Showing papers by "Instituto Superior Técnico published in 2001"

Image classification for content-based indexing

Abstract: Grouping images into (semantically) meaningful categories using low-level visual features is a challenging and important problem in content-based image retrieval. Using binary Bayesian classifiers, we attempt to capture high-level concepts from low-level image features under the constraint that the test image does belong to one of the classes. Specifically, we consider the hierarchical classification of vacation images; at the highest level, images are classified as indoor or outdoor; outdoor images are further classified as city or landscape; finally, a subset of landscape images is classified into sunset, forest, and mountain classes. We demonstrate that a small vector quantizer (whose optimal size is selected using a modified MDL criterion) can be used to model the class-conditional densities of the features, required by the Bayesian methodology. The classifiers have been designed and evaluated on a database of 6931 vacation photographs. Our system achieved a classification accuracy of 90.5% for indoor/outdoor, 95.3% for city/landscape, 96.6% for sunset/forest and mountain, and 96% for forest/mountain classification problems. We further develop a learning method to incrementally train the classifiers as additional data become available. We also show preliminary results for feature reduction using clustering techniques. Our goal is to combine multiple two-class classifiers into a single hierarchical classifier.

Cosmology from MAXIMA-1, BOOMERANG, and COBE DMR cosmic microwave background observations.

Abstract: Recent results from BOOMERANG-98 and MAXIMA-1, taken together with COBE DMR, provide consistent and high signal-to-noise measurements of the cosmic microwave background power spectrum at spherical harmonic multipole bands over 2

Scalar, electromagnetic and Weyl perturbations of BTZ black holes: Quasinormal modes

Abstract: We calculate the quasinormal modes and associated frequencies of the Ba\~nados-Teitelboim-Zanelli (BTZ) nonrotating black hole. This black hole lives in 2+1 dimensions in an asymptotically anti--de Sitter spacetime. We obtain exact results for the wave function and quasinormal frequencies of scalar, electromagnetic and Weyl (neutrino) perturbations.

Integrability of Lie brackets

Abstract: In this paper we present the solution to a longstanding problem of differential geometry: Lie's third theorem for Lie algebroids. We show that the integrability problem is controlled by two computable obstructions. As applications we derive, explain and improve the known integrability results, we establish integrability by local Lie groupoids, we clarify the smoothness of the Poisson sigma-model for Poisson manifolds, and we describe other geometrical applications.

Quasinormal modes of Schwarzschild anti-de Sitter black holes: Electromagnetic and gravitational perturbations

Abstract: We study the quasinormal modes (QNM) of electromagnetic and gravitational perturbations of a Schwarzschild black hole in an asymptotically anti--de Sitter (AdS) spacetime. Some of the electromagnetic modes do not oscillate; they only decay, since they have pure imaginary frequencies. The gravitational modes show peculiar features: the odd and even gravitational perturbations no longer have the same characteristic quasinormal frequencies. There is a special mode for odd perturbations whose behavior differs completely from the usual one in scalar and electromagnetic perturbations in AdS spacetime, but has a similar behavior to the Schwarzschild black hole in an asymptotically flat spacetime: the imaginary part of the frequency goes as ${1/r}_{+},$ where ${r}_{+}$ is the horizon radius. We also investigate the small black-hole limit showing that the imaginary part of the frequency goes as ${r}_{+}^{2}.$ These results are important to the AdS/CFT conjecture since, according to it, the QNM's describe the approach to equilibrium in the conformal field theory.

A High Spatial Resolution Analysis of the MAXIMA-1 Cosmic Microwave Background Anisotropy Data

Abstract: We extend the analysis of the MAXIMA-1 cosmic microwave background data to smaller angular scales. MAXIMA, a bolometric balloon-borne experiment, mapped a 124 deg2 region of the sky with 10' resolution at frequencies of 150, 240, and 410 GHz during its first flight. The original analysis, which covered the multipole range 36 ≤ l ≤ 785 using a 100 deg2 map, is extended to l = 1235 using a subset of the data from three 150 GHz photometers in the fully cross-linked central 60 deg2 of the map. The main improvement over the original analysis is the use of 3' square pixels in the calculation of the map. The new analysis is consistent with the original for l 785, where inflationary models predict a third acoustic peak, the new analysis shows power with an amplitude of 56 ± 7 μK at l 850 in excess to the average power of 42 ± 3 μK in the range 441 < l < 785.

A High Spatial Resolution Analysis of the MAXIMA-1 Cosmic Microwave Background Anisotropy Data

Abstract: We extend the analysis of the MAXIMA-1 cosmic microwave background (CMB) data to smaller angular scales. MAXIMA, a bolometric balloon experiment, mapped a 124 deg$^2$ region of the sky with 10\arcmin resolution at frequencies of 150, 240 and 410 GHz during its first flight. The original analysis, which covered the multipole range $36 \leq \ell \leq 785$, is extended to $\ell = 1235$ using data from three 150 GHz photometers in the fully cross-linked central 60 deg$^2$ of the map. The main improvement over the original analysis is the use of 3\arcmin square pixels in the calculation of the map. The new analysis is consistent with the original for $\ell 785$, where inflationary models predict a third acoustic peak, the new analysis shows power with an amplitude of $56 \pm 7$ \microk at $\ell \simeq 850$ in excess to the average power of $42 \pm 3$ \microk in the range $441 < \ell < 785$.

Transverse momentum dependence in gluon distribution and fragmentation functions

Abstract: We investigate the twist two gluon distribution functions for spin-1/2 hadrons, emphasizing the intrinsic transverse momentum of the gluons. These functions are relevant in leading order in the inverse hard scale in scattering processes such as inclusive leptoproduction or Drell-Yan scattering, or more generally in hard processes in which at least two hadrons are involved. They show up in azimuthal asymmetries. For future estimates of such observables, we discuss specific bounds on these functions. ©2001 The American Physical Society.

Direct Sequential Simulation and Cosimulation

Abstract: Sequential simulation of a continuous variable usually requires its transformation into a binary or a Gaussian variable, giving rise to the classical algorithms of sequential indicator simulation or sequential Gaussian simulation. Journel (1994) showed that the sequential simulation of a continuous variable, without any prior transformation, succeeded in reproducing the covariance model, provided that the simulated values are drawn from local distributions centered at the simple kriging estimates with a variance corresponding to the simple kriging estimation variance. Unfortunately, it does not reproduce the histogram of the original variable, which is one of the basic requirements of any simulation method. This has been the most serious limitation to the practical application of the direct simulation approach. In this paper, a new approach for the direct sequential simulation is proposed. The idea is to use the local sk estimates of the mean and variance, not to define the local cdf but to sample from the global cdf. Simulated values of original variable are drawn from intervals of the global cdf, which are calculated with the local estimates of the mean and variance. One of the main advantages of the direct sequential simulation method is that it allows joint simulation of N v variables without any transformation. A set of examples of direct simulation and cosimulation are presented.

The defect structure of sol–gel-derived silica/polytetrahydrofuran hybrid films by FTIR

Abstract: The structural perturbations induced by polytetrahydrofuran (PTHF) on sol–gel hybrid films were identified by Fourier-transform infrared (FTIR) spectroscopy. The films were prepared by spin-coating, from aged solutions containing tetraethylorthosilicate (TEOS) as the inorganic precursor and different concentrations of PTHF. All the spectra reveal a set of bands that may be associated with structural defects of the silica network. The hybrid films show an additional `defect' band at 560 cm −1 , assigned to a skeletal vibration of 4-fold siloxane rings, whose intensity grows as the polymer content or molecular weight increases. The relative intensity of two components of the νasSi–O–Si mode (resolved by deconvolution of the band at ∼ 1080 cm −1 ) grow accordingly. They were thus assigned to the longitudinal (LO) and transverse optical (TO) modes of that vibration in 4-fold siloxane rings. Simultaneously, the bands assigned to the νSi–O− mode and to the νSi–O(H) mode of unreacted silanol groups (obtained by deconvolution of the band at ∼ 950 cm −1 ) increase. These conjugated observations lead to the conclusion that the polymer hinders the condensation reactions, being responsible for a more porous structure, with retention of a larger proportion of 4-fold siloxane rings. For high concentrations of high molecular weight PTHF, the defect structure of the films indicates that a partial segregation of the polymer occurs.

Cosmological Implications of the MAXIMA-1 High-Resolution Cosmic Microwave Background Anisotropy Measurement

Abstract: We discuss the cosmological implications of the new constraints on the power spectrum of the cosmic microwave background (CMB) anisotropy derived from a new high-resolution analysis of the MAXIMA-1 measurement. The power spectrum indicates excess power at l ~ 860 over the average level of power at 411 ? l ? 785. This excess is statistically significant at the ~95% confidence level. Its position coincides with that of the third acoustic peak, as predicted by generic inflationary models selected to fit the first acoustic peak as observed in the data. The height of the excess power agrees with the predictions of a family of inflationary models with cosmological parameters that are fixed to fit the CMB data previously provided by BOOMERANG-LDB and MAXIMA-1 experiments. Our results therefore lend support for inflationary models and more generally for the dominance of adiabatic coherent perturbations in the structure formation of the universe. At the same time, they seem to disfavor a large variety of the nonstandard (but inflation-based) models that have been proposed to improve the quality of fits to the CMB data and the consistency with other cosmological observables. Within standard inflationary models, our results combined with the COBE/Differential Microwave Radiometer data give best-fit values and 95% confidence limits for the baryon density, ?bh2 simeq 0.033

A census of carbohydrate-active enzymes in the genome of Arabidopsis thaliana.

Abstract: The synthesis, modification, and breakdown of carbohydrates is one of the most fundamentally important reactions in nature. The structural and functional diversity of glycosides is mirrored by a vast array of enzymes involved in their synthesis (glycosyltransferases), modification (carbohydrate esterases) and breakdown (glycoside hydrolases and polysaccharide lyases). The importance of these processes is reflected in the dedication of 1-2% of an organism's genes to glycoside hydrolases and glycosyltransferases alone. In plants, these processes are of particular importance for cell-wall synthesis and expansion. starch metabolism, defence against pathogens, symbiosis and signalling. Here we present an analysis of over 730 open reading frames representing the two main classes of carbohydrate-active enzymes, glycoside hydrolases and glycosyltransferases, in the genome of Arabidopsis thaliana. The vast importance of these enzymes in cell-wall formation and degradation is revealed along with the unexpected dominance of pectin degradation in Arabidopsis, with at least 170 open-reading frames dedicated solely to this task.

A methodology for ontology integration

Abstract: Although ontology reuse is an important research issue only one of its subprocesses (merge) is fairly well understood. The time has come to change the current state of affairs with the other reuse subprocess: integration. In this paper we describe the activities that compose this process and describe a methodology to perform the ontology integration process..

Combined trajectory tracking and path following: an application to the coordinated control of autonomous marine craft

Abstract: The paper presents a solution to the problem of combined trajectory tracking and path following system design for autonomous marine craft. This problem is motivated by the practical need to develop control systems for marine craft that can yield good trajectory tracking performance while keeping some of the desired properties normally associated with path following. The solution described builds on and extends previous work by Hindman and Hauser (1992) on so-called maneuver modified trajectory tracking. An application is made to the problem of designing a control system for the coordinated operation of an autonomous surface craft (ASC) and an autonomous underwater vehicle (AUV). Simulations with nonlinear models of an underactuated marine craft and a fully actuated underwater vehicle illustrate the performance of the control system derived.

Adaptive Sparseness Using Jeffreys Prior

Abstract: In this paper we introduce a new sparseness inducing prior which does not involve any (hyper) parameters that need to be adjusted or estimated. Although other applications are possible, we focus here on supervised learning problems: regression and classification. Experiments with several publicly available benchmark data sets show that the proposed approach yields state-of-the-art performance. In particular, our method outperforms support vector machines and performs competitively with the best alternative techniques, both in terms of error rates and sparseness, although it involves no tuning or adjusting of sparseness-controlling hyper-parameters.

Algebra and algorithms for QoS path computation and hop-by-hop routing in the Internet

Abstract: Prompted by the advent of QoS routing in the Internet, we investigate the properties that path weight functions must have so that hop-by-hop routing is possible and optimal paths can be computed with a generalized Dijsktra's (1959) algorithm. For this purpose we define an algebra of weights which contains a binary operation, for the composition of link weights into path weights, and an order relation. Isotonicity is the key property of the algebra. It states that the order relation between the weights of any two paths is preserved if both of them are either prefixed or appended by a common, third, path. We show that isotonicity is both necessary and sufficient for a generalized Dijkstra's algorithm to yield optimal paths. Likewise, isotonicity is also both necessary and sufficient for hop-by-hop routing. However, without strict isotonicity, hop by-hop routing based on optimal paths may produce routing loops. They are prevented if every node computes what we call lexicographic-optimal paths. These paths can be computed with an enhanced Dijkstra's algorithm that has the same complexity as the standard one. Our findings are extended to multipath routing as well. As special cases of the general approach, we conclude that shortest-widest paths can neither be computed with a generalized Dijkstra's algorithm nor can packets be routed hop-by-hop over those paths. In addition, loop free hop by hop routing over widest and widest-shortest paths requires that each node computes lexicographic-optimal paths, in general.

Role played by the N2(A3Σu+) metastable in stationary N2 and N2-O2 discharges

Abstract: The role played by the N2(A3Σu+) metastable on the overall kinetics of N2 and N2-O2 stationary discharges is illustrated by using a kinetic model based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally and electronically excited molecules, atoms and charged particles, in which the sustaining electric field is self-consistently determined It is shown that together with the vibrational distribution of N2(X1Σg+,v) molecules, the metastable state N2(A3Σu+) plays a central role in the whole problem, since some important aspects of these discharges, such as ionization, gas phase chemistry and gas heating are associated with different processes involving the N2(A3Σu+) state

Hausdorff dimension of measures via poincar e recurrence

Abstract: We study the quantitative behavior of Poincar e recurrence. In particular, for an equilibrium measure on a locally maximal hyper- bolic set of a C 1+ dieomorphism f, we show that the recurrence rate to each point coincides almost everywhere with the Hausdor dimen- sion d of the measure, that is, inffk > 0 : f k x2 B(x;r)g r d . This result is a non-trivial generalization of work of Boshernitzan concerning the quantitative behavior of recurrence, and is a dimensional version of work of Ornstein and Weiss for the entropy. We stress that our ap- proach uses dierent techniques. Furthermore, our results motivate the introduction of a new method to compute the Hausdor dimension of measures.

Variational principles and mixed multifractal spectra

Abstract: We establish a \" conditional \" variational principle, which unifies and extends many results in the multifractal analysis of dynam-ical systems. Namely, instead of considering several quantities of local nature and studying separately their multifractal spectra we develop a unified approach which allows us to obtain all spectra from a new mul-tifractal spectrum. Using the variational principle we are able to study the regularity of the spectra and the full dimensionality of their irregular sets for several classes of dynamical systems, including the class of maps with upper semi-continuous metric entropy. Another application of the variational principle is the following. The multifractal analysis of dynamical systems studies multifractal spectra such as the dimension spectrum for pointwise dimensions and the en-tropy spectrum for local entropies. It has been a standing open problem to effect a similar study for the \" mixed \" multifractal spectra, such as the dimension spectrum for local entropies and the entropy spectrum for pointwise dimensions. We show that they are analytic for several classes of hyperbolic maps. We also show that these spectra are not necessarily convex, in strong contrast with the \" non-mixed \" multifractal spectra.

Testing neutrino mixing at future collider experiments

Abstract: Low energy supersymmetry with bilinear breaking of R parity leads to a weak-scale seesaw mechanism for the atmospheric neutrino scale and a radiative mechanism for the solar neutrino scale. The model has striking implications for collider searches of supersymmetric particles. Assuming that the lightest SUSY particle is the lightest neutralino we demonstrate that (i) the neutralino decays inside the detector even for tiny neutrino masses and (ii) measurements of the neutrino mixing angles lead to predictions for the ratios of various neutralino decay branching ratios implying an independent test of neutrino physics at future colliders, such as the CERN Large Hadron Collider or a Linear Collider. We study the lightest neutralino decay branching ratio predictions taking into account present supersymmetric particle mass limits as well as restrictions coming from neutrino physics, with emphasis on the solar and atmospheric neutrino anomalies.

An Improved RTN Continuous-Time Formulation for the Short-term Scheduling of Multipurpose Batch Plants

Abstract: This paper presents an improved general mathematical programming formulation for optimal scheduling of batch processes based on the resource−task network (RTN) representation. The formulation uses a continuous-time representation and results in a mixed integer linear programming problem. It is a relaxation of the problem presented by Schilling (Schilling, G. Optimal Scheduling of Multipurpose Plants. Ph.D. Thesis, University of London, London, U., K., 1997). By allowing, if possible, finite storage within the processing tasks resource equipments of the involved raw materials and/or products, the proposed approach leads to simpler and less degenerate mathematical models. These models can be solved in significantly less CPU time, when compared to other RTN continuous-time formulations. Three published example problems are presented to illustrate the effectiveness of the proposed formulation. Finally, we show that the STN-based continuous-time scheduling formulation of Ierapetritou and Floudas (Ierapetritou,...