Showing papers by "École Normale Supérieure published in 1991"
TL;DR: A lattice Boltzmann model for simulating immiscible binary fluids in two dimensions is introduced and a theoretical value of the surface-tension coefficient is derived and found to be in excellent agreement with values obtained from simulations.
Abstract: We introduce a lattice Boltzmann model for simulating immiscible binary fluids in two dimensions. The model, based on the Boltzmann equation of lattice-gas hydrodynamics, incorporates features of a previously introduced discrete immiscible lattice-gas model. A theoretical value of the surface-tension coefficient is derived and found to be in excellent agreement with values obtained from simulations. The model serves as a numerical method for the simulation of immiscible two-phase flow; a preliminary application illustrates a simulation of flow in a two-dimensional microscopic model of a porous medium. Extension of the model to three dimensions appears straightforward.
1,365 citations
TL;DR: In this article, the impact response of continuous fiber-reinforced composites is reviewed and an attempt is made to draw together much of the work published in the literature and to identify the fundamental parameters determining the impact resistance of continuous fibre reinforced composite materials.
1,276 citations
TL;DR: In this paper, a new standard plate theory, which accounts for cosine shear stress distribution and free boundary conditions for shear stresses upon the top and bottom surfaces of the plate, is presented.
932 citations
TL;DR: A cubic gallophosphate with a pore opening comprising 20 tetrahedrally coordinated atoms in the shape of a four-leafed clover was reported in this paper. But the structure of the clover is not known.
Abstract: THE most widely used catalyst in petroleum cracking and reforming processes is the synthetic zeolite Y. Its unique properties arise from the fact that the enormous inner surface area created by the three-dimensional channel system is accessible to sorbed molecules through 12-ring pore openings. These windows, with free diameters of 7-8A, allow both aliphatic and small aromatic molecules to enter the zeolite. Attempts to synthesize zeolitic structures with even wider pores, to accommodate larger molecules, have produced two aluminophosphates with one-dimensional channels: VPI-5 (ref. 1), which has 18-ring channels with free diameters of 12-13A, and AlPO48 (refs 2, 3), with oval 14-ring channels. We now report the structure of a new cubic gallophosphate with a pore opening comprising 20 tetrahedrally coordinated atoms in the shape of a four-leafed clover (Fig. 1) and a three-dimensional channel system. The unusual shape of the window is due to the presence of terminal hydroxyl groups in the framework, and provides new possibilities for shape-selective sorption. The supercage formed aHhe intersection of the channels has a body diagonal of 29-30 A, and could thus accommodate larger intermediates in zeolite catalytic processes.
702 citations
679 citations
TL;DR: The theory is developed for the case when orientation computations are necessary at all local neighborhoods of the n-dimensional Euclidean space and a certainty measure, based on the error of the fit, is proposed.
Abstract: The problem of detection of orientation in finite dimensional Euclidean spaces is solved in the least squares sense. The theory is developed for the case when such orientation computations are necessary at all local neighborhoods of the n-dimensional Euclidean space. Detection of orientation is shown to correspond to fitting an axis or a plane to the Fourier transform of an n-dimensional structure. The solution of this problem is related to the solution of a well-known matrix eigenvalue problem. The computations can be performed in the spatial domain without actually doing a Fourier transformation. Along with the orientation estimate, a certainty measure, based on the error of the fit, is proposed. Two applications in image analysis are considered: texture segmentation and optical flow. The theory is verified by experiments which confirm accurate orientation estimates and reliable certainty measures in the presence of noise. The comparative results indicate that the theory produces algorithms computing robust texture features as well as optical flow. >
590 citations
TL;DR: The lacunarity of a set is defined from the fluctuations of the mass distribution function, which is found using an algorithm the authors call the gliding-box method, and this definition is applied to characterize the geometry of random and deterministic fractal sets.
Abstract: The notion of lacunarity makes it possible to distinguish sets that have the same fractal dimension but different textures. In this paper we define the lacunarity of a set from the fluctuations of the mass distribution function, which is found using an algorithm we call the gliding-box method. We apply this definition to characterize the geometry of random and deterministic fractal sets. In the case of self-similar sets, lacunarity follows particular scaling properties that are established and discussed in relation to other geometrical analyses.
543 citations
TL;DR: A Ca2+ rise increases the sensitivity of Purkinje cells to GABA and induces a retrograde inhibition of presynaptic terminals, the latter effect may be due to a diffusible Ca2(+)-dependent messenger.
496 citations
TL;DR: Analysis of synaptic currents in terms of this model suggested that the recorded time course of decay was approximately correct, as well as the quality of the voltage clamp.
Abstract: 1. Postsynaptic currents originating from activation of the two major excitatory inputs to Purkinje cells were studied in thin slices of rat cerebellum, using the tight-seal whole-cell recording technique. Two types of excitatory postsynaptic currents were analysed: those evoked by stimulation of the granule cell-parallel fibre system (PF-EPSC) and those elicited by stimulation of the climbing fibres (CF-EPSC). 2. Both types of postsynaptic currents had a linear current-voltage relation, reversing at membrane potentials close to 0 mV. Their time course of activation was independent of the membrane potential. 3. For both types of postsynaptic currents, the time course of decay was well described by a single exponential function, with a time constant which increased as the membrane potential was made more positive. 4. Postsynaptic currents arising from stimulation of the climbing fibre generally had a slightly faster time course of onset and decay than those associated with stimulation of the granule cell-parallel fibre system. The average values of the 10-90% rise time were 1.8 +/- 0.4 ms (means +/- S.D., n = 7) for PF-EPSCs and 0.8 +/- 0.3 ms (n = 9) for CF-EPSCs. Time constants of decay, at a holding potential of -60 mV, had values of 8.3 +/- 1.6 ms (n = 7) and 6.4 +/- 1.1 ms (n = 9) for PF-EPSCs and CF-EPSCs respectively. 5. CF-EPSCs and PF-EPSCs had the characteristics described above in slices derived from animals aged 9-22 days old and 9-15 days old, respectively. The PF-EPSCs in animals older than 15 days had very slow time courses and positive apparent reversal potentials, suggesting that they originated from distal locations, not under accurate voltage control. 6. In order to assess the quality of the voltage clamp, responses to hyperpolarizing pulses from -70 mV were analysed. The capacitive currents could be fitted by the sum of two exponentials, and were interpreted with an equivalent electrical circuit comprising two main compartments (soma and proximal dendrites on one hand, distal dendrites on the other). Analysis of synaptic currents in terms of this model suggested that the recorded time course of decay was approximately correct. 7. CF-EPSCs as well as PF-EPSCs were insensitive to the NMDA receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), but were blocked in a dose-dependent reversible manner by the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX).(ABSTRACT TRUNCATED AT 400 WORDS)
481 citations
TL;DR: This paper used singular spectrum analysis to analyse the time series of global surface air tem-peratures for the past 135 years, allowing a secular warming trend and a small number of oscillatory modes to be separated from the noise.
Abstract: THE ability to distinguish a warming trend from natural variability is critical for an understanding of the climatic response to increasing greenhouse-gas concentrations. Here we use singular spectrum analysis1 to analyse the time series of global surface air tem-peratures for the past 135 years2, allowing a secular warming trend and a small number of oscillatory modes to be separated from the noise. The trend is flat until 1910, with an increase of 0.4 °C since then. The oscillations exhibit interdecadal periods of 21 and 16 years, and interannual periods of 6 and 5 years. The interannual oscillations are probably related to global aspects of the El Nino-Southern Oscillation (ENSO) phenomenon3. The interdecadal oscillations could be associated with changes in the extratropical ocean circulation4. The oscillatory components have combined (peak-to-peak) amplitudes of >0.2 °C, and therefore limit our ability to predict whether the inferred secular warming trend of 0.005 °Cyr−1 will continue. This could postpone incontrovertible detection of the greenhouse warming signal for one or two decades.
474 citations
TL;DR: A spatial iterative algorithm for electromagnetic imaging based on a Newton-Kantorovich procedure for the reconstruction of the complex permittivity of inhomogeneous lossy dielectric objects with arbitrary shape was proposed in this paper.
Abstract: The authors propose a spatial iterative algorithm for electromagnetic imaging based on a Newton-Kantorovich procedure for the reconstruction of the complex permittivity of inhomogeneous lossy dielectric objects with arbitrary shape. Starting from integral representation of the electric field and using the moment method, this technique has been developed for 2-D (for TM and TE polarization cases) objects as well as for 3-D objects. Its performance has been compared with spectral techniques of classical diffraction tomography, the modified Newton method, and the pseudo-inverse method. >
TL;DR: In this paper, a symmetry-breaking model for collective phenomena is presented, which combines a number of well-established social psychology hypotheses with recent concepts of statistical physics, and the conceptual analysis of the assumed mechanisms reveals that when we deal with phenomena that have until now been designated as polarization phenomena, we are faced with a whole class of phenomena.
Abstract: This study presents the outline of a model for collective phenomena. A symmetry-breaking model combines a number of well-established social psychology hypotheses with recent concepts of statistical physics. Specifically we start out from the regularities obtained in studies on the polarization of attitudes and decisions. From a strictly logical point of view, it is immediately clear that aggregation effects must be analysed separately from group effects as such. The conceptual analysis of the assumed mechanisms reveals that when we deal with phenomena that have until now been designated as polarization phenomena, we are faced not with a single phenomenon, as was believed hitherto, but with a whole class of phenomena. For this reason it would be appropriate to deal with them differentially both from an empirical and from a theoretical point of view. It is possible to show, moreover, that in principle polarization is a direct function of interaction and, beyond a critical threshold an inverse function of the differentiation between group members. A certain number of verifiable conjectures are presented on the basis of physio-mathematical-psychological considerations. It is to be hoped that these theoretical outlines will make it possible to give a new lease on life to a field of research that has established solid facts, but that became trapped in a dead-end road, for lack of a sufficiently broad analysis.
TL;DR: The paper presents the actual procedures used to mechanically produce such test sets, using Horn clause logic, embedded in an interactive system which, given some general hypotheses schemes and an algebraic specification, produces a test set and the corresponding hypotheses.
Abstract: This paper addresses the problem of constructing test data sets from formal specifications. Starting from a notion of an ideal exhaustive test data set which is derived from the notion of satisfaction of the formal specification, it is shown how to select by refinements a practicable test set, i.e. computable, not rejecting correct programs (unbiased), and accepting only correct programs (valid), assuming some hypotheses. The hypotheses play an important role: they formalize common test practices and they express the gap between the success of the test and correctness ; the size of the test set depends on the strength of the hypotheses. The paper shows an application of this theory in the case of algebraic specifications and presents the actual procedures used to mechanically produce such test sets, using Horn clause logic. These procedures are embedded in an interactive system which, given some general hypotheses schemes and an algebraic specification, produces a test set and the corresponding hypotheses.
TL;DR: In this paper, it was shown that the ferromagnetic ground state is unique if the number of electrons satisfies N>or=M. The results are valid also for a multi-band Hubbard model on a bipartite graph.
Abstract: Let L(G) be the line graph of a graph G=(V,E). The Hubbard model on L(G) has ferromagnetic ground states with a saturated spin if the interaction is repulsive (U>0) and if the number of electrons N satisfies N>or=M. M= mod E mod + mod V mod -1 if G is bipartite and M= mod E mod + mod V mod otherwise. The author shows that the ferromagnetic ground state is unique if N=M. Further he gives a sufficient condition for the existence of other ground states if N>M. The results are valid also for a multi-band Hubbard model on a bipartite graph. In the case of a periodic lattice, the results are related to the existence of a flat energy band.
TL;DR: In this paper, the authors discussed the properties of the Hubbard model on a line graph with n vertices and showed that the model has ferromagnetic ground states if the interaction is repulsive (U)0) and if the number of electrons N satisfies 2n>or=N>or =M.
Abstract: The author discusses some of the properties of the Hubbard model on a line graph with n vertices. It is shown that the model has ferromagnetic ground states if the interaction is repulsive (U)0) and if the number of electrons N satisfies 2n>or=N>or=M. M is a natural number that depends on the line graph. For example, the Kagome lattice is a line graph, it has M=5n/3-1.
03 Jan 1991
TL;DR: It is shown that to every pair of types in the subtype relation the authors can associate a term whose denotation is the uniquely determined coercion map between the two types, and derive an algorithm that can infer its least type whenever possible.
Abstract: We investigate the interactions of subtyping and recursive types, in a simply typed l-calculus. The two fundamental questions here are whether two (recursive)types are in the subtype relation and whether a term has a type. To address the first question, we relate various definitions of type equivalence and subtyping that are induced by a model, an ordering on infinite trees, an algorithm, and a set of type rules. We show soundness and completeness among the rules, the algorithm, and the tree semantics. We also prove soundness and a restricted form of completeness for the model. To address the second question, we show that to every pair of types in the subtype relation we can associate a term whose denotation is the uniquely determined coercion map between the two types. Moreover, we derive an algorithm that, when given a term with implicit coercions, can infer its least type whenever possible.
TL;DR: In this article, the lithium alkoxides formed as intermediates undergo an intramolecular addition to the neighboring CN group followed by an S to 0 benzothiazole transfer and simultaneous extrustion of sulfur dioxide and ejection of 2(3H)-benzothiazolone anion.
TL;DR: Theta‐bursting findings suggest that the optimal conditions for the induction of LTP occur naturally in behaving animals, time‐locked to behavioral events critical to learning.
Abstract: Recent studies have revealed 3 stimulation parameters that together comprise the temporal pattern of neuronal activation optimal for the induction of hippocampal LTP: high-frequency bursts, activity 100-200 ms prior to a burst, and burst delivery in phase with the ongoing hippocampal theta rhythm. The present paper reports that these 3 aspects of patterned neural activity, collectively referred to as "theta-bursting," are characteristic of the spike trains of CA1 pyramidal cells in rats during the sampling and analysis of learning cues in an odor discrimination task and during performances of a spatial memory task. In contrast, theta-bursting occurs relatively infrequently during behavioral events less directly related to task-relevant mnemonic processing. These findings suggest that the optimal conditions for the induction of LTP occur naturally in behaving animals, time-locked to behavioral events critical to learning.
TL;DR: In this article, the modifications of the state of aluminium when introducing protons into zeolite-β have been investigated by combining several analytical techniques, and it was concluded that the octahedrally coordinated and NMR invisible aluminium must be regarded as inherent parts of the framework of the Zeolite and their formation is explained by a distortion of aluminic sites caused by the high electron affinity of the proton.
TL;DR: In this paper, a prograde PT-path is constructed for pyrope and coesite in the country-rock gneisses, which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar, and finally up to 37 kbar at about 800° C where the Mg-rich metapelite was a pyropecite with phengite, kyanite, and talc still present.
Abstract: Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.
TL;DR: In this paper, a simple factorized scattering theory is suggested for the massless Goldstone fermions of the trajectory flowing from the tricritical Ising fixed point to the critical Ising one.
TL;DR: A new scaling theory is proposed which expresses all statistical properties in terms of ξ and shows that the average circulation of the vortices increases as tξ /2 and their average radius as t ξ /4 .
Abstract: Freely evolving two-dimensional turbulence is dominated by coherent vortices. The density of these vortices decays in time as \ensuremath{\rho}\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{-}}\ensuremath{\xi}}$ with \ensuremath{\xi}\ensuremath{\approxeq}0.75. A new scaling theory is proposed which expresses all statistical properties in terms of \ensuremath{\xi}. Thus the average circulation of the vortices increases as ${\mathit{t}}^{\ensuremath{\xi}/2}$ and their average radius as ${\mathit{t}}^{\ensuremath{\xi}/4}$. The total energy is constant, the enstrophy decreases as ${\mathit{t}}^{\mathrm{\ensuremath{-}}\ensuremath{\xi}/2}$, and the vorticity kurtosis increases as ${\mathit{t}}^{\ensuremath{\xi}/2}$. These results are supported both by numerical simulations of the fluid equations and by solutions of a modified point-vortex model.
Book•
01 Jan 1991TL;DR: This book introduces category theory at a level appropriate for computer scientists and provides practical examples in the context of programming language design and pursues the more complex mathematical semantics of data types and programs as objects and morphisms of categories.
Abstract: Category theory is a mathematical subject whose importance in several areas of computer science, most notably the semantics of programming languages and the design of programmes using abstract data types, is widely acknowledged. This book introduces category theory at a level appropriate for computer scientists and provides practical examples in the context of programming language design. "Categories, Types and structures" provides a self-contained introduction to general category theory and explains the mathematical structures that have been the foundation of language design for the past two decades. The authors observe that the language of categories could provide a powerful means of standardizing of methods and language, and offer examples ranging from the early dialects of LISP, to Edinburgh ML, to work in polymorphisms and modularity. The book familiarizes readers with categorical concepts through examples based on elementary mathematical notions such as monoids, groups and toplogical spaces, as well as elementary notions from programming-language semantics such as partial orders and categories of domains in denotational semantics. It then pursues the more complex mathematical semantics of data types and programs as objects and morphisms of categories.
TL;DR: In this article, a broad classification of nematic, cholesteric and smectic phases of LCPs is given, and the emphasis is on providing a broad perspective of the remarkable properties of these materials rather than a detailed and rigorous treatment of selected topics.
11 Aug 1991
TL;DR: A protocol for quantum oblivious transfer, utilizing faint pulses of polarized light, by which one of two mutually distrustful parties transmits two one-bit messages in such a way that the other party can choose which message he gets but cannot obtain information about both messages.
Abstract: We describe a protocol for quantum oblivious transfer, utilizing faint pulses of polarized light, by which one of two mutually distrustful parties ("Alice") transmits two one-bit messages in such a way that the other party ("Bob") can choose which message he gets but cannot obtain information about both messages (he will learn his chosen bit's value with exponentially small error probability and may gain at most exponentially little information about the value of the other bit), and Alice will be entirely ignorant of which bit he received. Neither party can cheat (ie deviate from the protocol while appearing to follow it) in such a way as to obtain more information than what is given by the description of the protocol. Our protocol is easy to modify in order to implement the All-or-Nothing Disclosure of one out of two string messages, and it can be used to implement bit commitment and oblivious circuit evaluation without complexity-theoretic assumptions, in a way that remains secure even against cheaters that have unlimited computing power. Moreover, this protocol is practical in that it can be realized with available opto-electronic apparatus while being immune to any technologically feasible attack for the foreseeable future.
TL;DR: The results show that each finger spans three nucleotides and indicate two positions in Krox-20 zinc fingers that are important for base-pair selectivity, and modelling with molecular graphics suggests that these residues could bind directly with the bases and that other amino acid–base contacts are also possible.
Abstract: Zinc fingers constitute important eukaryotic DNA-binding domains, being present in many transcription factors. The Cys2/His2 zinc-finger class has conserved motifs of 28-30 amino acids which are usually present as tandem repeats. The structure of a Cys2/His2 zinc finger has been determined by nuclear magnetic resonance, but details of its interaction with DNA were not established. Here we identify amino acids governing DNA-binding specificity using in vitro directed mutagenesis guided by similarities between the zinc fingers of transcription factors Sp1 and Krox-20. Krox-20 is a serum-inducible transcription activator which is possibly involved in the regulation of hindbrain development; it contains three zinc fingers similar to those of Sp1 and binds to a 9-base-pair target sequence which is related to that of Sp1. Our results show that each finger spans three nucleotides and indicate two positions in Krox-20 zinc fingers that are important for base-pair selectivity. Modelling with molecular graphics suggests that these residues could bind directly with the bases and that other amino acid-base contacts are also possible.
TL;DR: In this paper, the authors consider the field theory formulation for manifolds in random media using the replica method and use a variational (Hartree-Fock like) method which shows that replica symmetry is spontaneously broken.
Abstract: We consider the field theory formulation for manifolds in random media using the replica method. We use a variational (Hartree-Fock like) method which shows that replica symmetry is spontaneously broken. A hierarchical breaking of symmetry allows one to take into account the existence of many metastable states for the manifold, and to recover the results of the Flory scaling arguments for the wandering exponent. This field theoretic derivation of Flory results opens the way to computing corrections to these exponents.