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Search Results: 1 - 10 of 559489 matches for " J. Andrés Christen "
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A Generic Multivariate Distribution for Counting Data
Marcos Capistrán,J. Andrés Christen
Statistics , 2011,
Abstract: Motivated by the need, in some Bayesian likelihood free inference problems, of imputing a multivariate counting distribution based on its vector of means and variance-covariance matrix, we define a generic multivariate discrete distribution. Based on blending the Binomial, Poisson and Negative-Binomial distributions, and using a normal multivariate copula, the required distribution is defined. This distribution tends to the Multivariate Normal for large counts and has an approximate pmf version that is quite simple to evaluate.
Bayesian Analysis of ODE's: solver optimal accuracy and Bayes factors
Marcos Capistrán,J. Andrés Christen,Sophie Donnet
Statistics , 2013,
Abstract: In most relevant cases in the Bayesian analysis of ODE inverse problems, a numerical solver needs to be used. Therefore, we cannot work with the exact theoretical posterior distribution but only with an approximate posterior deriving from the error in the numerical solver. To compare a numerical and the theoretical posterior distributions we propose to use Bayes Factors (BF), considering both of them as models for the data at hand. We prove that the theoretical vs a numerical posterior BF tends to 1, in the same order (of the step size used) as the numerical forward map solver does. For higher order solvers (eg. Runge-Kutta) the Bayes Factor is already nearly 1 for step sizes that would take far less computational effort. Considerable CPU time may be saved by using coarser solvers that nevertheless produce practically error free posteriors. Two examples are presented where nearly 90% CPU time is saved while all inference results are identical to using a solver with a much finer time step.
An analysis of the interaction between influenza and respiratory syncytial virus based on acute respiratory infection records
Yendry N. Arguedas-Flatts,Marcos A. Capistrán,J. Andrés Christen,Daniel E. Noyola
Quantitative Biology , 2013,
Abstract: Under the hypothesis that both influenza and respiratory syncytial virus (RSV) are the two leading causes of acute respiratory infections (ARI), in this paper we have used a standard two-pathogen epidemic model as a regressor to explain, on a yearly basis, high season ARI data in terms of the contact rates and initial conditions of the mathematical model. The rationale is that ARI high season is a transient regime of a noisy system, e.g., the system is driven away from equilibrium every year by fluctuations in variables such as humidity, temperature, viral mutations and human behavior. Using the value of the replacement number as a phenotypic trait associated to fitness, we provide evidence that influenza and RSV coexists throughout the ARI high season through superinfection.
Towards Uncertainty Quantification and Inference in the stochastic SIR Epidemic Model
Marcos A. Capistrán,J. Andrés Christen,Jorge X. Velasco-Hernández
Statistics , 2011,
Abstract: In this paper we introduce a novel method to conduct inference with models defined through a continuous-time Markov process, and we apply these results to a classical stochastic SIR model as a case study. Using the inverse-size expansion of van Kampen we obtain approximations for first and second moments for the state variables. These approximate moments are in turn matched to the moments of an inputed generic discrete distribution aimed at generating an approximate likelihood that is valid both for low count or high count data. We conduct a full Bayesian inference to estimate epidemic parameters using informative priors. Excellent estimations and predictions are obtained both in a synthetic data scenario and in two Dengue fever case studies.
On optimal direction gibbs sampling
J. Andrés Christen,Colin Fox,Diego Andrés Pérez-Ruiz,Mario Santana-Cibrian
Statistics , 2012,
Abstract: Generalized Gibbs kernels are those that may take any direction not necessarily bounded to each axis along the parameters of the objective function. We study how to optimally choose such directions in a Directional, random scan, Gibbs sampler setting. The optimal direction is chosen by minimizing to the mutual information (Kullback-Leibler divergence) of two steps of the MCMC for a truncated Normal objective function. The result is generalized to be used when a Multivariate Normal (local) approximation is available for the objective function. Three Gibbs direction distributions are tested in highly skewed non-normal objective functions.
On the Origin of the Common Bean (Phaseolus vulgaris L.)  [PDF]
Andrés J. Cortés
American Journal of Plant Sciences (AJPS) , 2013, DOI: 10.4236/ajps.2013.410248

Phylogeographic methods provide the tools to accurately access the geographic origin and diversification of crop species. In the present commentary, I urge the common bean community to face those methods and a tree-thinking mentality with regards to the long standing debate of the origin of common bean. Such efforts will ultimately bring back interest into wild bean studies and reinforce the uniqueness of this species as a system to study diversification, domestication and adaptive processes across the two most diverse hotspots in the world.

Stress-induced R-MA-MC-T symmetry changes in BiFeO3 films
J. H. Nam,H. S. Kim,A. J. Hatt,N. A. Spaldin,H. M. Christen
Physics , 2010, DOI: 10.1103/PhysRevB.83.144107
Abstract: The recent discovery of a stress-induced structural phase transition in the single-component perovskite BiFeO3 has revived interest in this lead-free ferroelectric. The coexistence of different phases may lead to large piezoelectric coefficients, a property typically associated with complex solid solutions of lead-based perovskites. Here we report combined experimental and computational results showing that the stress-induced phase transitions in BiFeO3 follow the path of rhombohedral(R)-to-monoclinic(MA)-to-monoclinic(MC)-to-tetragonal(T), where both MC and T show highly enhanced c/a ratios. This R-MA-MC-T sequence is otherwise observed only near morphotropic phase boundaries (MPBs) in lead-based perovskites (i.e. near a compositionally induced phase instability), where it is controlled by electric field, temperature, or composition. Our results represent the first time that this evolution has been induced in a single component system using strain alone, and show that substrate- imposed symmetry lowering results in a similar phase instability as the proximity to a MPB in solid solutions.
Primer registro de Taxidea taxus berlandieri Baird, 1858 (Mammalia: Carnivora: Mustelidae) para el Estado de Veracruz, México
A. González Christen,A. González Romero,J. S. Rodríguez Colmenares
Acta zoológica mexicana , 2006,
Abstract: Examines the first record of the badger in the state of Veracruz in 1858 by Baird. The nearest known locality for this species outside of Veracruz is the state of Puebla, 80 kilometres in a straight line southwest. Standard and skull morphometric measures of the specimen are included, as well as data about the animal s habitat. The badger is protected in Mexico as an endangered species under the NOM-059-SEMARNAT-2001.
El teorema de Cayley revisitado
Revista Colombiana de Matemáticas , 2009,
Abstract: en este artículo probamos que no existe un n∈ n tal que todo grupo finito puede ser embebido en gl\mathbbc( n).
El teorema de Cayley revisitado The theorem of Cayley revisited
Revista Colombiana de Matemáticas , 2009,
Abstract: En este artículo probamos que no existe un N∈ N tal que todo grupo finito puede ser embebido en GLmathbbC( N). In this paper we prove that there not exists N∈ N such that any finite group can be embbeded into GLmathbbC( N).
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