12/2013 |
On Sliding Periodic Solutions for Piecewise Continuous Systems Defined on the 2-Cylinder Douglas D. Novaes, Mike R. Jeffrey, Marco A. Teixeira This paper deals with discontinuous differential equations defined on the 2--dimensional cylinder. The main goal is to exhibit conditions for the existence of typical periodic solutions of such systems. An averaging method for computing sliding periodic solutions is developed, subject to convenient assumptions. We also apply the method to example problems. The main tools used are structural stability theory for discontinuous differential systems and Brouwer degree theory. rp-2013-12.pdf |

11/2013 |
Birth of limit cycles bifurcating from a nonsmooth center Cláudio A. Buzzi, Tiago de Carvalho, Marco A. Teixeira This paper is concerned with a codimension analysis of a two-fold singularity of piecewise smooth planar vector fields, when it behaves itself like a center of smooth vector fields (also called nondegenerate Σ-center). We prove that any nondegenerate Σ-center is Σ-equivalent to a particular normal form Z0 . Given a positive integer number k we explicitly construct families of piecewise smooth vector fields emerging from Z0 that have k hyperbolic limit cycles bifurcating from the nondegenerate Σ-center of Z0 (the same holds for k = ∞).Moreover, we also exhibit families of piecewise smooth vector fields of codimension k emerging from Z0 . As a consequence we prove that Z0 has infinite codimension. rp-2013-11.pdf |

10/2013 |
Ruelle Operator Duality for Coupled Smooth Markov Maps of the Circle Vincent Pit Let TL and TR be two smooth surjective Markov maps of the circle, with TR expansive, coupled in such a way that there exists an extension (C, TC ) whose first factor is TL−1 and the second factor of TC is TR . Let AL piecewise continuous and AR piecewise absolutelycontinuous be two respective potentials. We show that, when those potentials are in involutionby a smooth kernel W on C, there is an explicit isomorphism between eigenfunctions of the Ruelle operator of (TL , AL ) and eigendistributions of the Ruelle operator of (TR , AR ) for the same eigenvalue. This gives a regularity result for eigendistributions of transfer operators associated with non-maximal eigenvalues. rp-2013-10.pdf |

9/2013 |
Academic performance of students from entrance to graduation via quasi U-statistics: a study at a Brazilian research university Rafael Pimentel Maia, Hildete P. Pinheiro, Aluísio Pinheiro We present novel methodology to assess undergraduate students’ performance. The proposed methods are based on measures of diversity and on the decomposability of quasi U-statistics to define average distances between and within groups. They have been employed as an alternative to the classic analysis of variance especially when the assumption of normality is not met. The quasi U-statistics nonparametric method can handle tests for interaction and uses jackknife to get p-values for the tests. The nonparametric method also results in smaller error variances, illustrating its robustness against model misspecication. rp-2013-9.pdf |

8/2013 |
A Nonsmooth Two-Sex Population Model Eduardo Garibaldi, Marcelo Sobottka This paper considers a two-dimensional logistic model to study populations with two genders.The growth behavior of a population is guided by two coupled ordinary differential equations given by a non-differentiable vector field whose parameters are the secondary sex ratio (the ratio of males to females at time of birth), inter- and intra-gender competitions, fertility rates and a mating function. Using geometrical techniques, we analyze the singularities and the basin of attraction of the system, determining the relationships between the parameters for which the system presents an equilibrium point. In particular, we describe conditions on the secondary sex ratio and discuss the role of the median number of female sexual partners of each male for the conservation of a two-sex species. rp-2013-8.pdf |

7/2013 |
Application of prediction models using fuzzy sets: an Bayesian inspired approach Felipo Bacani, Laécio C. Barros A fuzzy inference framework based on fuzzy relations is explored and applied to a real set of simulated forecasts and experimental data referring to temperature and humidity in specific coffee crop sites in Brazil. In short, the used model consists of fuzzy relations over possibility distributions, resulting in a fuzzy model analog to a Bayesian inference process. The application of the fuzzy model to temperature and humidity data resulted in a set of revised forecasts, which were later compared to the correspondent set of experimental data using two different statistical measures of accuracy, MAPE (mean absolute percentage error) and Willmott D. Statistical results were confronted to the original simulated forecast fit to experimental data, showing that the methodology was, in most cases, able to improve the specialist’s forecasts in both statistical measures. rp-2013-7.pdf |

6/2013 |
Augmented mixed beta regression models for periodontal proportion data Diana M. Galvis, Dipankar Badyophadyay, Víctor H. Lachos Continuous (clustered) proportion data often arise in various domains of medicine andpublic health where the response variable of interest is a proportion (or percentage) quantify-ing disease status for the cluster units, ranging between zero and one. However, due to thepresence of relatively disease-free as well as highly diseased subjects in any study, the proportion values can lie in the interval [0, 1]. While Beta regression can be adapted for assessing covariate effects here, it’s versatility is often challenged due to the presence/excess of zeros and ones because the Beta support lies in the interval (0, 1). To circumvent this, we augment the probabilities of zero and one with the Beta density, controlling for the clustering effect.Our approach is Bayesian with the ability to borrow information across various stages of thecomplex model hierarchy, and produces a computationally convenient framework amenable toavailable freeware. The marginal likelihood is tractable, and can be used to develop Bayesiancase-deletion influence diagnostics based on q-divergence measures. Both simulation studiesand application to a real dataset from a clinical periodontology study quantify the gain in modelfit and parameter estimation over other adhoc alternatives, and provide quantitative insight intoassessing the true covariate effects on the proportion responses. rp-2013-6.pdf |

5/2013 |
La Construcción de la Definición Axiomática de Probabilidad Mario A. Gneri rp-2013-5.pdf |

4/2013 |
On Non-Smooth Perturbations of Degenerate or Non-Degenerate Planar Centers Douglas D. Novaes We provide sufficient conditions for the existence of limit cycles of non–smooth perturbed planar centers, when the set of discontinuity is an algebraic variety. It is introduced a mechanism which allows us to deal with such system, even in higher dimension. The main tool used in this paper is the averaging method. Two applications are given in careful detail. rp-2013-4.pdf |

3/2013 |
Generalized Linear Mixed Models for Correlated Binary Data with T-link Denise Reis Costa, Marcos O. Prates A critical issue in modelling binary response data is the choice of the links. We introducea new link based on the Student t-distribution (t-link) for correlated binary data. The t-link relates to the common probit-normal link adding one additional parameter which purely controlsthe heaviness of the tails of the link. We propose an interesting EM algorithm for computingthe maximum likelihood for generalized linear mixed t-link models for correlated binary data.In contrast with recent developments (Tan et al., 2007; Meza et al., 2009), this algorithm usesclosed-form expressions at the E-step, as opposed to Monte Carlo simulation. Our proposedalgorithm rely on available formulas for the mean and variance of a truncated multivariatet-distribution. To illustrate the new methodology, a real data set on respiratory infection inchildren and a simulation study are presented. rp-2013-3.pdf |

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