Showing papers by "Instituto Superior Técnico published in 1997"
TL;DR: In this article, the authors presented the analytic next-to-next-to leading perturbative QCD corrections in the leading twist approximation for the moments N = 2, 4, 6, 8 of the flavour singlet deep inelastic structure functions F2 and FL.
226 citations
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TL;DR: In this paper, a 1-parameter family of extremal metrics of non-constant scalar curvature is investigated using Guillemin's construction, and the Euler-Lagrange condition for such metrics being extremal is derived.
Abstract: Recently Guillemin gave an explicit combinatorial way of constructing "toric" Kahler metrics on (symplectic) toric varieties, using only data on the moment polytope. In this paper, differential geometric properties of these metrics are investigated using Guillemin's construction. In particular, a nice combinatorial formula for the scalar curvature is given, and the Euler-Lagrange condition for such "toric" metrics being extremal (in the sense of Calabi) is derived. A construction, due to Calabi, of a 1-parameter family of extremal metrics of non-constant scalar curvature is recast very simply and explicitly. Finally, a curious combinatorial formula for convex polytopes, that follows from the relation between the total integral of the scalar curvature and the wedge product of the first Chern class with a suitable power of the Kahler class, is presented.
193 citations
TL;DR: In this article, the representation of quark distribution and fragmentation functions in terms of non-local operators is combined with a simple spectator model, which allows us to estimate these functions for the nucleon and the pion ensuring correct crossing and support properties.
192 citations
TL;DR: In this paper, the effects of spontaneous CPT violation in a string-based scenario were examined and it was shown that certain CPT-violating terms can produce a large baryon asymmetry at the grand-unified scale that reduces to the observed value via sphaleron or other dilution mechanisms.
176 citations
TL;DR: The developments that make MPEG-4 necessary, what the standard does to address the new needs, and an overview of how the standard is developed are described.
Abstract: The MPEG-4 audiovisual representation standard currently under development addresses the needs and requirements, arising from the increasing availability of audiovisual content in digital form. It goes beyond digitizing linear audio and video, specifying a description of digital material in the form of ‘objects’, that can be flexibly and interactively used and re-used. This paper describes the developments that make MPEG-4 necessary (‘Why’), what MPEG-4 does to address the new needs (‘What’), and gives an overview of how the standard is developed (‘How’).
171 citations
TL;DR: The results indicate that the vacuole also plays a role in homeostasis of the intracellular pH, and the decrease in cytosolic pH caused by external acidification was much more important when the permeant acetic acid was used instead of HCl as the acidulant.
161 citations
TL;DR: The authors reported the crystal structures of porcine seminal plasma PSP-I/PSP-II, a heterodimer of two glycosylated spermadhesins, and bovine aSFP at 2.4 A and 1.9 A resolution respectively.
Abstract: Spermadhesins, 12,000-14,000 M(r) mammalian proteins, include lectins involved in sperm-egg binding and display a single CUB domain architecture. We report the crystal structures of porcine seminal plasma PSP-I/PSP-II, a heterodimer of two glycosylated spermadhesins, and bovine aSFP at 2.4 A and 1.9 A resolution respectively.
141 citations
TL;DR: In this article, a variable-cell-shape molecular dynamics algorithm is proposed where the dynamical variables associated with the cell are the six independent dot products between the vectors defining the cell instead of the nine Cartesian components of those vectors.
Abstract: We propose a variable-cell-shape molecular dynamics algorithm where the dynamical variables associated with the cell are the six independent dot products between the vectors defining the cell instead of the nine Cartesian components of those vectors. Our choice of the metric tensor as the dynamical variable automatically eliminates the cell orientation from the dynamics. Furthermore, choosing for the cell kinetic energy a simple scalar that is quadratic in the time derivatives of the metric tensor makes the dynamics invariant with respect to the choice of the simulation cell edges. Choosing the tensorial density of that scalar allows us to have a dynamics that obeys the virial theorem. We derive the equations of motion for the two conditions of constant external pressure and constant thermodynamic tension. We also show that using the metric as a variable is convenient for structural optimization under those two conditions. We use simulations for Ar with Lennard-Jones parameters and for Si with forces and stresses calculated from first principles of density-functional theory to illustrate the applications of the method. {copyright} {ital 1997} {ital The American Physical Society}
137 citations
07 Jul 1997
TL;DR: A complete coverage path planning and guidance methodology for a mobile robot, having the automatic floor cleaning of large industrial areas as a target application, and the capability of the path planner to deal with a priori mapped or unexpected obstacles in the middle of the working space.
Abstract: This paper describes a complete coverage path planning and guidance methodology for a mobile robot, having the automatic floor cleaning of large industrial areas as a target application. The proposed algorithms rely on the a priori knowledge of a 2D map of the environment and cope with unexpected obstacles not represented on the map. A template based approach is used to control the path execution, thus incorporating, in a natural way, the kinematic and the geometric model of the mobile robot on the path planning procedure. The novelty of the proposed approach is the capability of the path planner to deal with a priori mapped or unexpected obstacles in the middle of the working space. If unmapped obstacles permanently block the planned trajectory, the path tracking control avoids these obstacles. The paper presents experimental results with a LABMATE mobile robot, confirming the feasibility of the total coverage path and the robustness of the path tracking behaviour based control.
130 citations
22 Jun 1997
TL;DR: In this article, the authors presented the control circuit of a shunt active power filter based on the instantaneous active and reactive current i/sub d/-i/sub q/ method.
Abstract: This paper presents the control circuit of a shunt active power filter based on the instantaneous active and reactive current i/sub d/-i/sub q/ method The presented method is compared with the instantaneous active and reactive power p-q method under several mains voltage conditions and for different harmonic injection highpass filters The i/sub d/-i/sub q/ method as an excellent harmonic compensation performance and presents the advantage of being frequency-independent The DC voltage regulation system is analysed and its synthesis is performed Extensive simulation results are used to show the stability of the voltage regulation system and to establish the superior performance of i/sub d/-i/sub q/ method based compensator
129 citations
TL;DR: In this article, diffuse reflectance infrared spectroscopy (DRIFT) was used to characterize the structures of different celluloses (microcrystalline and native) by analyzing the bands due to O−H stretching, C−O stretching, and C−H bending modes.
Abstract: The structures of different celluloses (microcrystalline and native) were characterized by diffuse reflectance infrared spectroscopy (DRIFT), analyzing the bands due to O−H stretching, C−O stretching, and C−H bending modes. The respective crystallinities were determined as 73 ± 2% and 40 ± 2%, and the crystalline structure in both was characterized as type Iβ. A comparative study of adsorption of an aromatic ketone, benzophenone, on these two substrates was made, using the same technique. Through the modifications observed in the carbonyl stretching band of benzophenone, it was possible to distinguish adsorption in three different environments: entrapped between the glycosidic chains in crystalline domains, in amorphous domains, and as crystallites adsorbed at the cellulose surface. By deconvolution of spectra and subsequent analysis of the main components' relative intensities, it has been possible to calculate the approximate amounts of entrapped molecules per gram of cellulose as a function of total b...
TL;DR: In this paper, the surface tension of a solid from contact angle measurements is discussed and compared, based on the contact angles of water, glycerol, and diiodomethane measured at 20°C on the surface of a sidechain liquid crystalline polymer.
TL;DR: Wettability studies suggest that this surface film is heterogeneous and porous, similar to the thicker films formed in albumin-free HBSS, while XPS analysis strongly indicates that the inhibition of apatite formation by BSA is only partial.
TL;DR: It is shown that, if the multifractal spectra for dimensions of two maps coincide, then the maps are differentiably equivalent, and this helps to obtain a "physical" classification of dynamical systems.
Abstract: We introduce the mathematical concept of multifractality and describe various multifractal spectra for dynamical systems, including spectra for dimensions and spectra for entropies. We support the study by providing some physical motivation and describing several nontrivial examples. Among them are subshifts of finite type and one-dimensional Markov maps. An essential part of the article is devoted to the concept of multifractal rigidity. In particular, we use the multifractal spectra to obtain a “physical’’ classification of dynamical systems. For a class of Markov maps, we show that, if the multifractal spectra for dimensions of two maps coincide, then the maps are differentiably equivalent.
TL;DR: In this paper, loss estimation techniques are used to support decision-making on upgrading strategies of historic city centres, suitable for application to masonry buildings, such as office buildings and railway stations.
Abstract: To support decision-making on upgrading strategies of historic city centres, loss estimation techniques are needed, suitable for application to masonry buildings. This paper describes the developme...
TL;DR: In this paper, the results indicated the existence of diverse copper species in CuMFI catalysts, such as isolated CU2+, Cu+ ions and CuO species, whose concentrations depend on the catalyst Si/Al ratio and copper loading.
Abstract: Copper MFI zeolites with different Si/Al ratios and different copper loadings, prepared by ion exchange, were characterised by XRD, H2-TPR and NO TPD. The results indicated the existence of diverse copper species in CuMFI catalysts, such as isolated CU2+, Cu+ ions and CuO species, whose concentrations depends on the catalyst Si/Al ratio and copper loading. In underexchanged catalysts (catalysts with low Si/Al ratio or copper loading) in the copper mainly present as isolated Cu2+ ions, while in overexchanged catalysts (catalysts with high Si/Al ratio or copper loading) CuO species, Cu2+ and Cu+ ions were detected. CuMFI with high Si/Al ratio or high copper loading are more easily reduced than CuMFI with low Si/Al ratio or low copper loading. An easier reducibility of the copper species has an effect on the operating temperature window of the catalysts by decreasing the temperature of the maximum NO conversion. CuMFI zeolites with a high concentration of isolated Cu2+ ions exhibit the highest NO adsorption capacity. The results make evident that the isolated Cu2+ ions are the most active species for NO SCR by propene.
TL;DR: Fusarium solani pisi recombinant cutinase, solubilized in bis(2-ethylhexyl) sodium sulfosuccinate (AOT)/isooctane-reversed micelles, was used to catalyze the transesterification reaction of butyl acetate with hexanol as discussed by the authors.
TL;DR: Comparison to NMR-spectra of lysozyme with a bound inhibitor shows that the interaction with sorbitol affects spatially disparate regions of the protein, and most significant changes are manifest in the anomalous relaxation properties of Ala and Thr methyl groups.
TL;DR: Most of the hydrophobic core residue characteristics are conserved in the jellyroll topology of certain icosahedral virus capsid proteins, indicating that the CUB domain and the viral proteins share a minimal structural core.
TL;DR: The robot motion is directly driven by the first-order time-space image derivatives, which can be estimated robustly and fast and is operating in real time and the performance is robust both in theego-docking andeco-d dockingparadigms.
TL;DR: In this article, the up-shifts of ultrashort laser pulses propagating in opposite directions with respect to a relativistic ionization front (interface gas plasma) were measured for the first time.
Abstract: The frequency up-shifts of ultrashort laser pulses (65 fs) propagating in opposite directions (20° and 160°) with respect to a relativistic ionization front (interface gas plasma) are measured for the first time. Up-shifts of the order of 25 nm are observed. A very good agreement is found with a two-dimensional ray-tracing theory.
TL;DR: In this paper, a series of impact and compression after impact tests were carried out on composite laminates made of carbon fiber reinforced epoxy resin matrix, and four stacking sequences representative of four different elastic behaviours were used.
TL;DR: In this paper, the results showed that copper is mainly in the form of isolated Cu 2+ ions in CuMOR catalysts with copper exchange ≤ 20%, whereas at higher copper exchange CuO species are also present.
Abstract: NO TPD, H 2 -TPR and XRD have been used to characterise copper-exchanged mordenites with different Si/Al ratios, copper contents and cocations. The results showed that copper is mainly in the form of isolated Cu 2+ ions in CuMOR catalysts with copper exchange ≤20%, whereas at higher copper exchange CuO species are also present. These results were obtained with H and Na as cocation and were achieved by changing either the catalyst Si/Al ratio or the copper content. The data also indicate that the cocation mainly affects copper location and that copper is more easily reduced in sodium form catalysts than in protonic form. It was found that the isolated Cu 2+ ions are the most effective species for NO adsorption and the most active species for NO SCR.
TL;DR: The existence of thermally activated delayed fluorescence in C is demonstrated by the study of the temperature 60... dependence 291-353 K of the fluorescence intensity of C degassed solutions as discussed by the authors.
TL;DR: In this paper, a new concept of contact angle measurements by the immersed method is applied to membrane surface characterisation, where a droplet of CCl4 is deposited on the membrane surface immersed in an aqueous solution.
TL;DR: In this paper, a self-contained formulation was developed to solve the steady-state spatially inhomogeneous electron Boltzmann equation (EBE) in a plasma positive column, taking into account the spatial gradient and the space charge field terms.
Abstract: In this paper we develop a self-contained formulation to solve the steady-state spatially inhomogeneous electron Boltzmann equation (EBE) in a plasma positive column, taking into account the spatial gradient and the space-charge field terms. The problem is solved in cylindrical geometry using the classical two-term approximation, with appropriate boundary conditions for the electron velocity distribution function, especially at the tube wall. A condition for the microscopic radial flux of electrons at the wall is deduced, and a detailed analysis of some limiting situations is carried out. The present formulation is self-contained in the sense that the electron particle balance equation is exactly satisfied, that is, the ionization rate exactly compensates for the electron loss rate to the wall. This condition yields a relationship between the applied maintaining field and the gas pressure, termed the discharge characteristic, which is obtained as an eigenvalue solution to the problem. By solving the EBE we directly obtain the isotropic and the anisotropic components of the electron distribution function (EDF), from which we deduce the radial distributions of all relevant macroscopic quantities: electron density, electron transport parameters and rate coefficients for excitation and ionization, and electron power transfer. The results show that the values of these quantities across the discharge are lower than those calculated for a homogeneous situation, due to the loss of electrons to the wall. The solutions for the EDF reveal that, for sufficiently low maintaining fields, the radial anisotropy at some radial positions can be negative, that is, directed toward the discharge axis, for energies above a collisional barrier around the inelastic thresholds. However, at the wall, the radial anisotropy always points to the wall, due to the strong electron drain occuring in this region. We further present pertinent comparisons with other formulations recently proposed in the literature to model the present inhomogeneous problem.
TL;DR: In this paper, the application of the general friction model, developed by Wanheim and Bay, involves a major improvement in the ability to simulate processes where low tool-workpiece interface stresses may prevail.
TL;DR: Improved efficiency in the integration process of the equations of motion is obtained using the augmented Lagrange formulation and the biomechanical model is finally applied indifferent situations of passive human motion such as in vehicle occupants during a crash or in an athlete during impact.
Abstract: Based on a general methodology using naturalco-ordinates, a three-dimensional whole body responsemodel for the articulated human body is presented inthis paper. The joints between biomechanical segmentsare defined by forcing adjacent bodies to share commonpoints and vectors that are used in their definition.A realistic relative range of motion for the bodysegments is obtained introducing a set of penaltyforces in the model rather than setting up newunilateral constraints between the system components.These forces, representing the reaction momentsbetween segments of the human body model when thebiomechanical joints reach the limit of their range ofmotion, prevent the biomechanical model from achievingphysically unacceptable positions. Improved efficiencyin the integration process of the equations of motionis obtained using the augmented Lagrange formulation.The biomechanical model is finally applied indifferent situations of passive human motion such asthat observed in vehicle occupants during a crash orin an athlete during impact.
TL;DR: A general formulation for the calculation of the first order analytical designs sensitivities based on the direct differentiation method is presented and the accuracy and characteristics of the sensitivities are analyzed and discussed.
Abstract: An important step in the application of automated design techniques to rigid-flexible multibody systems is the calculation of the sensitivities with respect to design variables. Thispaper presents a general formulation for thecalculation of the first order analytical designsensitivities based on the direct differentiationmethod. The analytical sensitivities are comparedwith the numerical results obtained by the finitedifferences method and the accuracy and validity ofboth methods is discussed. Cartesian co-ordinates areused for the dynamic analysis of rigid-flexiblemultibody systems. To reduce the number ofco-ordinates associated with the flexible bodies, thecomponent mode synthesis method is used. Theequations of the sensitivities are obtainedsymbolically and integrated in time simultaneouslywith the dynamic equations. Examples of 2Dsensitivity analysis of the transient response of aslider-crank and of a vehicle with a flexible chassisare presented, and the accuracy and characteristics ofthe sensitivities are analyzed and discussed.
TL;DR: It is shown that 4-hydroxytamoxifen quinone methide reacts with DNA to form covalent adducts, which are characterized as (E)- and (Z)-alpha-(deoxyguanosin-N2-yl)-4-hydroxifen.
Abstract: Tamoxifen is a liver carcinogen in rats and has been shown to increase the risk of endometrial cancer in women. Recent reports of DNA adducts in leucocyte and endometrial samples from women treated with tamoxifen indicate that it may be genotoxic to humans. One of the proposed pathways for the metabolic activation of tamoxifen involves oxidation to 4-hydroxytamoxifen, which may be further oxidized to an electrophilic quinone methide. In the present study we show that 4-hydroxytamoxifen quinone methide reacts with DNA to form covalent adducts. The major products, which result from 1,8-addition of the exocyclic nitrogen of deoxyguanosine to the conjugated system of 4-hydroxytamoxifen quinone methide, are characterized as (E)- and (Z)-alpha-(deoxyguanosin-N2-yl)-4-hydroxytamoxifen.