 Mathematics Subject Classification: 05C62, 05C75, 05C70, 05C65

Let P be an induced hereditary property and L(P) denote the class of all graphs that satisfy the property P locally. The purpose of the present paper is to describe the minimal forbidden subgraphs of L(P) and the structure of local properties. Moreover, we prove that L(P) is irreducible for any hereditary property P. c © 2001 Elsevier Science B.V. All rights reserved.