Didier G. Arquès

A purine-pyrimidine motif verifying an identical presence in almost all gene taxonomic groups*

A statistical parameter identifies, with a high degree of significance, a motif which is present in protein-coding sequences of eukaryotes, prokaryotes, chloroplasts, mitochondria, viral introns, ribosomal RNA genes, and transfer RNA genes. The random probability of occurrence of such a situation is 10(-12). This motif has the following properties: (i) its significant presence in almost all present-day genes explains why it can be considered as primitive oligonucleotide, (ii) its nucleotide order is: YRY (N)6YRY, R being a purine base, Y a pyrimidine one and N any base, (iii) its length and its terminal trinucleotides YRY suggest a primordial function related to the spatial structure of the DNA sequences. This motif is found in some viral protein-coding genes, but not in eukaryotic introns.

Parallel ray tracing based upon a multilevel topological knowledge acquisition of the scene

Including the standard parallelization by grouping primary rays, this paper presents a new parallel ray‐timing method based upon a topological knowledge acquisition of the scene. This topological knowledge focuses on relative positions between objects and processes and uses a new type of message. Indeed, instead of exchanging database pages or rays, processes exchange topological information. This information is used by each process to decrease its own list of objects to test against rays The acquisition of information about relative positions between objects and processes is obtained by a careful ordering of he pixel calculation. The processes are dispatched on a computer network including a parallel computer The organization of the processes on this network is a multilevel one leading to different levels of topological message exchanges This method is characterized by topological messages describing the scene, dynamic optimization of the database, easy parallelization on any network (no deadlock, fault tolerance, easily expandable and simple routing), and gives interesting results with true or simulated parallelism.

Combinatoire des mots et étude quantitative de la sérialisabilité: application à la concurrence d'accès à une base de données

Abstract In this paper we study the serializability and the concurrent access to a database by making use of the tools of the combinatorics of words. We first define a model in which the behavior of the concurrent execution of p transactions is reduced to a word over the alphabet defined by the access of the transactions to the entities. After having studied the free, the conflictless and the serializable systems by measuring the degree of parallelism that they allow, we characterize a scheduler (algorithm controlling the concurrent access to the database), by three quantities, respectively evaluating the mean number (among p ) of active, inactive and aborted transactions that it allows. We compute these quantities for the timestamp ordering (in the general case of p concurrent transactions) and for two-phase locking (in the case of p = 2 transactions) and then we compare these two schedulers. We also evaluate the frequency of deadlocked executions of two transactions in the case of the control by the two-phase locking algorithm. Some of the above results are generalized to the usual case where the transactions can lock some entities before the beginning of their concurrent execution and a method is proposed to generalize the study of the two-phase locking algorithm in the case of an arbitrary number of transactions. Then these results are compared with the statistical results obtained by Franaszek and Robinson by making use of a simulation study.