Let K denote a field and let V denote a vector space over K with finite positive dimension. We consider a pair of linear transformations A : V → V and A∗ : V → V that satisfy (i)–(iv) below: (i) Each of A, A∗ is diagonalizable. (ii) There exists an ordering V0, V1, . . . , Vd of the eigenspaces of A such that A ∗Vi ⊆ Vi−1 + Vi + Vi+1 for 0 ≤ i ≤ d, where V−1 = 0, Vd+1 = 0. (iii) There exists an...

An (r, n)-split coloring of a complete graph is an edge coloring with r colors under which the vertex set is partitionable into r parts so that for each i, part i does not contain K, in color i. This generalizes the notion of split graphs which correspond to (2, 2)-split colorings. The smallest N for which the complete graph Ks has a coloring which is not (r,n)-split is denoted by f,.(n). Balan...

SUMMARY The design and implementation of an interference graph is critical to the performance of a graph-coloring register allocator. The cost of constructing and manipulating the interference graph dominates the overall cost of allocation. The literature on graph-coloring register allocation suggests the use of a bit matrix coupled with lists of edges to represent the graph. 1-3 Recently, Geor...

In this paper, we prove that for every connected graph G, there exists a split graph G' with the same independence number and the same order. Then we propose a first algorithm for finding this graph, given the degree sequence of the input graph G. Further, we propose a second algorithm for finding the independence number of G, given the adjacency matrix of G.

Given an undirected graph G and a positive integer k, the NP-hard Sparse Split Graph Editing problem asks to transform G into a graph that consists of a clique plus isolated vertices by performing at most k edge insertions and deletions; similarly, the P3-Bag Editing problem asks to transform G into a graph which is the union of two possibly overlapping cliques. We give a simple linear-time 3-a...

A CIS-graph is defined as a graph whose every maximal clique and stable set intersect. These graphs have many interesting properties, yet, it seems difficult to obtain an efficient characterization and/or polynomial-time recognition algorithm for CIS-graphs. An almost CIS-graph is defined as a graph that has a unique pair (C, S) of disjoint maximal clique C and stable sets S. We conjecture that...

Say a set H of graphs is heroic if there exists k such that every graph containing no member of H as an induced subgraph has cochromatic number at most k. (The cochromatic number of G is the minimum number of stable sets and cliques with union V (G).) Assuming an old conjecture of Gyárfás and Sumner, we give a complete characterization of the finite heroic sets. This is a consequence of the fol...

A circle graph is the intersection graph of a set of chords of a circle. If a circle graph is prime for the split (or join) decomposition defined by Cunnigham, it has a unique representation as a set of intersecting chords, and we prove that this representation can be defined by monadic second-order formulas. By using the (canonical) split decomposition of a circle graph, one can define in mona...

A λ-coloring of a graph G is an assignment of colors from the integer set {0, . . . , λ} to the vertices of the graph G such that vertices at distance of at most two get different colors and adjacent vertices get colors which are at least two apart. The problem of finding λ-colorings with optimal or near-optimal λ arises in the context of radio frequency assignment. We show that the problem of ...

A λ-coloring of a graph G is an assignment of colors from the integer set {0, . . . , λ} to the vertices of the graph G such that vertices at distance at most two get different colors and adjacent vertices get colors which are at least two apart. The problem of finding λ-coloring with small or optimal λ arises in the context of radio frequency assignment. We show that the problem of finding the...