Dynamics and Structure of the 30 Largest North American Companies

In this paper we describe a method to analyze the structure and dynamics of the 30 largest North American companies. The method combines the tools of symbolic time series analysis (Daw et al. in Rev Sci Instrum 74:916–930, 2003) with the nearest neighbor single linkage clustering algorithm (Mantegna and Stanley in An introduction to econophysics: Correlations and complexity in finance, Cambridge University Press, UK, 2000). Data symbolization allows to obtain a metric distance between two different time series that is used to construct a minimal spanning tree allowing to compute an ultrametric distance. From the analysis of time series data of companies included in Dow Jones Industrial Average, we derive a hierarchical organization of these companies. In particular, we detect different clusters of companies which correspond with their common production activities or their strong interrelationship. The obtained classification of companies can be used to study deep relationships among different branch of economic activities and to construct financial portfolios.     

On 1-Laplacian Elliptic Equations Modeling Magnetic Resonance Image Rician Denoising

Modeling magnitude magnetic resonance images (MRI) Rician denoising in a Bayesian or generalized Tikhonov framework using total variation (TV) leads naturally to the consideration of nonlinear elliptic equations. These involve the so called 1-Laplacian operator and special care is needed to properly formulate the problem. The Rician statistics of the data are introduced through a singular equation with a reaction term defined in terms of modified first-order Bessel functions. An existence theory is provided here together with other qualitative properties of the solutions. Remarkably, each positive global minimum of the associated functional is one of such solutions. Moreover, we directly solve this nonsmooth nonconvex minimization problem using a convergent Proximal Point Algorithm. Numerical results based on synthetic and real MRI demonstrate a better performance of the proposed method when compared to previous TV-based models for Rician denoising which regularize or convexify the problem. Finally, an application on real Diffusion Tensor Images, a strongly affected by Rician noise MRI modality, is presented and discussed.     

A discontinuous‐Galerkin‐based immersed boundary method

A numerical method to approximate partial differential equations on meshes that do not conform to the domain boundaries is introduced. The proposed method is conceptually simple and free of user‐defined parameters. Starting with a conforming finite element mesh, the key ingredient is to switch those elements intersected by the Dirichlet boundary to a discontinuous‐Galerkin approximation and impose the Dirichlet boundary conditions strongly. By virtue of relaxing the continuity constraint at those elements, boundary locking is avoided and optimal‐order convergence is achieved. This is shown through numerical experiments in reaction–diffusion problems. Copyright © 2008 John Wiley & Sons, Ltd.     

Structures, Mechanisms, and Kinetics of Selective Ammoxidation and Oxidation of Propane over Multi-metal Oxide Catalysts

In order to determine the chemical mechanism for the (amm)oxidation of propane and propene on multi-metal oxide (MMO) catalysts, we have carried out quantum mechanical (QM) calculations for model reactions on small clusters that we have used to train the parameters for the ReaxFF reactive force field, which enables molecular dynamics (MD) simulations for reactions on the complex reconstructed surfaces of MMO. We report here insights from the QM on the reaction mechanisms of selective (amm)oxidation of propene on bismuth molybdate catalysts and the oxidative dehydrogenation of propane on vanadium oxide catalysts. We also report the application of ReaxFF to predict the stable surfaces of the M1 phases of the MoVTeNbO catalysts.     

Control of Polymorphism and Morphology in Solution Sheared Organic Field‐Effect Transistors

During the last decades, small molecule organic semiconductors have been successfully used as active layer in organic field‐effect transistors (OFETs). Despite the high mobility achieved so far with organic molecules, in order to progress in the field it is crucial to find techniques to process them from solution. The device reproducibility is one of the principal weak points of organic electronics for further commercialization. To achieve a high device‐to‐device reproducibility it is essential to control the morphology and polymorphism of the active layer for OFET application. In this work, the preparation of thin films is reported based on blends of the organic semiconductor dibenzo‐tetrathiafulvalene (DB‐TTF) and polystyrene by a solution shearing technique compatible with upscaling. Here, it is demonstrated that varying the deposition parameters (i.e., speed and temperature) or the solution formulation (i.e., semiconductor/binder polymer ratio) is possible to control the film morphology and semiconductor polymorphism and, hence, the different intermolecular interactions. It is demonstrated that the control of the thermodynamics and kinetics of the crystallization process is key for the device performance optimization. Further, this is the first time that DB‐TTF thin films of the α‐polymorph are reported.     

On the stability of the troposphere/lower stratosphere and its relationships with cirrus clouds and three mandatory levels over Buenos Aires

An unrotated principal components analysis was carried out to establish the most representative modes for the joint variability between the heights of the upper and lower boundaries of cirrus clouds and three different mandatory levels (850, 500, and 100 hPa), and the associated stability of the troposphere over Buenos Aires. Discussion is limited to the first three most representative structures found, which consists of spatial patterns (or empirical orthogonal functions, EOFs) and their time-evolving coefficients (or principal components, PCs). EOF₁ shows a direct (indirect) mode that encompasses the cirrus slightly below (above) its mean position, with 500 and 100 hPa exhibiting a similar behaviour and 850 hPa acting the opposite way. EOF₁ is associated with above-normal stability (instability) for direct (indirect) modes (i.e. positive (negative) values of PC₁). On a monthly average, this occurs in the austral winter (summer) months. Regarding EOF₂, all three mandatory levels experience positive (negative) height anomalies in direct (indirect) modes and cirrus goes up (down) under mild stability (instability). Monthly averages show that PC₂ is approximately positive in summer and in early fall and negative the rest of the year. As to EOF₃, it is characterized by a stability similar to that of EOF₂, with direct (indirect) modes showing lowered (raised) cirrus and all three mandatory levels above (below) normal conditions; on a monthly basis, PC₃ is best described as having a semi-annual evolution, with maxima (minima) in March and October (January and August). Overall, EOF₁ has the highest stability or instability, depending upon the sign of PC₁. These results are the first of their kind worldwide.     

IDEWEP: Web service for astronomical parallel image deconvolution

Image restoration is a significant process commonly applied in many research fields. In particular, image deconvolution algorithms play a very important role in the research methodology in astrophysics, where recorded images are frequently submitted to deconvolution processes. In this paper, we introduce a novel image deconvolution algorithm that is competitive in terms of the restored image quality when compared to classical approaches. We present parallelizations of this algorithm to make it competitive in terms of processing speeds as well. We also present the image deconvolution web portal (IDEWEP) that, using web services technologies, primarily aims to make this general parallel deconvolution method accessible through a web interface. Both the quality of the restored images and running times of the sequential and parallel version have been successfully tested in several sequential and parallel architectures. The IDEWEP portal greatly eases the access and use of the parallel algorithm in high performance architectures. As a contribution to the scientific community, open source sequential and parallel codes are provided and can be freely downloaded from our web portal.     

On the mobile‐to‐mobile linear time‐variant shallow‐water acoustic channel response

We expose some concepts concerning the channel impulse response (CIR) of linear time‐varying (LTV) channels to give a proper characterization of the mobile‐to‐mobile underwater channel. We find different connections between the linear time‐invariant (LTI) CIR of the static channel and 2 definitions of LTV CIRs of the dynamic mobile‐to‐mobile channel. These connections are useful to design a dynamic channel simulator from the static channel models available in the literature. Such feature is particularly interesting for overspread channels, which are hard to characterize by a measuring campaign. Specifically, the shallow water acoustic (SWA) channel is potentially overspread because of the signal low velocity of propagation, which prompts long delay spread responses and great Doppler effect. Furthermore, from these connections between the LTI static CIRs and the LTV dynamic CIRs, we find that the SWA mobile‐to‐mobile CIR does not only depend on the relative speed between transceivers, but also on the absolute speed of each of them referred to the velocity of propagation. Nevertheless, publications about this topic do not consider it and formulate their equations in terms of the relative speed between transceivers. We illustrate our find using 2 couples of examples where, even though the relative speed between the mobiles is the same, their CIRs are not.     

PyDTNN: A user-friendly and extensible framework for distributed deep learning

The Journal of Supercomputing - We introduce a framework for training deep neural networks on clusters of computers with the following appealing properties: (1) It is developed in Python, exposing...