Showing papers by "École Normale Supérieure published in 1999"

Strategies for Theorizing from Process Data

Abstract: In this article I describe and compare a number of alternative generic strategies for the analysis of process data, looking at the consequences of these strategies for emerging theories. I evaluate the strengths and weaknesses of the strategies in terms of their capacity to generate theory that is accurate, parsimonious, general, and useful and suggest that method and theory are inextricably intertwined, that multiple strategies are often advisable, and that no analysis strategy will produce theory without an uncodifiable creative leap, however small. Finally, I argue that there is room in the organizational research literature for more openness within the academic community toward a variety of forms of coupling between theory and data.

Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis

Abstract: Although the effect of biodiversity on ecosystem functioning has become a major focus in ecology, its significance in a fluctuating environment is still poorly understood. According to the insurance hypothesis, biodiversity insures ecosystems against declines in their functioning because many species provide greater guarantees that some will maintain functioning even if others fail. Here we examine this hypothesis theoretically. We develop a general stochastic dynamic model to assess the effects of species richness on the expected temporal mean and variance of ecosystem processes such as productivity, based on individual species’ productivity responses to environmental fluctuations. Our model shows two major insurance effects of species richness on ecosystem productivity: (i) a buffering effect, i.e., a reduction in the temporal variance of productivity, and (ii) a performance-enhancing effect, i.e., an increase in the temporal mean of productivity. The strength of these insurance effects is determined by three factors: (i) the way ecosystem productivity is determined by individual species responses to environmental fluctuations, (ii) the degree of asynchronicity of these responses, and (iii) the detailed form of these responses. In particular, the greater the variance of the species responses, the lower the species richness at which the temporal mean of the ecosystem process saturates and the ecosystem becomes redundant. These results provide a strong theoretical foundation for the insurance hypothesis, which proves to be a fundamental principle for understanding the long-term effects of biodiversity on ecosystem processes.

Plant diversity and productivity experiments in european grasslands

Abstract: At eight European field sites, the impact of loss of plant diversity on primary productivity was simulated by synthesizing grassland communities with different numbers of plant species. Results differed in detail at each location, but there was an overall log-linear reduction of average aboveground biomass with loss of species. For a given number of species, communities with fewer functional groups were less productive. These diversity effects occurred along with differences associated with species composition and geographic location. Niche complementarity and positive species interactions appear to play a role in generating diversity-productivity relationships within sites in addition to sampling from the species pool.

A universal lattice code decoder for fading channels

Abstract: We present a maximum-likelihood decoding algorithm for an arbitrary lattice code when used over an independent fading channel with perfect channel state information at the receiver. The decoder is based on a bounded distance search among the lattice points falling inside a sphere centered at the received point. By judicious choice of the decoding radius we show that this decoder can be practically used to decode lattice codes of dimension up to 32 in a fading environment.

High-order contrasts for independent component analysis

Abstract: This article considers high-order measures of independence for the independent component analysis problem and discusses the class of Jacobi algorithms for their optimization. Several implementations are discussed. We compare the proposed approaches with gradient-based techniques from the algorithmic point of view and also on a set of biomedical data.

Resistance against Differential Power Analysis for Elliptic Curve Cryptosystems

Abstract: Differential Power Analysis, first introduced by Kocher et al. in [14], is a powerful technique allowing to recover secret smart card information by monitoring power signals. In [14] a specific DPA attack against smart-cards running the DES algorithm was described. As few as 1000 encryptions were sufficient to recover the secret key. In this paper we generalize DPA attack to elliptic curve (EC) cryptosystems and describe a DPA on EC Diffie-Hellman key exchange and EC EI-Gamal type encryption. Those attacks enable to recover the private key stored inside the smart-card. Moreover, we suggest countermeasures that thwart our attack.

The Sociology of Critical Capacity

Abstract: This article argues that many situations in social life can be analyzed by their requirement for the justification of action. It is in particular in situations of dispute that a need arises to explicate the grounds on which responsibility for errors is distributed and on which new agreement can be reached. Since a plurality of mutually incompatible modes of justification exists, disputes can be understood as disagreements either about whether the accepted rule of justification has not been violated or about which mode of justification to apply at all. The article develops a grammar of such modes of justification, called orders of worth (grandeur), and argues that the human capacity for criticism becomes visible in the daily occurrence of disputes over criteria for justification. At the same time, it is underlined that not all social situations can be interpreted with the help of such a sense of justice, which resides on a notion of equivalence. Regimes of love, of violence or of familiarity are systematic...

Fast global oscillations in networks of integrate-and-fire neurons with low firing rates

Abstract: We study analytically the dynamics of a network of sparsely connected inhibitory integrate-and-fire neurons in a regime where individual neurons emit spikes irregularly and at a low rate. In the limit when the number of neurons N tends to infinity,the network exhibits a sharp transition between a stationary and an oscillatory global activity regime where neurons are weakly synchronized. The activity becomes oscillatory when the inhibitory feedback is strong enough. The period of the global oscillation is found to be mainly controlled by synaptic times, but depends also on the characteristics of the external input. In large but finite networks, the analysis shows that global oscillations of finite coherence time generically exist both above and below the critical inhibition threshold. Their characteristics are determined as functions of systems parameters, in these two different regimes. The results are found to be in good agreement with numerical simulations.

Fast global oscillations in networks of integrate-and-fire neurons with low firing rates

Abstract: We study analytically the dynamics of a network of sparsely connected inhibitory integrate-and-fire neurons in a regime where individual neurons emit spikes irregularly and at a low rate. In the limit when the number of neurons N!1, the network exhibits a sharp transition between a stationary and an oscillatory global activity regime where neurons are weakly synchronized. The activity becomes oscillatory when the inhibitory feedback is strong enough. The period of the global oscillation is found to be mainly controlled by synaptic times but depends also on the characteristics of the external input. In large but finite networks, the analysis shows that global oscillations of finite coherence time generically exist both above and below the critical inhibition threshold. Their characteristics are determined as functions of systems parameters in these two different regimes. The results are found to be in good agreement with numerical simulations.

Distant Suffering: Morality, Media and Politics

Abstract: Distant Suffering, first published in 1999, examines the moral and political implications for a spectator of the distant suffering of others as presented through the media. What are the morally acceptable responses to the sight of suffering on television, for example, when the viewer cannot act directly to affect the circumstances in which the suffering takes place? Luc Boltanski argues that spectators can actively involve themselves and others by speaking about what they have seen and how they were affected by it. Developing ideas in Adam Smith's moral theory, he examines three rhetorical 'topics' available for the expression of the spectator's response to suffering: the topics of denunciation and of sentiment and the aesthetic topic. The book concludes with a discussion of a 'crisis of pity' in relation to modern forms of humanitarianism. A possible way out of this crisis is suggested which involves an emphasis and focus on present suffering.

A moral economy of corruption in Africa

Abstract: As far as corruption in Africa is both conspicuous and generalised, it has to be studied from the viewpoint of the participants. This article starts with six general theses on corruption in Africa, which place it within a broader ‘corruption complex’, and emphasise its routine nature, the stigmatisation of corruption despite the absence of effective sanctions, its apparent irreversibility, the absence of correlation with regime types and its legitimacy to its perpetrators. Corruption is then shown to be socially embedded in ‘logics’ of negotiation, gift-giving, solidarity, predatory authority and redistributive accumulation. Any anti-corruption policy must face up to these realities.

A Lie-Backlund approach to equivalence and flatness of nonlinear systems

Abstract: A new system equivalence relation, using the framework of differential geometry of jets and prolongations of infinite order, is studied. In this setting, two systems are said to be equivalent if any variable of one system may be expressed as a function of the variables of the other system and of a finite number of their time derivatives. This is a Lie-Backlund isomorphism. The authors prove that, although the state dimension is not preserved, the number of input channels is kept fixed. They also prove that a Lie-Backlund isomorphism can be realized by an endogenous feedback. The differentially flat nonlinear systems introduced by the authors (1992) via differential algebraic techniques, are generalized and the new notion of orbitally flat systems is defined. They correspond to systems which are equivalent to a trivial one, with time preservation or not. The endogenous linearizing feedback is explicitly computed in the case of the VTOL aircraft to track given reference trajectories with stability.

Stochastic Analysis of the Fractional Brownian Motion

Abstract: Since the fractional Brownian motion is not a semi-martingale, the usual Ito calculus cannot be used to define a full stochastic calculus. However, in this work, we obtain the Ito formula, the Ito–Clark representation formula and the Girsanov theorem for the functionals of a fractional Brownian motion using the stochastic calculus of variations.

Making components contract aware

Abstract: Components have long promised to encapsulate data and programs into a box that operates predictably without requiring that users know the specifics of how it does so. Many advocates have predicted that components will bring about widespread software reuse, spawning a market for components usable with such mainstream software buses as the Common Object Request Broker Architecture (CORBA) and the Distributed Component Object Model (DCOM). In the Windows world, at least, this prediction is becoming a reality. Yet recent reports indicate mixed results when using and reusing components in mission-critical settings. Such results raise disturbing questions. How can you trust a component? What if the component behaves unexpectedly, either because it is faulty or simply because you misused it? Before we can trust a component in mission-critical applications, we must be able to determine, reliably and in advance, how it will behave. In this article the authors define a general model of sofware contracts and show how existing mechanisms could be used to turn traditional components into contract-aware ones.

A wavelet-based joint estimator of the parameters of long-range dependence

Abstract: A joint estimator is presented for the two parameters that define the long-range dependence phenomenon in the simplest case. The estimator is based on the coefficients of a discrete wavelet decomposition, improving a wavelet-based estimator of the scaling parameter (Abry and Veitch 1998), as well as extending it to include the associated power parameter. An important feature is its conceptual and practical simplicity, consisting essentially in measuring the slope and the intercept of a linear fit after a discrete wavelet transform is performed, a very fast (O(n)) operation. Under well-justified technical idealizations the estimator is shown to be unbiased and of minimum or close to minimum variance for the scale parameter, and asymptotically unbiased and efficient for the second parameter. Through theoretical arguments and numerical simulations it is shown that in practice, even for small data sets, the bias is very small and the variance close to optimal for both parameters. Closed-form expressions are given for the covariance matrix of the estimator as a function of data length, and are shown by simulation to be very accurate even when the technical idealizations are not satisfied. Comparisons are made against two maximum-likelihood estimators. In terms of robustness and computational cost the wavelet estimator is found to be clearly superior and statistically its performance is comparable. We apply the tool to the analysis of Ethernet teletraffic data, completing an earlier study on the scaling parameter alone.

Detection of older people at increased risk of adverse health outcomes after an emergency visit: the ISAR screening tool.

Abstract: OBJECTIVES: To develop a self-report screening tool to identify older people in the emergency department (ED) of a hospital at increased risk of adverse health outcomes, including: death, admission to a nursing home or long-term hospitalization, or a clinically significant decrease in functional status. DESIGN: Prospective (6-month) follow-up study of a cohort of ED patients aged 65 and older. SETTING: The EDs of four acute-care hospitals in Montreal, Quebec, Canada. PARTICIPANTS: Community-dwelling patients aged 65 and older who came to the EDs during the weekday shift over a 3-month recruitment period. Patients were excluded if they could not be interviewed either because of their medical condition or because of cognitive impairment and no other informant was available. MEASUREMENTS: Measures ascertained at the ED visit included: 27 self-report screening questions on social, physical, and mental risk factors; medical history; use of hospital services, medications, and alcohol; and the Older American Resources and Services (OARS) activities of daily living (ADL) scale. At follow-up, the OARS scale was readministered by telephone, and other adverse health outcomes were ascertained. RESULTS: Among 1673 patients who completed the follow-up measures, 488 (29.2%) had an adverse health outcome. Scale development and selection methods included logistic regression, receiver operating characteristic curves, and expert judgment. The proposed screening tool (ISAR) comprises six self-report questions on functional dependence (premorbid and acute change), recent hospitalization, impaired memory and vision, and polymedication. The tool performed well in the total cohort aged 65 and older, and in sub-groups defined by disposition (admitted or released from ED), language of questionnaire administration (French or English), information source (patient or other), and other characteristics. CONCLUSIONS: The ISAR is a short self-report questionnaire that can quickly identify older patients in the ED at increased risk of several adverse health outcomes and those with current disability.

Corporate environmental disclosure strategies: Determinants, costs and benefits

Abstract: In response to investors' and other stakeholders' concerns about corporate environmental policies, many firms are voluntarily increasing their level of environmental disclosure since there is a scarcity of alternative information sources. Using a cost-benefit framework, this study intends to identify determinants of corporate environmental reporting by Canadian firms subject to water pollution compliance regulations during the 1986–1993 period. Results suggest that information costs and a firm's financial condition are key determinants of environmental disclosure. Firm size, the regulatory regime governing corporate disclosure, and industry, also contribute to explaining environmental disclosure.

A time and frequency synchronization scheme for multiuser OFDM

Abstract: We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system.

Coherent Operation of a Tunable Quantum Phase Gate in Cavity QED

Abstract: We have realized a quantum phase gate operating on quantum bits carried by a single Rydberg atom and a zero- or one-photon field in a high- $Q$ cavity. The gate operation is based on the dephasing of the atom-field state produced by a full cycle of quantum Rabi oscillation. The dephasing angle, conditioned to the initial atom-field state, can be adjusted over a wide range by tuning the atom-cavity frequency difference. We demonstrate that the gate preserves qubit coherence and generates entanglement. This gate is an essential tool for the nondestructive measurement of single photons and for the manipulation of many-qubit entanglement in cavity QED.

Systems of Conservation Laws 1: Hyperbolicity, Entropies, Shock Waves

Abstract: Systems of conservation laws arise naturally in physics and chemistry. To understand them and their consequences (shock waves, finite velocity wave propagation) properly in mathematical terms requires, however, knowledge of a broad range of topics. This book sets up the foundations of the modern theory of conservation laws, describing the physical models and mathematical methods, leading to the Glimm scheme. Building on this the author then takes the reader to the current state of knowledge in the subject.The maximum principle is considered from the viewpoint of numerical schemes and also in terms of viscous approximation. Small waves are studied using geometrical optics methods. Finally, the initial-boundary problem is considered in depth.Throughout, the presentation is reasonably self-contained, with large numbers of exercises and full discussion of all the ideas. This will make it ideal as a text for graduate courses in the area of partial differential equations.

Different functions for the thyroid hormone receptors TRα and TRβ in the control of thyroid hormone production and post‐natal development

Abstract: The biological activities of thyroid hormones are thought to be mediated by receptors generated by the TRα and TRβ loci. The existence of several receptor isoforms suggests that different functions are mediated by specific isoforms and raises the possibility of functional redundancies. We have inactivated both TRα and TRβ genes by homologous recombination in the mouse and compared the phenotypes of wild‐type, and single and double mutant mice. We show by this method that the TRβ receptors are the most potent regulators of the production of thyroid stimulating hormone (TSH). However, in the absence of TRβ , the products of the TRα gene can fulfill this function as, in the absence of any receptors, TSH and thyroid hormone concentrations reach very high levels. We also show that TRβ, in contrast to TRα, is dispensable for the normal development of bone and intestine. In bone, the disruption of both TRα and TRβ genes does not modify the maturation delay observed in TRα−/− mice. In the ileum, the absence of any receptor results in a much more severe impairment than that observed in TRα−/− animals. We conclude that each of the two families of proteins mediate specific functions of triiodothyronin (T3), and that redundancy is only partial and concerns a limited number of functions.

Seeing a single photon without destroying it

Abstract: Light detection is usually a destructive process, in that detectors annihilate photons and convert them into electrical signals, making it impossible to see a single photon twice. But this limitation is not fundamental—quantum non-demolition strategies1,2,3 permit repeated measurements of physically observable quantities, yielding identical results. For example, quantum non-demolition measurements of light intensity have been demonstrated4,5,6,7,8,9,10,11,12,13,14, suggesting possibilities for detecting weak forces and gravitational waves3. But such experiments, based on nonlinear optics, are sensitive only to macroscopic photon fluxes. The non-destructive measurement of a single photon requires an extremely strong matter–radiation coupling; this can be realized in cavity quantum electrodynamics15, where the strength of the interaction between an atom and a photon can overwhelm all dissipative couplings to the environment. Here we report a cavity quantum electrodynamics experiment in which we detect a single photon non-destructively. We use atomic interferometry to measure the phase shift in an atomic wavefunction, caused by a cycle of photon absorption and emission. Our method amounts to a restricted quantum non-demolition measurement which can be applied only to states containing one or zero photons. It may lead to quantum logic gates16 based on cavity quantum electrodynamics, and multi-atom entanglement17.