Showing papers by "University of Rennes published in 1980"

Robust identification of a nonminimum phase system: Blind adjustment of a linear equalizer in data communications

Abstract: Consider an unknown linear time-invariant system without control, driven by a white noise with known distribution. We are interested in the identification of this system, observing only the output. This problem is well known under the major assumption: the system is minimum (or maximum!) phase, in which the very popular least squares method gives an identification of the system in an autoregressive form. However, we are Interested in the case where the system is nonminimum (nor maximum!) phase, i.e., we want identification of both gain and phase of the system. The literature gives only a negative result: the idenfication of the phase of the system is impossible in the case of a Gaussian driving noise (hence, second-order statistics are irrelevant to our problem). For a large class of other input distributions, we present an identification procedure, and give some numerical results for a concrete case origin of our study: the blind adjustment of a transversal equalizer without any startup period prior to data transmission.

Une méthode multipas implicite-explicite pour l'approximation des équations d'évolution paraboliques

Abstract: Let us consider a linear parabolic equation which is associated with a time dependent operatorA(t). In this paper, we present a method, which is founded on linear multistep methods, which discretize a time-independent part of the operatorA(t) in an implicit way, and the other part in an explicit way. We study stability and convergence for this method. Considerons une equation d'evolution parabolique lineaire associee a un operateur lineaireA(t) dependant du tempst. Nous developpons dans cet article une methode de discretisation, basee sur les methodes lineaires a pas multiples, traitant de maniere implicite une partie de l'operateurA(t) independante du temps, l'autre partie est traitee de maniere explicite. Nous etudions la stabilite et la convergence de cette methode.

A comparison of the diets of three major groups of primary consumers of Gabon (primates, squirrels and ruminants).

Abstract: The diets of three major groups of frugivores-folivores of an African rain forest (squirrels, ruminants, primates) are compared and the relationship of food habits to body weight, habitat and foraging height examined. A number of common trends in the trophic patterns are found in the three groups. Over half of the fruit species identified in the diets of any one taxon are exploited in common with one or more of the others. The parts of fruit usually eaten are different for each group but for a number of species, the same fruit parts are searched for by the species of the three taxa.

Is the Hercynien belt of Brittany a major shear zone

Abstract: In the Hercynian belt of Brittany (France), large localized shear zones indicate dextral transcurrent shearing. In this letter regional Hercynian structures and their relationships with shearing are considered and information obtained by quartz fabric analysis in the central part of the belt, outside the major shear zones, is presented. Fabric patterns show that a dextral shear component is associated with Hercynian penetrative deformation. Existing geological data are reconsidered in the light of these results, and the extent to which the Hercynian belt of Brittany can be regarded as a ‘major shear zone’ is discussed.

Weak and strong solutions of stochastic differential equations

Abstract: (1980). Weak and strong solutions of stochastic differential equations. Stochastics: Vol. 3, No. 1-4, pp. 171-191.

Pretty Printing of Trees

Abstract: A pleasing layout of printed tree structures is difficult to achieve automatically. The paper points out three main problems. First, the horizontal position of a node on the printed page depends on global consideration of the position of other nodes; secondly, the physical characteristics of printers require scanning the tree in left-to-right top-to-bottom sequence; finally, page overflow for wide trees must be handled. These problems are illustrated by analysing the shortcomings of a simple printing algorithm. A suitable general binary tree printing algorithm is presented and its adaptation to other types of trees is shown.

Nonlinear frequency-dependent diffraction effect in intracavity resonance asymmetries

Abstract: The asymmetry observed in intracavity resonances (especially of the Lamb-dip type) is attributed to a resonant nonlinear diffraction effect. Direct evidence of this effect is obtained by a new experiment and careful analysis of the diffracted light, in the geometrical shadow of the laser beam inside the cavity, which reveals a reversed asymmetry of the resonance. A modified Lamb-dip formula gives better agreement between experiment and theory. The effect occurs in many experiments with Gaussian beams, particularly in recent high-sensitivity intracavity laser spectroscopy.

Fluoride glasses in the ZrF4BaF2YF3AlF3 quaternary system

Abstract: The quaternary system ZrF4BaF2YF3AlF3 has been investigated in order to determine the nature of the vitreous domain Addition of aluminium fluoride to the basic ZrF4BaF2YF3 ternary system results in an increase in the size of the vitreous area and a lowering of the crystallization rate The change of density, refractive index and TG with respect to composition has been studied It has been shown that the substitution of Zr4+/Al3+ involves a change in the cationic distribution rather than in the anionic packing

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais, Southern Armorican Massif, France

Abstract: Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Desert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.

Archaean Mantle Heterogeneity: Evidence from Chemical and Isotopic Abundances in Archaean Igneous Rocks

Abstract: The problem of compositional heterogeneity in the Archaean upper mantle is evaluated from the existing data of isotopic compositions (Sr, Pb and Nd), trace element abundances (rare earth elements (r.e.e.)) and some critical major element chemistry in Archaean igneous rocks. To assess a large-scale mantle heterogeneity, we have emphasized the chemical characteristics of undoubtedly mantle-derived komatiites.The initial isotopic compositions of Sr and Pb (I$\_{\text{Sr}}$ and I$\_{\text{Pb}}$) in Archaean igneous rocks show a significant variation, suggesting that a long-term heterogeneity in Rb/Sr and U/Pb ratios might have existed in their mantle sources. In contrast, the available I$\_{\text{Nd}}$ values show little variation from the expected evolution in a chondritic uniform reservoir. These contrasting results, i.e. the variable I$\_{\text{Sr}}$ and I$\_{\text{Pb}}$ values as against the uniform I$\_{\text{Nd}}$ values, are in part attributed to the fact that mobile-refractory pairs in the Rb/Sr and the U/Pb systems but only the refractory-refractory pair in the Sm/Nd system have been involved in geochemical differentiation. The r.e.e. distributions in komatiitic rocks are not uniform. Many show strong depletion in light r.e.e. The degree of light r.e.e. depletion in these komatiitic rocks is comparable with that found in present mid-ocean ridge basalts. Regarding the major element chemistry, we have demonstrated that the CaO/Al$\_{2}$O$\_{3}$ ratios are significantly different in the komatiitic suites of two adjacent cratons (Kaapvaal and Rhodesia) in southern Africa. Both trace and major elements data suggest that the Archaean upper mantle was heterogeneous, but whether the heterogeneity truly represents a long-term phenomenon remains to be resolved by more isotopic analyses.