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The detection of performance bugs, like those causing an unexpected execution time, has gained much attention in the last years due to their potential impact in safety-critical and resource-constrained applications. Much effort has been put on trying to understand the nature of performance bugs in different domains as a starting point for the development of effective testing techniques. However, the lack of a widely accepted classification scheme of performance faults and, more importantly, the lack of well-documented and understandable datasets makes it difficult to draw rigorous and verifiable conclusions widely accepted by the community. In this paper, we present TANDEM, a dual contribution related to real-world performance bugs. Firstly, we propose a taxonomy of performance bugs based on a thorough systematic review of the related literature, divided into three main categories: effects, causes and contexts of bugs. Secondly, we provide a complete collection of fully documented real-world performance bugs. Together, these contributions pave the way for the development of stronger and reproducible research results on performance testing.

In the last decade, the advertising industry has experienced a quantum leap, powered by recent advances in neuroscience, a large investment in artificial intelligence, and a high degree of consumer expertise. Within this context, opinion mining, sentiment analysis, and emotion understanding bring us closer to one of the most sought-after objectives of advertising: to offer relevant ads at scale. The importance of studies about opinion mining, sentiment analysis, and emotion understanding in advertising has been rising exponentially over the last years. The peak of this new situation has been the interest of the research community in studying the relationship between such innovations and the spread of smart and contextual advertising. This article analyzes those works that address the relationship between sentiment analysis, opinion mining, and emotion understanding in advertising. The main objective is to clarify the current state of these studies, explore issues, methods, findings, themes, and gaps as well as to define their significance within the current convergence advertising research scenario. To reach such objectives, a bibliometric analysis was conducted, retrieving and analyzing 919 research works published between 2010 and 2019 based on results from Web of Science (WoS)., Programa Operativo FEDER Andalucia UMA 18-FEDERJA-148

Falls can have serious consequences for people, leading to restrictions in mobility or, in the worst case, to traumatic-based cases of death. To provide rapid assistance, a portable fall detection system has been developed that is capable of detecting fall situations and, if necessary, alerting emergency services without any user interaction. The prototype is designed to facilitate reliable fall detection and to classify several fall types and human activities. This solution represents a life-saving service for every person that will significantly improve assistance in the case of fall events, which are a part of daily life. Additionally, this approach facilitates independent system operation, since the system does not depend on sensor or network units located within a building structure. This article also introduces fall analysis. To guarantee functional safety, a hazard analysis method named system-theoretic accident model and processes (STAMP) is applied.

In the recent years, the use of Information and Communication Technologies (ICTs) to enhance the educational and developmental interventions of individuals with intellectual disabilities has increased drastically. Current literature about the efficacy of assistive technologies for people with intellectual disabilities includes numerous examples of co-design and research involving people with intellectual disabilities and their stakeholders. However, literature regarding the challenges and opportunities presented by this research is scarce. The discussion and reflection presented in this work is based on the authors' previous experience and ground-breaking literature in the field. After a review of relevant work, the the authors' research is described in detail, in order to reflect on the challenges encountered, and on opportunities for new researchers and open questions. The paper concludes with a reflection on the use of technologies to support people with intellectual disabilities, adopted strategies and limitations., This work was supported in part by the Tenure of an ERCIM Alain Bensoussan Postdoctoral Fellowship, and in part by the Project e-Madrid under Grant P2018/TCS-4307

Mutation testing is becoming increasingly widely used to evaluate the quality of test suites, especially to test programs coded in widely used programming languages in the industry. Mutation tools have arisen to automate the technique in different languages, including C++. With the increasing use of this technique, new mutation operators modeling possible faults often emerge to improve its abilities and adapt the tools to new advanced features. In this work, mutation operators for the new C++ standards, defined in previous work, are implemented and applied to generate and execute mutants in real programs. With this study, the MuCPP mutation tool is updated with the inclusion of these new operators. In addition, the improvements suggested in the definition of those operators can be finally tested, and conclusions about their utility in practice can be drawn. The implemented operators are checked on a set of four C++ programs that use these advanced features. The results show significant differences with the previous manual analysis: the number of invalid mutants was reduced by 64%, and we found fewer alive mutants (88%) and an increase in dead mutants (31%). In summary, both the number of mutants incorrectly classified in the previous manual analysis and the number of mutants generated (particularly equivalent mutants) have been reduced.

Testing is a crucial activity in the development of software systems. With the increasing complexity of software projects, the industry requires incorporating graduates with adequate testing skills and preparation in this field. A challenge in software testing education is to make students perceive the benefits of writing tests and assess their quality with advanced testing techniques. In this paper, we present an experience integrating both mutation testing and self/peer assessment –two of the most used techniques to that end in the past– into a software testing course during three years. This experience allowed us to analyze the effect of applying these strategies on the students’ perception of their manually-written test suites. Noticeably, the computation of the mutation score significantly undermined the initial expectations they had on the developed test suites. Also, the application of peer testing helped them estimate the relative quality of two comparable test suites, as we found a notable correspondence with their respective mutation coverage. Besides, a more in-depth analysis revealed that the students' test suites with more test cases did not always achieve the highest scores, that they found more readable their own tests, and that they tended to cover the basic operations while forgetting about more advanced features. An opinion survey confirmed the impact that the use of mutants had on their perception about testing, and they mostly supported paying a higher level of attention to testing concepts in software engineering degree plans., The work was partially funded by the European Commission (FEDER), the Spanish Ministry of Science, Innovation and Universities under the project FAME (RTI2018-093608-B-C33), the European project ASSETs (612678-EPP-1-2019-1-IT-EPPKA2-SSA-B), and the University of Cádiz.

Over the last years, the Internet of Things has fostered a growing interest in context-aware mobile applications; this fact is mainly due to highly favoring information provision from multiple Internetconnected devices. To identify user context, these applications collect information from the user and his/her environment and typically lter app information, so that the user receives only the interesting and relevant information. However, such a task usually implies further resource consumption on user mobile devices, not only regarding battery usage but also in terms of network traf c. Accordingly, although context-aware applications can improve user experiences in their daily lives, they must ensure the maintenance of lowlevel resource consumption; otherwise, the applications are promptly replaced by less consuming ones, and therefore, removed from the mobile market. In this paper, we evaluate and discuss several architectural styles for context-aware mobile applications, as well as, providing a set of guidelines to decide on the right architecture for a particular app depending on its characteristics. The use of such guidelines when choosing the right architectural style can strongly in uence the resource consumption of context-aware mobile applications. Following these guidelines, user satisfaction of a context-aware mobile application may be improved, thus guaranteeing the app success.

Skin cancer is a major health problem. There are several techniques to help diagnose skin lesions from a captured image. Computer-aided diagnosis (CAD) systems operate on single images of skin lesions, extracting lesion features to further classify them and help the specialists. Accurate feature extraction, which later on depends on precise lesion segmentation, is key for the performance of these systems. In this paper, we present a skin lesion segmentation algorithm based on a novel adaptation of superpixels techniques and achieve the best reported results for the ISIC 2017 challenge dataset. Additionally, CAD systems have paid little attention to a critical criterion in skin lesion diagnosis: the lesion's evolution. This requires operating on two or more images of the same lesion, captured at different times but with a comparable scale, orientation, and point of view; in other words, an image registration process should first be performed. We also propose in this work, an image registration approach that outperforms top image registration techniques. Combined with the proposed lesion segmentation algorithm, this allows for the accurate extraction of features to assess the evolution of the lesion. We present a case study with the lesion-size feature, paving the way for the development of automatic systems to easily evaluate skin lesion evolution, This work was supported in part by the Spanish Government (HAVideo, TEC2014-53176-R) and in part by the TEC department (Universidad Autonoma de Madrid)

For software quality assurance, many safety-critical industries appeal to the use of dynamic testing and structural coverage criteria. However, there are reasons to doubt the adequacy of such practices. Mutation testing has been suggested as an alternative or complementary approach but its cost has traditionally hindered its adoption by industry, and there are limited studies applying it to real safety-critical code. This paper evaluates the effectiveness of state-of-the-art mutation testing on safety-critical code from within the U.K. nuclear industry, in terms of revealing flaws in test suites that already meet the structural coverage criteria recommended by relevant safety standards. It also assesses the practical feasibility of implementing such mutation testing in a real setting. We applied a conventional selective mutation approach to a C codebase supplied by a nuclear industry partner and measured the mutation score achieved by the existing test suite. We repeated the experiment using trivial compiler equivalence (TCE) to assess the benefit that it might provide. Using a conventional approach, it first appeared that the existing test suite only killed 82% of the mutants, but applying TCE revealed that it killed 92%. The difference was due to equivalent or duplicate mutants that TCE eliminated. We then added new tests to kill all the surviving mutants, increasing the test suite size by 18% in the process. In conclusion, mutation testing can potentially improve fault detection compared to structural-coverage-guided testing, and may be affordable in a nuclear industry context. The industry feedback on our results was positive, although further evidence is needed from application of mutation testing to software with known real faults.

Mutation testing is a method used to assess and improve the fault detection capability of a test suite by creating faulty versions, called mutants, of the system under test. Evolutionary Mutation Testing (EMT), like selective mutation or mutant sampling, was proposed to reduce the computational cost, which is a major concern when applying mutation testing. This technique implements an evolutionary algorithm to produce a reduced subset of mutants but with a high proportion of mutants that can help the tester derive new test cases (strong mutants). In this paper, we go a step further in estimating the ability of this technique to induce the generation of test cases. Instead of measuring the percentage of strong mutants within the subset of generated mutants, we compute how much the test suite is actually improved thanks to those mutants. In our experiments, we have compared the extent to which EMT and the random selection of mutants help to find missing test cases in C++ object-oriented systems. We can conclude from our results that the percentage of mutants generated with EMT is lower than with the random strategy to obtain a test suite of the same size and that the technique scales better for complex programs.

Esta comunicación presenta un conjunto de trabajos de investigación sobre metodologías docentes, ludificación y calidad realizados en el seno del proyecto eMadrid, de la Comunidad Autónoma de Madrid. En primer lugar se resumen los trabajos realizados en los dos primeros años del proyecto. Posteriormente se presentan las líneas de trabajo previstas para los dos años restantes, Estos trabajos se han financiado parcialmente por el proyecto eMadrid (S2013/ICE-2715) de la Comunidad de Madrid, los proyectos FLEXOR (TIN2014-52129-R), RESET (TIN2014-53199-C3-1-R) e iProg (TIN2015-66731-C2-1-R) del Ministerio de Economía y Competitividad, y el proyecto “Adaptación de la metodología PhyMEL a la formación clínica mediante el uso de simuladores” financiado por la empresa Medical Simulator

Information and Communication Technologies offer more possibilities both for students with special needs and teachers who work with them. In recent years, research has been done about the possible applications and benefits of the use of social networks by people with disabilities, obtaining promising results. However, when reviewing the literature we observe that some professionals do not see social networks as suitable tools for this collective. In this paper, we present a learning experience in which a traditional methodology such as cooperative learning is combined with ClipIt, a social video-based learning platform. This work shows that the combination of traditional methodologies and new technologies is beneficial for students with special needs, enhancing both their motivation and their learning process, This work has been funded in part by the Spanish Economy and Competitiveness Ministry under project “e-Integra: e-Training y e-Coaching para la integración socio-laboral” (TIN2013-44586-R) and by the Region of Madrid under project “eMadrid – Investigación y Desarrollo de tecnologías para el e-learning en la Comunidad de Madrid (S2013/ICE-2715). ClipIt has been funded by the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 317964 JUXTALEARN. Thanks to the participants who were involved in the experiences presented in this paper and to the pedagogical team of the program PROMENTOR

We present a methodology for creating hypermedia materials derived from face-to-face tutoring sessions between a tutor and graduate students. To create the hypermedia materials, the tutor and the student used a smart pen which allowed to record the conversation and digitalize the notes being taken. The production of hypermedia material is based on the use of visual representations and text to help students go from concrete to abstract thinking and vice versa. We point out that hypermedia materials are audio-visual narratives (i.e., dynamic graphics, diagrams) that facilitate the representation of co-constructed shared knowledge and let participants navigate between oral and textual information. This methodology allows the production of individualized material without investing additional time in editing and designing. The hypermedia based tutoring (HBT) model is highly valued by students since it helps them to go over the discussions with the tutor and review the thinking process that both constructed during the session. HBT becomes a creative form of communicating and representing information that challenges the tutor and student to develop new skills and ways of thinking. The model that we propose here requires to change traditional tutor and student roles and to create learning experiences that do not overlook students’ needs, The eMadrid Excellence Network is funded by Madrid Regional Government (Comunidad de Madrid) grant no. P2013/ICE-2715. The FOMIX grant No. MOR-2013-C01-225102 is funded by Fondo Mixto-CONACYT (Morelos State Government and the Federal Agency CONACYT, Mexico)

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. J. de Lara, E. Guerra and J. Sánchez Cuadrado, "A-posteriori typing for Model-Driven Engineering," 2015 ACM/IEEE 18th International Conference on Model Driven Engineering Languages and Systems (MODELS), Ottawa, ON, 2015, pp. 156-165. doi: 10.1109/MODELS.2015.7338246, Model-Driven Engineering is founded on the ability to create and process models conformant to a meta-model. Hence, meta-model classes are used in two ways: as templates to create objects, and as classifiers for them. While these two aspects are inherently tied in most meta-modelling approaches, in this paper, we discuss the benefits of their decoupling. Thus, we rely on standard mechanisms for object creation and propose a-posteriori typing as a means to reclassify objects and enable multiple, partial, dynamic typings. This approach enhances flexibility, permitting unanticipated reutilization (as existing model management operations defined for a meta-model can be reused with other models once they get reclassified), as well as model transformation by reclassification.We show the underlying theory behind the introduced concepts, and illustrate its applicability using our METADEPTH meta-modelling tool., Work supported by the Spanish MINECO (TIN2011-24139 and TIN2014-52129-R), and the R&D programme of the Madrid Region (S2013/ICE-3006).

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. D. F. Barrero, M. D. R-Moreno, B. Castano, and D. Camacho, "Effects of the lack of selective pressure on the expected run-time distribution in genetic programming", in IEEE Congress on Evolutionary Computation, CEC 2013, pp. 1748 - 1755, Run-time analysis is a powerful tool to analyze algorithms. It is focused on studying the time required by an algorithm to find a solution, the expected run-time, which is one of the most relevant algorithm attributes. Previous research has associated the expected run-time in GP with the lognormal distribution. In this paper we provide additional evidence in that regard and show how the algorithm parametrization may change the resulting run-time distribution. In particular, we explore the influence of the selective pressure on the run-time distribution in tree-based GP, finding that, at least in two problem instances, the lack of selective pressure generates an expected run-time distribution well described by the Weibull probability distribution., This work has been partly supported by Spanish Ministry of Science and Education under project ABANT (TIN2010- 19872).

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. G. M. Sacha, "Influence of the Substrate and Tip Shape on the Characterization of Thin Films by Electrostatic Force Microscopy", IEEE Transactions on Nanotechnology, vol.12, no.2, pp.152-156, Marzo 2013., Electrostatic force microscopy has been shown to be a useful tool to determine the dielectric constant of nanoscaled thin films that play a key role in many electrical, optical and biological phenomena. Previous approaches have made use of simple analytical models to analyze the experimental data for these materials. Here we show that the electrostatic force shows a completely different behavior when the shape of the tip and sample are taken into account. We present a complete study of the interaction between the whole tip and the layers below the thin film. We demonstrate that physical magnitudes such as the surface charge density distribution and the size of the materials have a strong influence on the EFM signal. The EFM sensitivity to the substrate below the thin film decreases with the substrate thickness and saturates for thicknesses above two times the length of the tip, when it is close to that of an infinite medium., Author acknowledges interesting discussion from J. J. Sáenz, C. Gómez-Navarro, J. Gómez-Herrero and E. Castellano-Hernández. This work was supported by TIN2010- 19607. Author acknowledges support from the Spanish Ramón y Cajal Program.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.I. D. Carlos, R. Cobos, E. Guerra, J. de Lara, A. Pescador, and J. Sánchez Cuadrado, “Integrating Open Services for Building Educational Environments”, in Global Engineering Education Conference, Berlin (Germany), 2013, pp. 1147-1156, The increasing popularity of Massive Open Online Courses (MOOCs) has raised the need for highly scalable, customizable, open learning environments. At the same time, there is a growing trend to open the services that the companies offer on the web with open APIs and in the form of REST services, facilitating their integration in customized applications. The goal of this work is to show how such open services can be used for the support of on-line educational systems. These services were not created for an education context, so it is necessary to complement it with functionalities for supporting aspects such as evaluations, monitoring or collaboration. This article discusses on the strategies for integrating services for education and presents two cases studies: first, SMLearning, a collaborative learning environment supported by social media platforms Facebook and YouTube, and second, an application for project-based programming courses, customized through a generative architecture, making heavy use of Google services., This research was partly funded by the Spanish National Plan of R+D, project number TIN2011-24139; and by the Autonomous Community of Madrid, e-Madrid project, number S2009/TIC-1650.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. A. L. A. Dalhoum et al., "Digital Image Scrambling Using 2D Cellular Automata", IEEE MultiMedia, vol. 19, no. 4 pp. 28 – 36, oct-dec. 2012, A digital image scrambling method based on a 2D cellular automaton, specifically the well-known Game of Life, produces an effective image encryption technique., This work has been partially sponsored by the Spanish MICINN project TIN2011-28260-C03-02

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. R. Cobos, R. J. Martín, J. Moreno-Llorena, E. Guerra, and J. de Lara, “REUSES: Questionnaire-driven design for the automatic generation of web-based collaborative applications”, 7th International Conference on Collaborative Computing: Networking, Applications and Worksharing (CollaborateCom), 2011, Orlando (FL), 2011, pp. 9-18, In this paper, we present a questionnaire-driven collaborative design methodology for the automatic generation of collaborative applications. The methodology is based on a repository of collaborative components and a knowledge base of its application and use modes. The repository includes components built by our team, as well as third-party components that use heterogeneous technologies like JavaScript, Java, Flash, and services and components provided by companies like Google and Facebook. Based on this methodology, we have built a Web 2.0 tool called REUSES (Rapid End-User Synthesis of Collaborative Applications), which provides facilities for the automatic generation of Web-based collaborative applications tailored to any particular community of end users. Finally, we also report on some experimental work to test REUSES in the context of academic courses at our University., This work was supported in part by the Spanish National Plan of R+D, project number TIN2008-02081 (Meteoric), and by the CAM (Autonomous Community of Madrid), project number S2009/TIC-1650 (eMadrid)

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. J. C. SanMiguel, and J. M. Martínez, "Shadow detection in video surveillance by maximizing agreement between independent detectors", in 16th IEEE International Conference on Image Processing, ICIP 2009. p. 1141-1144, This paper starts from the idea of automatically choosing the appropriate thresholds for a shadow detection algorithm. It is based on the maximization of the agreement between two independent shadow detectors without training data. Firstly, this shadow detection algorithm is described and then, it is adapted to analyze video surveillance sequences. Some modifications are introduced to increase its robustness in generic surveillance scenarios and to reduce its overall computational cost (critical in some video surveillance applications). Experimental results show that the proposed modifications increase the detection reliability as compared to some previous shadow detection algorithms and performs considerably well across a variety of multiple surveillance scenarios., Work supported by the Spanish Government (TEC2007- 65400 SemanticVideo), by Cátedra Infoglobal-UAM for “Nuevas Tecnologías de video aplicadas a la seguridad”, by the Spanish Administration agency CDTI (CENIT-VISION 2007-1007), by the Comunidad de Madrid (S-050/TIC-0223 - ProMultiDis), by the Consejería de Educación of the Comunidad de Madrid and by the European Social Fund.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. F. Alonso-Fernández, J. Fiérrez, A. Gilpérez, J Ortega-García, "Impact of time variability in off-line writer identification and verification" in 6th International Symposium on Image and Signal Processing and Analysis (ISPA), Salzburg (Austria), 2009, pp. 540 - 545, One of the biggest challenges in person recognition using biometric systems is the variability in the acquired data. In this paper, we evaluate the effects of an increasing time lapse between reference and test biometric data consisting of static images of handwritten signatures and texts. We use for our experiments two recognition approaches exploiting information at the global and local levels, and the BiosecurlD database, containing 3,724 signature images and 532 texts of 133 individuals acquired in four acquisition sessions distributed along a 4 months time span. We report results of the recognition systems working both in verification (one-to-one) and identification (one-to-many) mode. The results show the extent of the impact that the time separation between samples under comparison has on the recognition rates, being the local approach more robust to the time lapse than the global one. We also observe in our experiments that recognition based on handwritten texts provides higher accuracy than recognition based on signatures., This work has been supported by Spanish MCYT TEC2006-13141-C03-03 project.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Á. Bayona, J. C. SanMiguel, and J. M. Martínez, "Comparative evaluation of stationary foreground object detection algorithms based on background subtraction techniques" in Sixth IEEE International Conference on Advanced Video and Signal Based Surveillance. AVSS 2009, p. 25 - 30, In several video surveillance applications, such as the detection of abandoned/stolen objects or parked vehicles,the detection of stationary foreground objects is a critical task. In the literature, many algorithms have been proposed that deal with the detection of stationary foreground objects, the majority of them based on background subtraction techniques. In this paper we discuss various stationary object detection approaches comparing them in typical surveillance scenarios (extracted from standard datasets). Firstly, the existing approaches based on background-subtraction are organized into categories. Then, a representative technique of each category is selected and described. Finally, a comparative evaluation using objective and subjective criteria is performed on video surveillance sequences selected from the PETS 2006 and i-LIDS for AVSS 2007 datasets, analyzing the advantages and drawbacks of each selected approach., This work has partially supported by the Cátedra UAMInfoglobal ("Nuevas tecnologías de vídeo aplicadas a sistemas de video-seguridad"), the Spanish Administration agency CDTI (CENIT-VISION 2007-1007), by the Spanish Government (TEC2007-65400 SemanticVideo), by the Comunidad de Madrid (S-050/TIC-0223- ProMultiDis), by the Consejería de Educación of the Comunidad de Madrid, and by The European Social Fund.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. S. Gomez-Moñivas, "Contrast and Resolution of Nanowires in Electrostatic Force Microscopy", IEEE Transactions on nanotechonology, vol.8, no.2, pp. 148-152, Marzo 2009, A detailed analysis of the contrast and lateral resolution between a dc-biased tip and metallic nanowires over a dielectric sample is presented. The theoretical technique used to analyze the interaction intrinsically includes the mutual polarization between the tip, the sample, and the metallic objects. A good selection of the dielectric constant of the sample is found to be critical since it can increase the contrast by more than an order of magnitude. On the other hand, the dielectric constant does not have any influence on the lateral resolution. In this case, the tip–sample distance and the tip radius are the most relevant parameters. For small tip–sample distances, the length of the nanowires must also be taken into account since it can produce differences of up to 20% in the lateral resolution.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. F. Alonso-Fernández, J. Fiérrez, M. Martínez-Diaz, J. Ortega-García, "Fusion of static image and dynamic information for signature verification" in 16th Conference on Image Processing (ICIP), Cairo (Egypt), 2009, pp. 2725 - 2728, This paper evaluates the combination of static image (off-line) and dynamic information (on-line) for signature verification. Two off-line and two on-line recognition approaches exploiting information at the global and local levels are used. Experimental results are given using the BiosecurID database (130 signers, 3,640 signatures). Fusion experiments are done using a trained fusion approach based on linear logistic regression. It is shown experimentally that the local systems outperform the global ones, both in the on-line and in the off-line case. We also observe a considerable improvement when combining the two on-line systems, which is not the case with the off-line systems. The best performance is obtained when fusing all the systems together, which is specially evident for skilled forgeries when enough training data is available., This work has been supported by Spanish MCYT TEC2006-13141-C03-03 project.

Proceedings of the 2008 IEEE International Conference on Semantic Computing, This paper is a theoretical analysis of formal annotation and ontology for the expression of the semantics of document. They are found wanting in this respect, not only for technical reasons, but because they embody a fundamentally misunderstood model of the process of signification. The author proposes an alternative model in which the interpretation context plays a fundamental role, and briefly discuss it and its current technical embodiment.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. E. Maiorana, P. Campisi, M. Martínez-Díaz, J. Ortega-García, A. Neri, "Template protection for HMM-based on-line signature authentication" in IEEE Computer Society Conference on Computer Vision and Pattern Recognition Workshops CVPRW, Anchorage, AK (USA), 2008, pp. 1-6., The security of biometric data is a very important issue in the deployment of biometric-based recognition systems. In this paper, we propose a signature-based biometric authentication system, where signal processing techniques are applied to the acquired on-line signature in order to generate protected templates, from which retrieving the original data is computationally as hard as randomly guessing them. A hidden Markov model (HMM)-based matching strategy is employed to compare the transformed signatures. The proposed protected authentication system generates a score as the result of the matching process, thus allowing to implement protected multibiometric recognition systems, through the application of score-fusion techniques. The experimental results show that, at the cost of only a slight performance reduction, the desired protection for the employed biometric templates can be properly achieved.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. de Pedro, L.; Aracil, J.; Hernandez, J.A.; Garcia-Dorado, J.L., "Analysis of the processing and sojourn times of Burst Control Packets in Optical Burst Switches," Optical Network Design and Modeling, 2008. ONDM 2008. International Conference on , vol., no., pp.1,3, 12-14 March 2008, In Optical Burst Switched networks, when a Burst Control Packet arrives at a core node, its scheduling algorithm looks up its internal reservation table for an available gap at which to accomodate the incoming burst. The processing time of the BCP is variable, and depends on the number of available gaps over which to perform the search. This work analyses the gap distribution in such reservation table, and derives an expression for the waiting time of BCPs, assuming LAUC-VF scheduling. Our findings can be applied to the dimensioning of the offset time values in OBS networks, for a realistic case of BCP variable processing times., This work has been funded by the Spanish MEC under the project DIOR (TEC2006-03246).

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. M. Fernández, V. López, M. Sabou, V. S. Uren, D. Vallet, E. Motta, and P. Castells, "Semantic search meets the Web", 2008 IEEE International Conference on Semantic Computing, Santa Clara (CA), 2008, pp. 253 - 260, While semantic search technologies have been proven to work well in specific domains, they still have to confront two main challenges to scale up to the Web in its entirety. In this work we address this issue with a novel semantic search system that a) provides the user with the capability to query Semantic Web information using natural language, by means of an ontology-based Question Answering (QA) system [14] and b) complements the specific answers retrieved during the QA process with a ranked list of documents from the Web [3]. Our results show that ontology-based semantic search capabilities can be used to complement and enhance keyword search technologies., This work was funded by the Spanish Ministry of Science and Education (TIN2005-068 and FPU program) and OpenKnowledge (FP6-027253) projects

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. F. Alonso-Fernández, F. C. Fairhurst, J. Fiérrez, J. Ortega-García, "Impact of signature legibility and signature type in off-line signature verification" in Biometrics Symposium, Baltimore, (Maryland), 2007, 1-6., Proceedings of Biometrics Symposium, held in Baltimore (MD), 2007., The performance of two popular approaches for off-line signature verification in terms of signature legibility and signature type is studied. We investigate experimentally if the knowledge of letters, syllables or name instances can help in the process of imitating a signature. Experimental results are given on a sub-corpus of the MCYT signature database for random and skilled forgeries. We use for our experiments two machine experts, one based on global image analysis and statistical distance measures, and the second based on local image analysis and Hidden Markov Models. Verification results are reported in terms of Equal Error Rate (EER), False Acceptance Rate (FAR) and False Rejection Rate (FRR)., This work has been supported by Spanish MCYT TEC2006-13141-C03-03 and by European Commission IST-2002-507634 Biosecure NoE projects. Author F. A.-F. thanks Consejeria de Educacion de la Comunidad de Madrid and Fondo Social Europeo for supporting his PhD studies.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. J. Galbally, J. Fiérrez, M. R. Freire, J. Ortega-garcía, "Feature Selection Based on Genetic Algorithms for On-Line Signature Verification" in Workshop on Automatic Identification Advanced Technologies, 2007, 198 - 203., Two different genetic algorithm (GA) architectures are applied to a feature selection problem in on-line signature verification. The standard GA with binary coding is first used to find a suboptimal subset of features that minimizes the verification error rate of the system. The curse of dimensionality phenomenon is further investigated using a GA with integer coding. Results are given on the MCYT signature database comprising 330 users (16500 signatures). Signatures are represented by means of a set of 100 features which can be divided into four different groups according to the signature information they contain, namely: i) time, ii) speed and acceleration, iii) direction, and iv) geometry. The GA indicates that features from subsets i and iv are the most discriminative when dealing with random forgeries, while parameters from subsets ii and iv are the most appropriate to maximize the recognition rate with skilled forgeries., This work was supported by Spanish MEC under project TEC2006-13141-C03-03 and the European NoE Biosecure.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. P. Bottoni, J. de Lara, and E. Guerra, "Action Patterns for the Incremental Specification of the Execution Semantics of Visual Languages", IEEE Symposium on Visual Languages and Human-Centric Computing, 2007. VL/HCC 2007,Coeur d'Alene, ID, 2007, pp. 163-170, We present a new approach - based on graph transformation - to incremental specification of the operational (execution) semantics of visual languages. The approach combines editing rules with two meta-models: one to define the concrete syntax and one for the static semantics. We introduce the notion of action patterns, defining basic actions (e.g. consuming or producing a token in transition-based semantics), in a way similar to graph transformation rules. The application of action patterns to a static semantics editing rule produces a meta-rule, to be paired with the firing of the corresponding syntactic rule to incrementally build an execution rule. An execution rule is thus tailored to any active element (e.g. a transition in a Petri net model) in the model. Examples from Petri nets, state automata and workflow languages illustrate these ideas., Work sponsored by the EC with contract HPRN-CT-2002-00275, SegraVis, and the Spanish Ministry of Science and Education, projects MD2 (TIC200303654) and MOSAIC (TSI2005-08225-C07-06)

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. M. Martínez-Díaz, J. Fiérrez-Aguilar, F. Alonso-Fernández, J. Ortega-García, J.A. Siguenza, "Hill-Climbing and Brute-Force Attacks on Biometric Systems: A Case Study in Match-on-Card Fingerprint Verification" in Proceedings of 40th Annual IEEE International Carnahan Conferences Security Technology ICCST, Lexington, KY (USA), 2006, 151 - 159, In this paper, we study the robustness of state-of-the-art automatic fingerprint verification systems against hill climbing and brute-force attacks. We compare the performance of this type of attacks against two different minutiae-based systems, the NIST Fingerprint Image Software 2 (NFIS2) reference system and a Match-on-Card based system. In order to study their success rate, the attacks are analyzed and modified in each scenario. We focus on the influence of initial conditions in hill-climbing attacks, like the number of minutiae in the synthetically generated templates or the performance of each type of modification in the template. We demonstrate how slight modifications in the hill-climbing algorithm lead to very different success rates, This work has been supported by the Spanish Ministry of Defense, BioSecure NoE and the TIC2003-08382-C05-01 project of the Spanish Ministry of Science and Technology

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. E. Martin, N. Andueza, and R.M. Carro, “Architecture of a system for context-based adaptation in m-learning”, in Conference on Advanced Learning Technologies, 2006. Sixth International, Kerkrade, 2006, pp. 252-254, In this paper, the architecture of a system that supports context-based adaptation for m-learning is presented. This system manages data about users and activities so that the most suitable activities to be accomplished at each time are proposed to each user. This decision is not only based on the user's personal features, preferences or previous actions but also on information about the specific user's context, including spare time, location and available devices, This work has been funded by the Spanish Ministry of Science and Education, project TIN2004-03140

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. J. A. Hernández, J. Aracil, V. López, J. F. Palacios, and O. G. de Dios, "A resilience-based comparative study between optical burst switching and optical circuit switching technologies", in International Conference on Transparent Optical Networks, 2006, p. 231-234, Internet operators and ISP providers have traditionally designed network resources following an over-planning policy, on attempts to maintain a desired grade of service and network availability, regardless of network failures. This work presents a comparative study of two resilience mechanisms in the design of optical networks either based on optical burst switching (OBS) or optical circuit switching (OCS): the M:N protection scheme with dedicated backup channels. It is further analysed and discussed the benefits and disadvantages of such mechanism based on an analytical model

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. C. H. Ho, P. H. W. Leong, W. Luk, S. J. E. Wilton, and S. Buedo, "Virtual embedded blocks: A methodology for evaluating embedded elements in FPGAs", in 14th Annual IEEE Symposium on Field-Programmable Custom Computing Machines, 2006. FCCM '06. 35-44, Embedded elements, such as block multipliers, are increasingly used in advanced field programmable gate array (FPGA) devices to improve efficiency in speed, area and power consumption. A methodology is described for assessing the impact of such embedded elements on efficiency. The methodology involves creating dummy elements, called virtual embedded blocks (VEBs), in the FPGA to model the size, position and delay of the embedded elements. The standard design flow offered by FPGA and CAD vendors can be used for mapping, placement, routing and retiming of designs with VEBs. The speed and resource utilisation of the resulting designs can then be inferred using the FPGA vendor's timing analysis tools. We illustrate the application of this methodology to the evaluation of various schemes of involving embedded elements that support floating-point computations

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. E. Todorovich, and E. Boemo, "A-B nodes classification for power estimation", in International Conference on Field Programmable Logic and Applications, 2006. FPL '06, 2006, p. 803-808, In this paper, an optimization for the classical statistical power estimation method is proposed. This technique is applied to the individual nodes. The optimization is based on two observations. Firstly, a small percentage of both the nodes and the estimated power requires nearly a half of the total simulation time. On the other hand, the statistical method produces results with better accuracy than those specified by the user. This additional precision enables to reduce the run time for the slow convergence nodes with no loss of accuracy. A simple partitioning of the nodes into two groups, A and B, with normal and high computational cost respectively, leads to a modified stopping criterion with dramatic savings in the run time.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. K. Koroutchev, and M. Cebrian, “Detecting the same text in different languages”, in Information Theory Workshop, 2006. ITW '06 Punta del Este. IEEE, Punta del Este, Uruguay, 2006, pp. 337-341, Compression based similarity distances have the main drawback of needing the same coding scheme for the objects to be compared. When two texts are translated, there exists significant similarity with no literal coincidence. In this article, we present an algorithm that compares the redundancy structure of the data extracted by means of a Lempel- Ziv compression scheme. Each text is presented as a graph and two texts are considered similar with our measure if they have the same referential topology when compressed. We give empirical evidence that this measure detects similarity between data coded in different languages., This work was partially supported by grant TIN 2004-07676-G01 of the Spanish Ministry of Education and Culture. Partially supported by grant TSI 2005-08255-C07-06 of the Spanish Ministry of Education and Culture

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. D. Ramos, J. González-Rodríguez, A. Montero-Asenjo, J. Ortega-García, "Suspect-Adapted MAP Estimation of Within-Source Distributions in Generative Likelihood Ratio Estimation" in Odyssey 2006: the Speaker and Language Recognition Workshop, San Juan (Puerto Rico), 2006, 1-5, In this paper, a novel suspect-adaptive technique for robust Bayesian forensic speaker recognition via maximum a posteriori (MAP) estimation is presented, which addresses likelihood ratio (LR) computation in limited suspect speech data conditions obtaining good calibration performance. Robustness is achieved by the use of speaker-independent information, adapting it to the specificities of the suspect involved in the process. Thus, this procedure allows the system to weight the relevance of the suspect specificities depending on the amount of suspect data available via MAP estimation. Experimental results show robustness to suspect data scarcity and stable performance for any amount of suspect material. Also, the proposed technique outperforms other previously proposed non-adaptive approaches. Results are presented as discrimination capabilities (DET plots), distributions of LRs (Tippett plots) and expected cost of wrong decisions over any prior or decision cost (Cllr). The use of such evaluation metrics allows us to highlight the importance of LR calibration in the performance of a forensic system, This work has been supported by the Spanish Ministry for Science and Technology under project TIC2003-09068-C02-01.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. F. Muñoz, and A. Ortigosa, “Using adaptive hypermedia to support diversity in secondary schools”, in Sixth International Conference on Advanced Learning Technologies, 2006, Kerkrade, 2006, pp. 1055-1059, Attention to diversity is growing concern in public secondary schools in Spain. This paper presents an assessment about the use of adaptive hypermedia technology for supporting diversity. Three experiences were carried out with an adaptive course on mathematics to test the effects of this technology on the heterogeneous population of secondary schools. Their results and conclusions are also presented, This work has been funded by the Spanish Ministry of Science and Education, TIN2004-03140.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. E. Guerra et al. "Generación de entornos de modelado avanzados mediante técnicas de transformación de grafos", IEEE Latin America Transactions, vol. 4, no. 2, pp. 107-114, april 2006, En este artículo mostramos nuestro enfoque para la generación automática de entornos de modelado que soporten Lenguajes Visuales de Dominio Específico (LVDEs) con múltiples vistas. El enfoque está basado en la herramienta de metamodelado AToM3. La herramienta es capaz de generar un entorno de modelado a partir de un meta-modelo del LVDE y sus vistas. La consistencia entre las vistas se consigue mediante la generación automática de reglas de gramáticas de grafos triples. Éstas construyen un único modelo que combina los distintos modelos de las vistas. El diseñador del LVDE puede especificar métricas y rediseños de manera visual para el entorno generado. Valores extremos de estas métricas pueden disparar la aplicación de rediseños, los cuales se han formalizado mediante reglas de gramáticas de grafos. Mostramos la aplicación de estos conceptos en el área del diseño hipermedia mediante la generación de un entorno de modelado para el LVDE Labyrinth., In this paper we show our approach to the automatic generation of modelling environments for Visual Languages (VLs) supporting multiple views. The approach is based on the meta-modelling tool AToM3. Starting from a meta-model description of the VL and its views, a customized modelling environment is generated. Consistency between views is supported by the automatic generation of triple graph grammar rules that build a unique model “gluing” all the view models. Moreover, the generated environments allow taking basic static metrics of the models. Extreme values of the metrics may trigger heuristics for the refactoring of the models. These heuristics are formalized in the form of graph grammar rules. We show the application of these concepts in the area of hypermedia design by generating an environment for the Labyrinth VL., Este trabajo ha sido financiado en parte por el Ministerio de Ciencia y Tecnología con los Proyectos TIC2002-01948 y TIC2003-03654.

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. I. Cantador, P. Castells, and D. Vallet, "Enriching group profiles with ontologies for knowledge-driven collaborative content retrieval", WETICE '06. 15th IEEE International Workshops on Enabling Technologies: Infrastructure for Collaborative Enterprises, 2006., Manchester (United Kingdom), 2006, pp. 358 - 363, This paper proposes several strategies for the combination of ontology-based user profiles to generate a shared semantic profile for a group of users. The performance of the strategies is theoretically and empirically evaluated in an existing personalization framework from a knowledge-driven multimedia retrieval system. Early experiments are reported, which show the benefits of using semantic user preferences representations and providing initial evidence as to which profiles combination strategies are most appropriate for collaborative content retrieval tasks, This research was supported by the EC (FP6- 001765–aceMedia), and the Spanish Ministry of Science and Education (TIN2005-06885).

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. H. Vangheluwe, and J. de Lara, "Computer automated multi-paradigm modelling for analysis and design of traffic networks", Proceedings of the 2004 Winter Simulation Conference, 2004, Volumen 1, Heidelberg, Germany, 2004, Computer automated multiparadigm modelling (CAMPaM) is an enabler for domain-specific analysis and design. Traffic, a new untimed visual formalism for vehicle traffic networks, is introduced. The syntax of traffic models is meta-modelled in the entity-relationship diagrams formalism. From this, augmented with concrete syntax information, a visual modelling environment is synthesized using our CAMPaM tool AToM3, a tool for multiformalism and meta-modelling. The semantics of the traffic formalism is subsequently modelled by mapping traffic models onto Petri net models. As models' abstract syntax is graph-like, graph rewriting can be used to transform models. The advantages of a domain-specific formalism such as traffic as opposed to a generic formalism such as Petri nets are presented. We demonstrate how mapping onto Petri nets allows one to employ the vast array of Petri net analysis techniques. A coverability graph is generated and conservation analysis is automated by transforming this graph into an integer linear programming specification, Juan de Lara’s work has been partially sponsored by the Spanish Interdepartmental Commission of Science and Technology (CICYT), project number TIC2002-01948. Hans Vangheluwe gratefully acknowledges partial support for this work by a National Sciences and Engineering Research Council of Canada (NSERC) Individual Research Grant. The authors wish to thank Ms. Sokhom Pheng for her work on the Petri Net conservation analysis during her “Modelling and Simulation Based Design” project at McGill University

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. M. M. Peiró, J. Valls, T. Sansaloni, A. P. Pascual, and E. I. Boemo, "A comparison between lattice, cascade and direct form FIR filter structures by using a FPGA bit-serial distributed arithmetic implementation", in 6th IEEE International Conference on Electronics, Circuits and Systems, 1999. Proceedings of ICECS '99, 2009, p. 241 - 244, In this paper, several bit-serial, high-order implementations of cascade, lattice and direct-form FIR filters using Distributed Arithmetic (DA) are studied. Although lattice and cascade structures present many interesting properties related to quantification error and stability, the DA versions have not been thoroughly compared. Three types of filters with their particular bit-serial DA model error have been built using an ALTERA 10K50 FPGA and their area-time figure is analysed. The results show that a 60th order bit-serial cascade and direct-form implementation at nearly 4 MHz and a 40th order lattice structure at 7.5 MHz can be implemented. Moreover, the lattice filter presents the lower quantification error

Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. M. Alfonseca, and, H. Vangheluwe, "Web-based simulation of systems described by partial differential equations", in Proceedings of the Winter Simulation Conference, 2001, pp. 629 - 636., This paper describes how to take advantage of Internet services and object technology to solve 2D partial differential equations (PDEs) in a distributed manner. This is accomplished by means of a distributed object oriented continuous simulation language designed by our research group, called OOCSMP, and a Java (and C++) generating compiler for this language (called C-OOL). We also describe a graphical mesh generator, which produces OOCSMP code. The mesh generator may be invoked from the simulation model allowing the generation of domains and meshes during model execution. The simulation of heating moving pieces is shown, in the single machine case, and in the distributed case, This paper has been sponsored by the Spanish Interdepartmental Commission of Science and Technology (CICYT), project number TEL1999-018 1