A Poisson Mixed Model with Nonnormal Random Effect Distribution

We propose in this paper a random intercept Poisson model in which the random effect distribution is assumed to follow a generalized log-gamma (GLG) distribution. We derive the first two moments for the marginal distribution as well as the intraclass correlation. Even though numerical integration methods are in general required for deriving the marginal models, we obtain the multivariate negative binomial model for a particular parameter setting of the hierarchical model. An iterative process is derived for obtaining the maximum likelihood estimates for the parameters in the multivariate negative binomial model. Residual analysis are proposed and two applications with real data are given for illustration.Comment: Submitted in the Computational Statistics & Data Analysis journa

The Privatization and Project Finance Adventure: Acquiring a Colombian Public Utility Company

Colombia is an exciting new arena for the international practitioner structuring project finance and privatization transactions. The sectors of the economy that are attractive targets for most multinationals (MNCs) entering Colombia are the energy, oil and gas, mining, water, and telecommunications sectors. In most cases, MNCs in these sectors consider investments in Colombia when the Colombian authorities announce a plan for the privatization or capitalization of state-run companies that have a monopoly on business. Often the MNC becomes aware of the privatization, concession, or capitalization project via an official announcement by the Colombian government calling for bids or an invitation informing the MNC that it is qualified for the particular project or a specific aspect of the project. When a client is interested in participating in the bid process, it will most certainly, unless it has participated in other bids in Colombia, approach counsel and its other advisors with assumptions of how the bid should be structured, from both a Colombian and U.S. angle. In these instances, the only option for counsel and other advisors is to listen and take note. The following key questions must be considered under this scenario: what type of restrictions exist under current privatization laws; what form of business association should be used; what is the tax treatment regarding business associations, dividend treatment, technical services and assistance contracts, and repatriation of profits; do foreign exchange regulations hinder the availability of using inter-company loans and foreign currency operations; how should the due diligence review be performed; and how should local counsel be chosen

The Privatization and Project Finance Adventure: Acquiring a Colombian Public Utility Company

Colombia is an exciting new arena for the international practitioner structuring project finance and privatization transactions. The sectors of the economy that are attractive targets for most multinationals (MNCs) entering Colombia are the energy, oil and gas, mining, water, and telecommunications sectors. In most cases, MNCs in these sectors consider investments in Colombia when the Colombian authorities announce a plan for the privatization or capitalization of state-run companies that have a monopoly on business. Often the MNC becomes aware of the privatization, concession, or capitalization project via an official announcement by the Colombian government calling for bids or an invitation informing the MNC that it is qualified for the particular project or a specific aspect of the project. When a client is interested in participating in the bid process, it will most certainly, unless it has participated in other bids in Colombia, approach counsel and its other advisors with assumptions of how the bid should be structured, from both a Colombian and U.S. angle. In these instances, the only option for counsel and other advisors is to listen and take note. The following key questions must be considered under this scenario: what type of restrictions exist under current privatization laws; what form of business association should be used; what is the tax treatment regarding business associations, dividend treatment, technical services and assistance contracts, and repatriation of profits; do foreign exchange regulations hinder the availability of using inter-company loans and foreign currency operations; how should the due diligence review be performed; and how should local counsel be chosen

Pseudospectral versus finite-differences schemes in the numerical integration of stochastic models of surface growth

We present a comparison between finite differences schemes and a pseudospectral method applied to the numerical integration of stochastic partial differential equations that model surface growth. We have studied, in 1+1 dimensions, the Kardar, Parisi and Zhang model (KPZ) and the Lai, Das Sarma and Villain model (LDV). The pseudospectral method appears to be the most stable for a given time step for both models. This means that the time up to which we can follow the temporal evolution of a given system is larger for the pseudospectral method. Moreover, for the KPZ model, a pseudospectral scheme gives results closer to the predictions of the continuum model than those obtained through finite difference methods. On the other hand, some numerical instabilities appearing with finite difference methods for the LDV model are absent when a pseudospectral integration is performed. These numerical instabilities give rise to an approximate multiscaling observed in the numerical simulations. With the pseudospectral approach no multiscaling is seen in agreement with the continuum model.Comment: 13 single column pages, RevTeX, 6 eps fig

Scaling of Local Slopes, Conservation Laws and Anomalous Roughening in Surface Growth

We argue that symmetries and conservation laws greatly restrict the form of the terms entering the long wavelength description of growth models exhibiting anomalous roughening. This is exploited to show by dynamic renormalization group arguments that intrinsic anomalous roughening cannot occur in local growth models. However some conserved dynamics may display super-roughening if a given type of terms are present.Comment: To appear in Phys. Rev. Lett., 4 pages in RevTeX style, no fig

Predictability: Can the turning point and end of an expanding epidemic be precisely forecast?

No, they can't. Epidemic spread is characterized by exponentially growing dynamics, which are intrinsically unpredictable. The time at which the growth in the number of infected individuals halts and starts decreasing cannot be calculated with certainty before the turning point is actually attained; neither can the end of the epidemic after the turning point. An SIR model with confinement (SCIR) illustrates how lockdown measures inhibit infection spread only above a threshold that we calculate. The existence of that threshold has major effects in predictability: A Bayesian fit to the COVID-19 pandemic in Spain shows that a slow-down in the number of newly infected individuals during the expansion phase allows to infer neither the precise position of the maximum nor whether the measures taken will bring the propagation to the inhibition regime. There is a short horizon for reliable prediction, followed by a dispersion of the possible trajectories that grows extremely fast. The impossibility to predict in the mid-term is not due to wrong or incomplete data, since it persists in error-free, synthetically produced data sets, and does not necessarily improve by using larger data sets. Our study warns against precise forecasts of the evolution of epidemics based on mean-field, effective or phenomenological models, and supports that only probabilities of different outcomes can be confidently given.Comment: 13 pages, 4 figure + Supplementary Informatio

Self-Organized Ordering of Nanostructures Produced by Ion-Beam Sputtering

We study the self-organized ordering of nanostructures produced by ion-beam sputtering (IBS) of targets amorphizing under irradiation. By introducing a model akin to models of pattern formation in aeolian sand dunes, we extend consistently the current continuum theory of erosion by IBS. We obtain new non-linear effects responsible for the in-plane ordering of the structures, whose strength correlates with the degree of ordering found in experiments. Our results highlight the importance of redeposition and surface viscous flow to this nanopattern formation process.Comment: 4 pages, 2 figure

2b-RAD genotyping for population genomic studies of Chagas disease vectors: Rhodnius ecuadoriensis in Ecuador

Background: Rhodnius ecuadoriensis is the main triatomine vector of Chagas disease, American trypanosomiasis, in Southern Ecuador and Northern Peru. Genomic approaches and next generation sequencing technologies have become powerful tools for investigating population diversity and structure which is a key consideration for vector control. Here we assess the effectiveness of three different 2b restriction site-associated DNA (2b-RAD) genotyping strategies in R. ecuadoriensis to provide sufficient genomic resolution to tease apart microevolutionary processes and undertake some pilot population genomic analyses. Methodology/Principal findings: The 2b-RAD protocol was carried out in-house at a non-specialized laboratory using 20 R. ecuadoriensis adults collected from the central coast and southern Andean region of Ecuador, from June 2006 to July 2013. 2b-RAD sequencing data was performed on an Illumina MiSeq instrument and analyzed with the STACKS de novo pipeline for loci assembly and Single Nucleotide Polymorphism (SNP) discovery. Preliminary population genomic analyses (global AMOVA and Bayesian clustering) were implemented. Our results showed that the 2b-RAD genotyping protocol is effective for R. ecuadoriensis and likely for other triatomine species. However, only BcgI and CspCI restriction enzymes provided a number of markers suitable for population genomic analysis at the read depth we generated. Our preliminary genomic analyses detected a signal of genetic structuring across the study area. Conclusions/Significance: Our findings suggest that 2b-RAD genotyping is both a cost effective and methodologically simple approach for generating high resolution genomic data for Chagas disease vectors with the power to distinguish between different vector populations at epidemiologically relevant scales. As such, 2b-RAD represents a powerful tool in the hands of medical entomologists with limited access to specialized molecular biological equipment. Author summary: Understanding Chagas disease vector (triatomine) population dispersal is key for the design of control measures tailored for the epidemiological situation of a particular region. In Ecuador, Rhodnius ecuadoriensis is a cause of concern for Chagas disease transmission, since it is widely distributed from the central coast to southern Ecuador. Here, a genome-wide sequencing (2b-RAD) approach was performed in 20 specimens from four communities from Manabí (central coast) and Loja (southern) provinces of Ecuador, and the effectiveness of three type IIB restriction enzymes was assessed. The findings of this study show that this genotyping methodology is cost effective in R. ecuadoriensis and likely in other triatomine species. In addition, preliminary population genomic analysis results detected a signal of population structure among geographically distinct communities and genetic variability within communities. As such, 2b-RAD shows significant promise as a relatively low-tech solution for determination of vector population genomics, dynamics, and spread