Showing papers by "Tunis University published in 1995"

Moral hazard, financial constraints and sharecropping in El Oulja

Abstract: This paper develops a theory of sharecropping which emphasizes the dual role of moral hazard in the provision of effort and financial constraints. The model is compatible with a large variety of contracts as observed in the region of El Ouija in Tunisia. Using an original set of data including financial data, various tests of the theory are realized. The role of financial constraints in the explanation of which type of contract is selected (as well as its implications that financial constraints affect effort and therefore output) are strongly supported by the data.

Femmes dans la Ville Mamlūke

Abstract: Medieval Islamic jurists strictly regulated the comings and goings of women in the street, as well as their code of dress. But the historical sources show that the baths, markets, cementaries, and tombs of the saints were actively visited by women, who were equally present in the great popular assemblies originating in religious or secular festivities. Therefore, women's behaviour in the great cities of the Mamluk empire totally differed from the ideal put forth by the jurists.

Ab initio pseudopotential calculations of the band lineups at strained ZnS/ZnSe interfaces: Including the 3d electrons of Zn as valence states.

Abstract: A first-principles pseudopotential approach has been used to calculate the valence-band offset (${\mathrm{\ensuremath{\Lambda}}}_{\mathit{v}}$) at strained ZnS/ZnSe (001) interfaces, and the structural and electronic structure properties of ZnS and ZnSe. In these calculations the semicore 3d electrons of Zn are included as relaxed valence states. Three strain configurations, corresponding to growth on ${\mathrm{ZnS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (001) (with x=0, 0.5, and 1) substrates were considered. We found that ${\mathrm{\ensuremath{\Lambda}}}_{\mathit{v}}$ varies almost linearly between 0.43 and 0.78 eV in the above strain range, with the state at the top of the valence band in ZnSe being higher in energy. The averages of ${\mathrm{\ensuremath{\Lambda}}}_{\mathit{v}}$ and the conduction-band offset are found to be independent of the strain state and with values of 0.50 and 0.23 eV, respectively. These results and that of the other studied properties are found to be in excellent agreement with experiment and all-electron calculations. The charge density associated with the d bands shows some deviations from being spherically symmetric around the Zn ions and has appreciable magnitude in the anions regions, which suggests that the dp hybridization is quite strong.

Theoretical spectroscopic data for the x2b1 and a2a1 states of h2s

Abstract: Coupled electron pair (CEPA) electronic wave functions have been used to calculate the three-dimensional potential energy functions of the two lowest doublet states X2B1 and A2A1 of H2S+. The near-equilibrium potential for the X2B1 state has been used in variational calculations of the ro-vibrational energy levels up to 10 000 cm−1. Rotationally resolved absorption spectra of the three fundamental transitions have been calculated ab initio using the three-dimensional electric dipole moment functions and the ro-vibrational eigenfunctions. The absolute line strengths are given for the most intense transitions.

Designing control and diagnosis for flexible manufacturing systems as a multi-agent system using blackboard and object Petri nets

Abstract: When creating and setting up assistance to diagnosis, it is important to have the necessary means to analyse the physical systems in order to extract and formalize the various pieces of knowledge. Hence the idea of integrating in the SA-RT/OPN approach these two aspects; because representation is a pre-requisite for the solution, our approach constitutes a real guide for information and knowledge analysis and representation. The main idea is that the use of this information and its organisation lead to strategies which may be the base for the diagnosis of an industrial system. The final implementation of the control/monitoring, and in particular the diagnosis, is conceived as a multi-agent system exploiting knowledge modelled by the blackboard.

Method for Knowledge Acquisition from Multiple Experts

Abstract: In this paper, we present a method for knowledge acquisition from several experts. This method is based on a set of generic models which serve as a template to the knowledge engineer when acquiring knowledge from multiple experts. Experts are described as a society of interacting cognitive agents by instantiating the models. Our work can be viewed as an extension of KADS (Knowledge-based system Analysis and Design Structured methodology) [Schreiber &al. 94] to the multi-expertise.

Exciton states and relaxation dynamics in shallow quantum wells

Abstract: Optical properties of GaAs/AlxGa1−xAs shallow quantum wells have been studied in the 2–200K range, both by c.w. and time-resolved experiments. The results agree with a variational calculation of the excitonic transitions. The LO-phonon-assisted carrier relaxation and a temperature-activated non-radiative channel are evidence.

A Parallel Logic Language for Transaction Specification in Multidatabase Systems

Abstract: The realization of truly heterogeneous database systems is hampered by two principal obstacles. One is the unsuitability of traditional transaction models; this has led to the proposal of a number of new, advanced transaction models. The second is the lack of appropriate programming support for these advanced concepts. This paper addresses these two issues by pointing out the advantages of using a logic-based approach for the integration of autonomous software systems.

Structural influence of the carbon chain length in hybrid materials obtained from zirconium n-propoxide and diols

Abstract: Zirconium n-propoxide Zr(OPr n )4 has been chemically modified by 1,3-propanediol and 1,3-butanediol with molar ratio r=0.5, 1 and 2. The properties of the hybrid organic-inorganic materials obtained after drying at 100°C for 24 hours have been measured by thermal analysis, X-ray diffraction, infra-red spectroscopy and 13C NMR with cross-polarisation at magic angle spinning. Chemical modification of the zirconium precursor at a molecular level has been clearly detected, with a bridging coordination mode for both diols. If the coordination mode of the diols cannot be changed by increasing the carbon chain length, it was found using 13C CP MAS NMR that the local order is much more well-defined when the reactivity of both OH groups are not the same. Thus, using 1,3-butanediol which has a primary and a secondary OH group, it is possible to obtain after complete modification (r=2) a highly homogeneous hybrid polymer which transforms directly into tetragonal zirconia below 400°C. With other diols (1,2-ethanediol and 1,3-propanediol), the hybrid materials transform into tetragonal zirconia above 400°C through a decomposition process involving several steps. Then playing with the carbon chain length and/or the steric hindrance around the OH groups provides an easy way to monitor the ultrastructure of these hybrid materials, and allows a better control of the gel → oxide transformation.

Quantification, identification et propriétés physiques des sédiments de la retenue du barrage Sidi Saad (centre de la Tunisie)

Abstract: La retenue du barrage de Sidi Saâd est sujette a un envasement rapide. Les terrains vulnerables s'etendent sur 300 000 ha sur une superficie totale du bassin versant de 8 650 km2 pratiquement depourvue de couvert vegetal. Le taux annuel de l'alluvionnement est passe de 2,24 hm3 a 3,54 hm3 a la suite d'une annee hydrologique exceptionnelle. Des echantillons superficiels, des carottes de sediments preleves dans la retenue du barrage et les produits des soutirages ont fait l'objet d'etudes granulometriques, mineralogiques, et geotechniques. Les courbes gamma-densimetriques des carottes de sediments nous ont revele la presence de plusieurs sequences de depots. Celles-ci correspondent a des niveaux turbiditiques type Bouma avec une predominance des termes a, b, d et e. Toutefois, les sediments superficiels montrent un tri granulometrique en allant des cones de dejection vers l'ouvrage. Ceci a ete bien defini par l'etude geotechnique (teneur en eau, plasticite, densite, cohesion, etc.). Le cortege mineralogique traduit un heritage detritique des formations geologiques vulnerables du bassin versant. Les essais de consolidation realises au laboratoire sur des mixtures de charge solides differentes nous ont permis d'identifier cinq phases de depots. La vitesse de consolidation est fonction de la charge solide initiale et des conditions de drainage. L'augmentation de la cohesion au cours du temps suit une loi exponentielle.

Minimal program covering based on the output variables

Abstract: This paper discusses how a program can be represented by a binary relation, R, and how to decompose the latter into a set of rectangular relations. Next, we present our methodology based on relational operators and dependence relations, to show how we can use these rectangles to obtain more interesting ones that describe the entire behavior of every variable in the program. The notion of lattice of maximal rectangles is effective in that it permits to have a particular representation of the program which shows all the different parts that constitute the original program. By looking at this lattice structure, we find that the set of the leaves of this lattice, which represent "pertinent" rectangles associated to output variables, gives a minimal program covering.

Electronic correlations in the quasi-2D organic superconductor κ-(ET)2Cu[N(CN)2]Br

Abstract: We present a 13C NMR study of the organic superconductor κ-(ET)2CCu[N(CN)2]Br in a wide temperature and pressure domain. The temperature profiles of the Knight shift and the nuclear relaxation rate in the normal phase show a striking similarity with that found in several high Tc superconductors. We suggest that nuclear relaxation is dominated by strong antiferromagnetic fluctuations due to Coulomb repulsion enhanced by nesting. From the pressure dependence of the Knight shift we estimate the U/W ratio to be of the order of unity.