Showing papers by "École Normale Supérieure published in 1990"

A New Optimization Algorithm for the Vehicle Routing Problem with Time Windows

Abstract: The vehicle routing problem with time windows VRPTW is a generalization of the vehicle routing problem where the service of a customer can begin within the time window defined by the earliest and the latest times when the customer will permit the start of service. In this paper, we present the development of a new optimization algorithm for its solution. The LP relaxation of the set partitioning formulation of the VRPTW is solved by column generation. Feasible columns are added as needed by solving a shortest path problem with time windows and capacity constraints using dynamic programming. The LP solution obtained generally provides an excellent lower bound that is used in a branch-and-bound algorithm to solve the integer set partitioning formulation. Our results indicate that this algorithm proved to be successful on a variety of practical sized benchmark VRPTW test problems. The algorithm was capable of optimally solving 100-customer problems. This problem size is six times larger than any reported to date by other published research.

Single-layer learning revisited: a stepwise procedure for building and training a neural network

Abstract: A stepwise procedure for building and training a neural network intended to perform classification tasks, based on single layer learning rules, is presented. This procedure breaks up the classification task into subtasks of increasing complexity in order to make learning easier. The network structure is not fixed in advance: it is subject to a growth process during learning. Therefore, after training, the architecture of the network is guaranteed to be well adapted for the classification problem.

Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases

Abstract: We report on calculations of intrasubband and intersubband phonon scattering in quantum-confined electron gases based on lattice-matched ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP quantum wells. Dimensionality effects on the emission of acoustic phonons are studied comparing the scattering times of two-, one-, and zero-dimensional electron gases as a function of the lateral confinement. Optical phonon scattering in quantum wells and wires is discussed using a phenomenological broadening of the one-dimensional density of states. The energy relaxation rates of heated electron gases due to phonon emission and absorption have been calculated for lattice temperatures ${\mathit{T}}_{\mathit{l}}$ between 0.3 and 20 K. For a given heating power per electron, the electron temperature ${\mathit{T}}_{\mathit{e}}$ in a quantum wire can be greater or smaller than that in the corresponding quantum well, depending on the electron density ${\mathit{n}}_{\mathit{s}}$, while the energy relaxation in quantum dots with significant quantization energies is always slower than in the corresponding wells and wires.

Formation of carbonate-apatite crystals after implantation of calcium phosphate ceramics

Abstract: The aims of this study were (1) to determine at the crystal level, the nonspecific biological fate of different types of calcium phosphate (Ca−P) ceramics after implantation in various sites (osseous and nonosseous) in animals and (2) to investigate the crystallographic association of newly formed apatitic crystals with the Ca−P ceramics. Noncommercial Ca−P ceramics identified by X-ray diffraction as calcium hydroxylapatite (HA), beta-tricalcium phosphate (β-TCP), and biphasic calcium phosphates (BCP) (consisting of β-TCP/HA=40/60) were implanted under the skin in connective tissue, in femoral lamellar cortical bone, articular spine bone, and cortical mandibular and mastoidal bones of animals (mice, rabbits, beagle dogs) for 3 weeks to 11 months. In humans, HA or β-TCP granules were used to fill periodontal pockets, and biposies of the implanted materials were recovered after 2 and 12 months. Results of this study demonstrated the following: (1) the presence of needle-like microcrystals (new crystals) associated with the Ca−P ceraiic macrocrystals in the microporous regions of the implants regardless of the sites of implantation (osseous or nonosseous), type of Ca−P ceramics (HA, β-TCP, BCP), type of species used (mice, rabbits, dogs, humans), or duration of implantation; (2) decrease in the area occupied by the ceramic crystals and the subsequent filling of the spaces between the ceramic crystals by the new crystals; (3) these new crystals were identified as apatite by electron diffraction and as carbonate-apatite by infrared absorption spectroscopy; (4) high resolution transmission electron microscopy (Hr TEM) revealed one family of apatite lattice fringes in the new crystals in continuity with the lattice planes of the HA of β-TCP ceramic crystals; (5) Hr TEM also demonstrated the presence of linear dislocations at the junction of the new apatite crystals and ceramic crystals. It is suggested that the formation of the CO3 apatite crystals associated with the implanted Ca−P ceramic is due to dissolution/precipitation and secondary nucleation involving an epitatic growing process and not to an osteogenic property of the ceramic.

Quantum nondemolition measurement of small photon numbers by Rydberg-atom phase-sensitive detection.

Abstract: We describe a new quantum nondemolition method to monitor the number N of photons in a microwave cavity. We propose coupling the field to a quasiresonant beam of Rydberg atoms and measuring the resulting phase shift of the atom wave function by the Ramsey separated-oscillatory-fields technique. The detection of a sequence of atoms reduces the field into a Fock state. With realistic Rydberg atom-cavity systems, small-photon-number states down to N=0 could be prepared and continuously monitored.

Manufacturing flow control and preventing maintenance: a stochastic control approach

Abstract: A stochastic control model for simultaneously planning production and maintenance in a flexible manufacturing system (FMS) is proposed, and an efficient technique for computing the optimal control policy is developed. The model extends previous formulations by including an age-dependent machine failure rate and by allowing the control to influence some jump rates (namely the preventive maintenance activities). By using an adaptation to the case of piecewise-deterministic systems of the approximation technique initially proposed by H.J. Kushner (1977) in the realm of the optimal control of diffusions, one shows how it is possible to computer the optimal control for a two-machine system. >

Estimate of organic matter turnover rate in a savanna soil by 13C natural abundance measurements.

Abstract: The medium-term dynamics of organic matter in a sandy savanna soil has been investigated at Lamto (Ivory Coast) through changes in the 13C/12C ratio induced by vegetation changes from the C4 to the C3 photosynthetic pathway. After 25 yr of protection from fire, a soil previously covered by C4 grassland vegetation had been progressively colonized by C3 woody plants. Although the total C content did not show significant changes. 52–70% of the original C4 carbon was turned over when vegetation cover was changed. Turnover of coarse organic debris (> 250 μm) was much greater (97%) than the mineralization (50%) of clay-associated fractions (< 20 μm).

Construction of solutions with exactly $k$ blow-up points for the Schrödinger equation with critical nonlinearity

Abstract: We consider the nonlinear Schrodinger equation: (1) $${{i\partial u} \mathord{\left/ {\vphantom {{i\partial u} {\partial t}}} \right. \kern- ulldelimiterspace} {\partial t}} = - \Delta u - \left| u \right|^{{4 \mathord{\left/ {\vphantom {4 N}} \right. \kern- ulldelimiterspace} N}} uandu\left( {0,.} \right) = \varphi \left( . \right),$$ whereu:[0,T)×ℝ N →ℂ. For any given pointsx 1,x 2,...,x k in ℝ N , we construct a solution of Eq. (1),u(t), which blows up in a finite timeT at exactlyx 1,x 2,...,x k . In addition, we describe the precise behavior of the solutionu(t) whent→T, at the blow-up points {x 1,x 2,...,x k } and in ℝ N −{x 1,x 2,...,x k }.

Short chain flavour esters synthesis by microbial lipases

Abstract: The peparative synthesis of 35 short chain flavour esters by lipases fromMucor miehi, Aspergillus sp.,Candida rugosa andRhizopus arrhizus was investigated in organic media. Acetic, propionic, butyric, valeric and caproic acids, as well as methanol, ethanol, butanol, i-pentanol, hexanol, citronellol and geraniol were used as substrates. Most of the esters were synthesized in good yield by at least one of the lipase preparations tested. Different conversion yields were observed according to the lipase specificity toward the acid or the alcohol moiety of the ester. Methyl- and ethyl acetates were also produced by changing the organic solvent. Enzymatic catalysis in organic solvent is thought to be a valuable method for preparative synthesis of flavour esters.

Quantum Limits in Interferometric Measurements

Abstract: Quantum noise limits the sensitivity of interferometric measurements. It is generally admitted that it leads to an ultimate sensitivity, the "standard quantum limit". Using a semi-classical analysis of quantum noise, we show that a judicious use of squeezed states allows one in principle to push the sensitivity beyond this limit. This general method could be applied to large-scale interferometers for gravitational wave detection.

Laser Cooling of Cesium Atoms Below 3 μK

Abstract: We have measured the temperature of cesium atoms released from optical molasses. For a wide range of laser intensity and detuning from resonance, the temperature depends only on the intensity-to-detuning ratio. The lowest temperature achieved is (2.5 ± 0.6) μK, which corresponds to an r.m.s. velocity of 12.5 mm/s or 3.6 times the single-photon recoil velocity. This is, to our knowledge, the coldest kinetic temperature ever measured for three-dimensional (3D) cooling.

Sol-gel condensation of rapidly hydrolyzed silicon alkoxides: A joint 29Si NMR and small-angle x-ray scattering study.

Abstract: We have studied by $^{29}\mathrm{Si}$ NMR the complete condensation kinetics in the conditions of rapid hydrolysis (acidic medium, water in excess) of three silicon alkoxides. The gelation of the tetravalent tetraethoxysilane (TEOS) takes several weeks, whereas the trivalent methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS) do not form gels. From a quantitative analysis of the data, we deduce that the first steps of the condensation proceed by progressive assembling of small organized units. This accounts for the very slow kinetics (logarithmic function of time), the occurrence of highly condensed agglomerates, and the absence of gelation in trivalent systems. For the tetravalent TEOS, this is followed by an aggregation phase, which has been studied both by NMR and small-angle x-ray scattering. The fractal dimension D=1.9 and the growing kinetics (cluster size increasing as a linear function of time) are consistent with reaction-limited cluster-cluster aggregation with preferential reactivity at the external cluster sites. Finally, we suggest that the progressive transformation of the sol phase into the gel phase after the gel time can be observed by comparing static and magic-angle-spinning NMR spectra.

Ductile extension in alpine Corsica

Abstract: Ductile deformation in high-pressure ( P )-low temperature ( T ) conditions due to the westward thrusting of oceanic material onto a continental basement in alpine Corsica is overprinted by a late deformation event with a reverse shear sense (eastward) that took place in less severe P-T conditions. We show that the late deformation can be linked to extension during rifting and spreading of the Liguro Provencal basin from late Oligocene to late-middle Miocene time. Major compressive thrust contacts were reactivated as ductile normal faults and, in some units, only a penetrative eastward shear can be observed. This extension following the thickening of the crust brought tectonic units which underwent very different P- T conditions during the earlier stage into close contact. The Balagne nappe, which shows neither significant ductile deformation nor metamorphism, directly overlies the high- P units. The extensional deformation is distributed through the entire thickness of the nappe stack but is more important along the major thrust contacts, which localize the strain. The geometry of the crustal extension is controlled by that of the early compressive thrusts. The latest structures are east-dipping brittle normal faults which bound the early to middle Miocene Saint Florent half graben.

Optical molasses

Abstract: Summary form only given, as follows. A three-dimensional configuration of counterpropagating laser beams tuned slightly below an atomic resonance frequency can provide strong 'viscous' damping of the atomic kinetic motion through the action of then photon momentum. This 'optical molasses', first observed in experiments at Bell Labs, not only cools the atoms to microkelvin temperatures, but also provides confinement and concentration as well. Experiments at NIST in Gaithersburg showed that the temperatures reached in optical molasses were much lower than believed possible on the basis of simple and widely accepted theoretical models. Groups at Stanford University and at Ecole Normale Superieure in Paris both confirmed these experiments and offered novel theoretical models to explain the low temperatures. These models have been qualitatively verified in further experiments at all three laboratories. Experiments with cesium atoms in optical molasses at the Ecole Normale laboratory have found temperatures as low as 5 mu K. It corresponds to an RMS velocity of less than 2 cm/s. Such slow atoms may have important applications in a variety of measurements, including atomic frequency standards where atomic motion is the major factor limiting performance. >

A comparative analysis of regional correlation, dynamic time warping, and skeletal tree matching for signature verification

Abstract: A report is presented on a comparative study of three different signal matching algorithms in the context of signature verification: regional correlation, dynamic time warping, and skeletal tree matching. The algorithm performances are compared in a single experimental protocol over the same database. Algorithm performance is analyzed in terms of verification error rates, execution time, and number and sensitivity of algorithm parameters. Three different script types (normal signatures, handwritten passwords, and initials) and three different signal representation spaces (position, velocity, and acceleration) are considered. Verification errors show that no algorithm consistently outperforms the others in all circumstances. >