Loading...
29 results
Search Results
Now showing 1 - 10 of 29
- Smarandache sequences: explorations and discoveries with a computer algebra systemPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We study Smarandache sequences of numbers, and related problems, via a Computer Algebra System. Solutions are discovered, and some conjectures presented.
- Codificação de fala por modelos variáveis no tempoPublication . Gouveia, Paulo D.F.O trabalho apresentado nesta tese representa uma contribuição para a optimização da codificação da fala. Utilizam-se para o efeito modelos de codificação baseados em filtros LP (filtros de Predição Linear) de parâmetros variáveis no tempo, contrastando com os modelos fixos utilizados nos métodos convencionais. Nestes, a adaptação dos filtros de predição realiza-se simplesmente através de actualizações periódicas dos seus parâmetros, não traduzindo por isso uma evolução gradual e contínua ao longo do tempo. A técnica utilizada na implementação dos modelos variáveis tem por base a utilização de funções do tipo B-spline na representação das formas de onda dos parâmetros LP. Para o estudo da viabilidade do modelo proposto, analisou-se o desempenho de um vocoder de predição linear incluindo, quer o modelo LP de parâmetros variáveis, quer o modelo LP de parâmetros fixos convencional, por forma a possibilitar a comparação de desempenhos. Dos resultados obtidos concluímos que a codificação de fala por modelos variáveis no tempo, embora não tenha evidenciado vantagens convincentes, pode ser encarada como outra forma de codificação, competindo por isso com as metodologias já existentes. The work presented in this thesis aims at to be a contribution to speech coding. To accomplish this objective, coding models based on LP filters (Linear Predictive Filters) with time-varying parameters are used, and compared with fixed models used in conventional methods. In these models, the predictive filters adaptation is carried on simply through periodic updatings of its parameters, therefore doesn’t representing a gradual and continuous evolution in time. The technique used in varying models implementation is based on the utilization of B-spline like functions to represent the LP parameters waveforms. In order to make a viability study of the proposed model, the performance of a linear predictive vocoder was analyzed, including both the LP model with varying parameters and the conventional LP model with fixed parameters, thus enabling the comparison of their performances. From the results, we concluded that speech coding by time-varying models, although it had not demonstrated clear benefits, can be viewed as another coding way, therefore competing with the already existing methodologies.
- A Maple interface for computing variational symmetries in optimal controlPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.A computer algebra package, for the automatic computation of variational symmetries in optimal control, was recently developed by the authors [2,3]. Now we present a graphical user interface which permit to interact, in a point-and-click environment, with all the previous symbolical tools.
- Problems of maximal mean resistance on the planePublication . Plakhov, Alexander; Gouveia, Paulo D.F.A two-dimensional body moves through a rarefied medium; the collisions of the medium particles with the body are absolutely elastic.The body performs both translational and slow rotational motion. It is required to select the body, from a given class of bodies, such that the average force of resistance of the medium to its motion is maximal. There are presented numerical and analytical results concerning this problem. In particular, the maximum resistance in the class of bodies contained in a convex body K is proved to be 1.5 times resistance of K. The maximum is attained on a sequence of bodies with very complicated boundary. The numerical study was made for somewhat more restricted classes of bodies. The obtained values of resistance are slightly lower, but the boundary of obtained bodies is much simpler, as compared to the analytical solutions.
- Computing ODE symmetries as abnormal variational symmetriesPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We give a new computational method to obtain symmetries of ordinary differential equations. The proposed approach appears as an extension of a recent algorithm to compute variational symmetries of optimal control problems [P.D.F. Gouveia, D.F.M. Torres, Automatic computation of conservation laws in the calculus of variations and optimal control, Comput. Methods Appl. Math. 5 (4) (2005) 387-409], and is based on the resolution of a first order linear PDE that arises as a necessary and sufficient condition of invariance for abnormal optimal control problems. A computer algebra procedure is developed, which permits one to obtain ODE symmetries by the proposed method. Examples are given, and results compared with those obtained by previous available methods.
- Modelling academic dropout in computer engineering using arti cial neural networksPublication . Camelo, Diogo; Santos, João C.C.; Martins, Maria Prudência; Gouveia, Paulo D.F.School dropout in higher education is an academic, economic, political and social problem, which has a great impact and is difficult to resolve. In order to mitigate this problem, this paper proposes a predictive model of classification, based on artificial neural networks, which allows the prediction, at the end of the first school year, of the propensity that the computer engineering students of a polytechnic institute in the interior of the country have for dropout. A differentiating aspect of this study is that it considers the classifications obtained in the course units of the first academic year as potential predictors of dropout. A new approach in the process of selecting the factors that foreshadow the dropout allowed isolating 12 explanatory variables, which guaranteed a good predictive capacity of the model (AUC = 78.5%). These variables reveal fundamental aspects for the adoption of management strategies that may be more assertive in the combat to academic dropout.
- Reflection of parallel rays by a two-dimensional body of nearly maximal resistancePublication . Gouveia, Paulo D.F.This Demonstration shows a class of nonconvex bodies that maximize resistance as they move forward while slowly rotating through a rarefied medium. This class of shapes yields a resistance very close to the theoretical maximum, improving all previous results found by the authors.
- On the two-dimensional rotational body of maximal Newtonian resistancePublication . Gouveia, Paulo D.F.; Plakhov, Alexander; Torres, Delfim F.M.We investigate, by means of computer simulations, shapes of nonconvex bodies that maximize resistance to their motion through a rarefied medium, considering that the bodies are moving forward and at the same time slowly rotating. A two-dimensional geometric shape that confers to the body a resistance very close to the theoretical supremum value is obtained, which improves previous results.
- Uma forma bidimensional que maximiza a resistência aerodinâmica newtonianaPublication . Gouveia, Paulo D.F.; Plakhov, Alexander; Torres, Delfim F.M.Um corpo bidimensional, apresentando um ligeiro movimento rotacional, desloca-se num meio rarefeito de partículas que colidem com ele de uma forma perfeitamente elástica. Em investigações que os dois primeiros autores realizaram anteriormente [Plakhov and Gouveia, 2007], procuraram-se formas de corpos que maximizassem a força de travagem do meio ao seu movimento. Dando continuidade a esse estudo, encetam-se agora novas investigações que culminam num resultado que representa um grande avanço qualitativo relativamente aos então alcançados.Esse resultado, que agora se apresenta, consiste numa forma bidimensional que confere ao corpo uma resistência muito próxima do seu limite teórico.
- Automatic computation of conservation laws in the calculus of variations and optimal controlPublication . Gouveia, Paulo D.F.; Torres, Delfim F.M.We present analytic computational tools that permit us to identify, in an automatic way, conservation laws in optimal control. The central result we use is the famous Noether's theorem, a classical theory developed by Emmy Noether in 1918, in the context of the calculus of variations and mathematical physics, and which was extended recently to the more general context of optimal control. We show how a Computer Algebra System can be very helpful in finding the symmetries and corresponding conservation laws in optimal control theory, thus making useful in practice the theoretical results recently obtained in the literature. A Maple implementation is provided and several illustrative examples given.
- «
- 1 (current)
- 2
- 3
- »