Greedy algorithms and Zipf laws

We consider a simple model of firm/city/etc. growth based on a multi-item criterion: whenever entity B fares better that entity A on a subset of $M$ items out of $K$, the agent originally in A moves to B. We solve the model analytically in the cases $K=1$ and $K \to \infty$. The resulting stationary distribution of sizes is generically a Zipf-law provided $M > K/2$. When $M \leq K/2$, no selection occurs and the size distribution remains thin-tailed. In the special case $M=K$, one needs to regularise the problem by introducing a small "default" probability $\phi$. We find that the stationary distribution has a power-law tail that becomes a Zipf-law when $\phi \to 0$. The approach to the stationary state can also been characterized, with strong similarities with a simple "aging" model considered by Barrat & M\'ezard

May's Instability in Large Economies

Will a large economy be stable? Building on Robert May's original argument for large ecosystems, we conjecture that evolutionary and behavioural forces conspire to drive the economy towards marginal stability. We study networks of firms in which inputs for production are not easily substitutable, as in several real-world supply chains. Relying on results from Random Matrix Theory, we argue that such networks generically become dysfunctional when their size increases, when the heterogeneity between firms becomes too strong or when substitutability of their production inputs is reduced. At marginal stability and for large heterogeneities, we find that the distribution of firm sizes develops a power-law tail, as observed empirically. Crises can be triggered by small idiosyncratic shocks, which lead to "avalanches" of defaults characterized by a power-law distribution of total output losses. This scenario would naturally explain the well-known "small shocks, large business cycles" puzzle, as anticipated long ago by Bak, Chen, Scheinkman and Woodford

The importance of early parenting for later child outcomes: A study with Spanish families with children with disabilities

Children with different disability conditions have varied developmental trajectories, which perhaps influences parent-child interaction. For these and all infants, an optimal home environment that includes good parenting and positive parent-child interactions predicts better developmental outcomes (Spiker et al., 2005). Interventions using a family-centered, parenting-focused approach result in better parenting behavior that contributes to improved early child development (Avellar & Suplee, 2013; Roggman & Cardia, 2016). Information on parent-child interaction patterns is needed to guide intervention in natural contexts of families (Fuligni & Brooks-Gunn, 2013). Parent-child interaction data were collected from 44 mothers and 35 fathers interacting with their children (23 to 47 months) who had a disability and were in one of eight Spanish Early Interventions Centers. Mothers and fathers, separately, auto-recorded 10-minute play sessions at home. Parental interactions were assessed using PICCOLO (Roggman et al., 2013), a standardized measure of parenting. Two independent observers coded the interactions; strong intra-class correlation coefficients were found (.82 to .97 for mothers, .77 to .89 for fathers). The BSID-III scales (Bayley, 2015) assessed children’s development. Mothers scored slightly higher than did fathers. Scores varied by PICCOLO domains but were consistent with other PICCOLO studies of typical developing children (Roggman et al., 2013). Mother and father’s scores correlated indicating that the higher the scores of mothers in any dimension correlated with those of the parents combined. Positive significant Pearson correlations were found between parenting and child’s development with different patterns of father and mother domains predicting better language and cognitive outcomes. Although barriers to optimal parenting need further investigation, PICCOLO can be a useful tool to support optimal parenting.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Modelo de planificación para acentuar la resistencia aerobia en los futbolistas juveniles

This research is directed from the State University of Milagros in Ecuador and it responds to the need to improve the aerobic resistance in youth soccer players based on a planning model that allows dosing without and with weights adapted exercises to the sport object of study, other physical abilities that complement the preparation of athletes are worked. Its objective is to elaborate a planning model to accentuate aerobic resistance in youth soccer players from alternative exercises to achieve satisfactory results in competitions. In its development, empirical and theoretical level methods were used that allowed interpreting the necessary data to prepare the proposal and corroborate the feasibility of the proposed model.La presente investigación está dirigida desde la Universidad Estatal de Milagros en Ecuador y responde a la necesidad de mejorar la resistencia aerobia en los futbolistas juveniles a partir de un modelo de planificación que permita dosificar sin y con pesas ejercicios adaptados al deporte objeto de estudio, se trabajan otras capacidades físicas que complementan la preparación de los atletas. Su objetivo consiste en elaborar un modelo de planificación para acentuar la resistencia aerobia en los futbolistas juveniles a partir de ejercicios alternativos para lograr resultados satisfactorios en competencias. En el desarrollo de la misma se utilizaron métodos del nivel empírico y teórico que permitieron interpretar los datos necesarios para elaborar la propuesta y corroborar la factibilidad del modelo propuesto

Out-of-Equilibrium Dynamics and Excess Volatility in Firm Networks

We study the conditions under which input-output networks can dynamically attain a competitive equilibrium, where markets clear and profits are zero. We endow a classical firm network model with minimal dynamical rules that reduce supply/demand imbalances and excess profits. We show that the time needed to reach equilibrium diverges to infinity as the system approaches an instability point beyond which the Hawkins-Simons condition is violated and competitive equilibrium is no longer admissible. We argue that such slow dynamics is a source of excess volatility, through accumulation and amplification of exogenous shocks. Factoring in essential physical constraints absent in our minimal model, such as causality or inventory management, we then propose a dynamically consistent model that displays a rich variety of phenomena. Competitive equilibrium can only be reached after some time and within some restricted region of parameter space, outside of which one observes spontaneous periodic and chaotic dynamics, reminiscent of real business cycles. This suggests an alternative explanation of excess volatility in terms of purely endogenous fluctuations. Diminishing return to scale and increased perishability of goods are found to ease convergence towards equilibrium.Comment: 59 pages, 20 figure

Enfermedades de la aorta y su tratamiento con endoprótesis aórticas

El aneurisma aórtico es una entidad de diagnóstico cada vez más frecuente y con unas indicaciones claras de reparación por su mala evolución en ciertos casos. El presente estudio descriptivo, de serie de casos, expone la experiencia en la utilización de endoprótesis aórticas para el tratamiento de aneurismas en 13 pacientes, intervenidos desde hace diez años en nuestro centro hospitalario. Los resultados obtenidos han sido satisfactorios, sobre todo al considerar la importante comorbilidad que presentaba la mayoría de pacientes. No se ha producido mortalidad tardía por ruptura del aneurisma ni se ha precisado conversión al procedimiento abierto en ningún caso. Pese a las limitaciones derivadas del tamaño muestral, los resultados obtenidos permiten plantear una ampliación de las indicaciones, muy restrictiva hasta ahora.Aortic aneurysm is a clinical entity with very frecuent diagnosis and clear repair indications due to bad evolution in many cases. This descriptive study presents the experience in using aortic endoprosthesys for aneurysm treatment in 13 patients, operated in our institution since 10 years ago. The results have been satisfactory, mainly due to the extreme comorbidity that was present in most of them. There has not been late mortality due to aneurysm rupture, nor late open conversion . Although the numbers are small, our good results allow to consider expansion of the indications for this procedure, very restrictive at this moment

Quasi-switched inverter using space vector pulse width modulation with triangular comparison for photovoltaic applications

Este trabajo analiza un prototipo para un inversor elevador cuasi-conmutado (qSBI) alimentando una carga resistiva aislada desde una fuente de CC. Se propone el uso de una modulación de ancho de pulso de vectores espaciales (SPWM) con comparación triangular que genera un incremento en el factor de ganancia del qSBI, y se contrasta su desempeño con otro tipo de modulaciones de vectores espaciales, tales como las modulaciones discontinuas. Para verificar la validez de la extensión de rango de tensión en el convertidor qSBI, se desarrolló una plataforma de pruebas semi-personalizada. Esta plataforma utiliza una tarjeta DSP de punto flotante (Analog Devices ADSP-21369) para el procesamiento de las estrategias de control, y una tarjeta de interfaz que incluye un arreglo lógico programable (FPGA) de Xilinx (Spartan-3), que permite desarrollar la modulación sincronizada que el qSBI necesita. Los resultados experimentales demuestran mejoras en el desempeño del convertidor qSBI en cuanto al factor de ganancia, reducción del estrés de voltaje en el capacitor y los perfiles de corriente de entrada. Las estrategias discontinuas de modulación del vector espacial no presentan un buen desempeño cuando se compara con las modulaciones continuas SVPWM o SPWM, ya que los niveles de rizado en las corrientes tomadas del módulo PV son de aproximadamente el doble que en el caso de las técnicas de modulación continuas. Finalmente, el uso del convertidor qSBI como microinversor es puesto en evidencia por dos casos experimentales prácticos de un sistema fotovoltaico PV con un algoritmo de ajuste del máximo punto de potencia (MPPT).This work analyzes a prototype of a quasi-switched boost inverter (qSBI) feeding an isolated resistive load from a DC source. The use of spatial vector pulse width modulation (SPWM) with triangular comparison is proposed to increase the qSBI gain factor, and its performance is contrasted with other types of spatial vector modulations, such as discontinuous modulations. To verify the validity of the method for voltage range extension in the qSBI converter, a semi-customized test platform was developed. This platform uses a DSP floating point card (Analog Devices ADSP-21369) for processing and control strategies and an interface card that includes a programmable logic array (FPGA) from Xilinx (Spartan-3), which allows to develop the synchronized modulation qSBI needs. The experimental results show improvements in the performance of the qSBI converter in terms of gain factor, voltage reduction in the capacitor, and input current profiles. Discontinuous space vector modulation strategies do not perform well when compared to continuous SVPWM or SPWM modulations, because the ripple levels in the currents taken from the PV module are approximately twice as great as in continuous modulation techniques. Finally, the usefulness of a qSBI as PV microinverter is confirmed by two practical experimental cases of a PV photovoltaic system with a maximum power point adjustment algorithm (MPPT)

O ensino superior a distância no Brasil

Brazil has effectively implemented distance learning courses only after the promulgation of the Directives and Bases for National Education Law (LDB) of 1996. We moved from a more experimental phase – learning and testing some different models – to the present phase of consolidation and greater regulation by the Ministry of Education. The predominant models are teleclasses, video classes and WEB, with a greater or lesser local support. There are private and public institutions acting in undergraduate and postgraduate programs. The public ones articulate themselves in the UAB (Open University of Brazil) – an articulation among state and federal universities, federal institutes of education, science, and technology (Ifets), with hundreds of distance learning programs in major, bachelor, technical, specialization or extension courses. The present laws in Brazil privilege the semi-presence courses, with the students studying near their homes (in poles) and shows some distrust of the on-line model, mainly in the undergraduate courses. The distance learning system is becoming an important reference for a deep change in higher education, moving from a complementary or special modality to specific situations. The distance learning system shall use more the semi-presence methodologies, flexibilizing the need for the physical presence, reorganizing the spaces and the time for teaching and learning.Brasil sólo después de la LDB – en 1996 – implantó efectivamente carreras superiores a distancia. Pasamos de una etapa más experimental – aprendiendo y probando algunos modelos diferentes – a la fase actual de consolidación y de mayor regulación por parte del Ministerio de Educación. Los modelos predominantes son los de teleclase, videoclase y WEB con mayor o menor apoyo local. Hay instituciones particulares y públicas actuando en la graduación y postgrado. Las públicas se articulan en la UAB – una articulación entre universidades estaduales, federales e institutos federales de educación, ciencia y tecnología (Ifets), con centenas de carreras superiores a distancia de licenciatura, bachillerato, cursos tecnológicos, especialización y extensión. La legislación actual en Brasil da prioridad al modelo semipresencial, con acompañamiento de los alumnos en las proximidades de donde viven (en polos) y muestra desconfianza por el modelo de acompañamiento on-line, principalmente en carreras del nivel universitario de graduación. La educación a distancia está transformándose, de una modalidad complementar o especial para situaciones específicas, en referencia importante para un cambio profundo de enseñanza superior como un todo. Este utilizará cada vez más metodologías semipresenciales, flexibilizando la necesidad de presencia física, reorganizando los espacios y tiempos de enseñanza y aprendizaje.Somente depois da LDB de 1996 o Brasil implantou efetivamente cursos superiores a distância. Passamos de uma fase mais experimental – aprendendo e testando alguns modelos diferentes – à fase atual de consolidação e de maior regulação por parte do Ministério da Educação. Os modelos predominantes são os de teleaula, videoaula e WEB, com maior ou menor apoio local. Temos instituições particulares e públicas atuando na graduação e pós-graduação. As públicas se articulam na Universidade Aberta do Brasil (UAB) – uma articulação entre universidades estaduais, federais e institutos federais de educação, ciência e tecnologia (Ifets), com centenas de cursos superiores a distância de licenciatura, bacharelado, tecnológicos, especialização e extensão. A legislação atual no Brasil privilegia o modelo semipresencial, com acompanhamento dos alunos perto de onde moram (em polos) e mostra desconfiança pelo modelo de acompanhamento on-line, principalmente em cursos de graduação. A educação a distância está se transformando de uma modalidade complementar ou especial para situações específicas em referência importante para uma mudança profunda do ensino superior como um todo. O ensino superior utilizará cada vez mais metodologias semipresenciais, flexibilizando a necessidade de presença física, reorganizando os espaços e tempos de ensino e aprendizagem.

Como utilizar a Internet na educação

Relato e análise de experiências pessoais e institucionais que utilizam a Internet na educação presencial como pesquisa, apoio ao ensino e como comunicação. Avalia os avanços e problemas que estão acontecendo atualmente e mostra que a Internet é mais eficaz, quando está inserida em processos de ensino-aprendizagem e de comunicação que integram as dimensões pessoais, as comunitárias e as tecnológicas

Modelos e avaliação do ensino superior a distância no Brasil

O Brasil se encontra em uma fase de consolidação da educação a distância em todos os setores e níveis de ensino. Depois de uma fase de experimentação, onde houve uma aprendizagem intensa e busca de modelos mais adequados para cada instituição, nos encontramos em uma fase de amadurecimento, de maior regulação governamental, de maior cuidado com o crescimento, a infra-estrutura, a metodologia de ensino, a avaliação. Os modelos predominantes são os de teleaula, videoaula e WEB com maior ou menor apoio local. A legislação atual no Brasil privilegia o modelo semi-presencial, com acompanhamento dos alunos perto de onde moram (em pólos) e mostra desconfiança pelo modelo de acompanhamento online, principalmente em cursos de graduação. A educação a distância está se transformando, de uma modalidade complementar ou especial para situações específicas, em referência importante para uma mudança profunda do ensino superior como um todo. Este utilizará cada vez mais metodologias semi-presenciais, flexibilizando a necessidade de presença física, reorganizando os espaços e tempos de ensino e aprendizagem.Brazil is in a consolidation phase of distance education in all sectors and levels of education. After a phase of experimentation, where there was an intense learning and searching for more appropriate models for each institution, we are in a phase of maturity, more government regulation, more concern about growth, infrastructure, methodology of teaching and learning. The main models are "tele-class" (class satellite), online, video-class with more or less local support. A current legislation in Brazil favors the bimodal model, with close monitoring of students and shows the pattern of mistrust the online monitoring, especially in undergraduate courses. Distance education is becoming an important reference for a change of higher education as a whole. It uses increasingly hybrid methodologies, making more flexible the need for physical presence, rearranging the spaces and times of teaching and learning