In this paper two methods for automatic generation of connected chordal graphs are proposed: the first one is based on new results concerning the dynamic maintenance of chordality under edge insertions; the second is based on expansion/merging of maximal cliques. Theoretical and experimental results are presented. In both methods, chordality is preserved along the whole generation process.In the solution of algorithmic problems, graphs can play different roles, being the very input for an algorithm or simply an auxiliary data structure handled by it. In the first case, the generation of suitable instances (i.e. input graphs satisfying given constraints) can be so complex that it constitutes a further problem, sometimes as hard to solve as the original one.Chordal graphs are a broadly studied class, as their peculiar clique-based structure allows the solution of many algorithmic problems. See, for instance, Chandran et al. (2003), Gavril (1972, and Kumar and Madhavan (2002). Since the generation of instances for testing these algorithms is often required, our goal in this paper is to develop procedures for automatically constructing large connected chordal graphs, with more than 10,000 vertices.Rather than generating connected graphs at random and testing whether they are chordal or not, we focus here on generation procedures in which graphs are constructed while chordality is maintained during the whole generation process. Two methods are presented.The incremental method is based on new results about the dynamic maintenance of chordality under edge insertions developed by Araujo (2004). It allows the generation of L. Markenzon's research is partially supported by grant 301068/2003-8, CNPq, Brazil. L. Markenzon ( ) · O. Vernet

In this paper, we discuss hamiltonian problems for reducible flowgraphs. The main result is finding, in linear time, the unique hamiltonian cycle, if it exists. In order to obtain this result, two other related problems are solved: finding the hamiltonian path starting at the source vertex and finding the hamiltonian cycle given the hamiltonian path.

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In this paper, we propose a new representation for k-trees -the compact code, which reduces the required memory space from O(nk) to O(n). The encoding and decoding algorithms, based on a simplification of a priority queue, are linear and very simple. As long as the k-tree is represented by its compact code, the exact vertex coloring problem can be solved in time O(n).Keywords: k-trees; Prüfer code; compact code. ResumoNeste artigo, propomos uma nova representação para k-árvores -o código compacto, que reduz o espaço de memória de armazenamento de O(nk) para O(n). Os algoritmos de codificação e decodificação, baseados em uma simplificação de uma lista de prioridades, são lineares e muito simples. Uma vez que a k-árvore esteja representada pelo seu código compacto, o problema da coloração exata de vértices pode ser resolvido em tempo O(n).