A renormalization group model for the stick-slip behavior of faults

A fault which is treated as an array of asperities with a prescribed statistical distribution of strengths is described. For a linear array the stress is transferred to a single adjacent asperity and for a two dimensional array to three ajacent asperities. It is shown that the solutions bifurcate at a critical applied stress. At stresses less than the critical stress virtually no asperities fail on a large scale and the fault is locked. At the critical stress the solution bifurcates and asperity failure cascades away from the nucleus of failure. It is found that the stick slip behavior of most faults can be attributed to the distribution of asperities on the fault. The observation of stick slip behavior on faults rather than stable sliding, why the observed level of seismicity on a locked fault is very small, and why the stress on a fault is less than that predicted by a standard value of the coefficient of friction are outlined

Utilización de metil jasmonato para la inducción de resistencia ante Fusarium oxysporum, F. circinatum y Ophiostoma novo-ulmi

Damping off is probably the most common disease affecting seedlings in forest nurseries. In south-western Europe, the pitch canker and the Dutch elm disease cause relevant economic looses in forests, mostly in adult trees. The ability of the chemical plant elicitor methyl jasmonate (MeJA) to induce resistance in Pinus pinaster against Fusarium oxysporum and F. circinatum, and in Ulmus minor against Ophiostoma novo-ulmi was examined. In a first experiment, an aqueous solution of MeJA 5 mM was applied to P. pinaster seeds by immersion or spray, and different concentrations of MeJA (0, 0.1, 0.5, 1, 5 and 10 mM) were tested in seedlings before inoculations with F. oxysporum (105 and 107 spores mL–1). In a second experiment, 6-months-old P. pinaster seedlings were sprayed with 0 and 25 mM of MeJA, and later challenged with mycelium of F. circinatum. Finally, 4-year-old U. minor trees were sprayed with 0, 50 and 100 mM of MeJA and subsequently inoculated with O. novo-ulmi (106 spores mL–1). MeJA did not protect P. pinaster seeds and seedlings against F. oxysporum, probably because plants were too young for the physiological mechanisms responsible for resistance to be induced. Based on the morphological changes observed in the treated 6-months-old P. pinaster seedlings (reduction of growth and increased resin duct density), there is evidence that MeJA could have activated the mechanisms of resistance. However, 25 mM MeJA did not reduce plant mortality, probably because the spread of the virulent F. circinatum strain within the tree tissues was faster than the formation of effective defense responses. Based on the lack of phenological changes observed in the treated elms, there is no evidence that MeJA would cause induction of resistance. These results suggest that the use of MeJA to prevent F. oxysporum and F. circinatum in P. pinaster seedlings in nurseries and O. novo-ulmi in U. minor trees should be discarded.El “damping off” es una de las enfermedades más comunes en los viveros forestales. En árboles adultos del suroeste de Europa, el chancro resinoso y la grafiosis del olmo son enfermedades que están causando importantes pérdidas económicas en los bosques. Se ha estudiado la capacidad del metil jasmonato (MeJA), un elicitador químico de plantas, para inducir resistencia en Pinus pinaster ante Fusarium oxysporum y F. circinatum, y en Ulmus minor ante Ophiostoma novo-ulmi. En un primer experimento se aplicó una solución acuosa de MeJA 5 mM a semillas de P. pinaster mediante inmersión o pulverización de las mismas, y diferentes concentraciones de MeJA (0, 0.1, 0.5, 1, 5 and 10 mM) fueron pulverizadas en plántulas de P. pinaster antes de las inoculaciones con F. oxysporum (105 y 107 esporas mL–1). En un segundo experimento, plántulas de P. pinaster de 6 meses de edad fueron pulverizadas con MeJA 0 y 25 mM, y posteriormente inoculadas con micelio de F. circinatum. Por último, brinzales de U. minor de 4 años de edad fueron pulverizados con MeJA a 0, 50 y 100 mM e inmediatamente inoculados con O. novo-ulmi (106 esporas mL–1). El MeJA no protegió a las semillas ni a las plántulas de P. pinaster ante F. oxysporum, quizá debido a que las plántulas eran demasiado jóvenes para inducir los mecanismos fisiológicos responsables de la resistencia. Basándonos en los cambios morfológicos observados en las plántulas de 6 meses de P. pinaster (reducción del crecimiento e incremento de la densidad de los canales resiníferos), hay evidencia de que el MeJA pudo haber activado los mecanismos de resistencia. El MeJA a 25 mM no consiguió reducir la mortalidad probablemente porque la dispersión de F. circinatum en el interior de los tejidos fue más rápida que la formación de respuestas defensivas efectivas. Basándonos en la falta de cambios fenológicos de los olmos tratados, no hay evidencias de que el MeJA pueda haber causado una inducción de resistencia. Los resultados sugieren que el uso del MeJA para prevenir los patógenos F. oxysporum y F. circinatum en plántulas de P. pinaster en viveros y O. novo-ulmi en brinzales de U. minor debe ser descartado

Alternatives to antibiotic growth promoters in rabbit feeding: a review

[EN] This review is focused on the most studied and developed substances which are commonly knownas alternatives to dietary antibiotics, particularly as far as rabbit feeds are concerned. After a reminder of thereason to be and success of antibiotic growth promoters, and why they lately came to be banned in the European Union, we successively deal with probiotics, prebiotics, enzymes and organic acids. Data on rabbitsare, as expected, quite scarce when compared to species such as pigs and poultry. Nevertheless, theavailable performance results are discussed together with the possible mechanisms of action. Special mentionis made of the effects of these substances on digestibility and caecal activity.Falcão-E-Cunha, L.; Castro-Solla, L.; Maertens, L.; Marounek, M.; Pinheiro, V.; Freire, J.; Mourão, JL. (2007). Alternatives to antibiotic growth promoters in rabbit feeding: a review. World Rabbit Science. 15(3). doi:10.4995/wrs.2007.597SWORD15

Live and Dead Nodes

In this paper, we explore the consequences of a distinction between `live' and `dead' network nodes; `live' nodes are able to acquire new links whereas `dead' nodes are static. We develop an analytically soluble growing network model incorporating this distinction and show that it can provide a quantitative description of the empirical network composed of citations and references (in- and out-links) between papers (nodes) in the SPIRES database of scientific papers in high energy physics. We also demonstrate that the death mechanism alone can result in power law degree distributions for the resulting network.Comment: 12 pages, 3 figures. To be published in Computational and Mathematical Organization Theor

Relación entre parámetros de intercambio gaseoso y los primeros síntomas de Ulmus minor Mill. a la grafiosis

Research has been carried out into the identification of easily measured anatomical or physiological parameters that are related to resistance, with the aim of screening elms resistant to Dutch elm disease. Given that the factors which encourage diffusion of this disease in elms also lead to an increase in symptoms, it is hypothesised that estomatic conductance (gs) and specific leaf hydraulic conductivity (CH) could be related to susceptibility. These parameters were determined in 75 U. minor seedlings, aged 7, 6, 3 and 2 years old (n . 15), which were subsequently inoculated with the Dutch elm disease fungus. In plants of 7, 6, and 3 years a positive relationship was observed between gs at 14 hours, and the amount of wilting shown after 15 days (r2 > 0,40; P * 0,01). This relationship was shown weaker with the amount of wilting after 60 days in 7 and 6 year old plants, whilst in 3 year old plants it became nonsignificant. Similar results were obtained between CH and the amount of wilting after 15 days in elms of 6 and 7 years. It is thought that the relationships observed occured because a greater gas exchange favours the colonisation or diffusion of the pathogen within the tree. The fact that neither gs nor CH showed relationships with susceptibility in two year old plants makes it inadvisable to use these parameters for the early screening of resistant U. minor trees.Con el objetivo de encontrar olmos resistentes a la grafiosis se han tratado de encontrar parámetros anatómicos o fisiológicos, fácilmente medibles, relacionados con la resistencia. Dado que los factores que favorecen la difusión de la enfermedad en el árbol conducen a una mayor sintomatología, se plantea la hipótesis de que la conductancia estomática (gs) y la conductividad hidráulica foliar específica (CH) puedan tener relación con la susceptibilidad. Estos parámetros se determinaron en 75 ejemplares de la especie U. minor, con edades de 7, 6, 3 y 2 savias (n . 15). Los ejemplares fueron inoculados posteriormente con el hongo de la grafiosis. En plantas de 7, 6, y 3 savias se observó una relación positiva entre gs a las 14:00 h y el marchitamiento manifestado a los 15 días (r2 > 0,40; P * 0,01). La relación se hizo más débil con el marchitamiento manifestado a los 60 días, y en plantas de 3 savias dejó de ser significativa. Resultados similares se obtuvieron entre CH y el marchitamiento a los 15 días en olmos de 6 y 7 savias. Se piensa que las relaciones observadas responden a que un mayor intercambio gaseoso favorece la colonización o difusión del patógeno en el árbol. La falta de relación entre gs o CH y la susceptibilidad, en planta de 2 savias, desaconseja la utilización de estos parámetros para la selección precoz de U. minor resistentes

The Geography of Scientific Productivity: Scaling in U.S. Computer Science

Here we extract the geographical addresses of authors in the Citeseer database of computer science papers. We show that the productivity of research centres in the United States follows a power-law regime, apart from the most productive centres for which we do not have enough data to reach definite conclusions. To investigate the spatial distribution of computer science research centres in the United States, we compute the two-point correlation function of the spatial point process and show that the observed power-laws do not disappear even when we change the physical representation from geographical space to cartogram space. Our work suggests that the effect of physical location poses a challenge to ongoing efforts to develop realistic models of scientific productivity. We propose that the introduction of a fine scale geography may lead to more sophisticated indicators of scientific output.Comment: 6 pages, 3 figures; minor change

Variabilidad en la brotación de las yemas vegetativas de los olmos europeos

The bud-burst dates of clones of Ulmus minor, Ulmus glabra, and Ulmus laevis were recorded in the field during the spring of 2000, 2001 and 2002 at six locations in five European countries. Meteorological data were obtained from stations close to the plots. Thermal time to bud-burst (T) and chilling (C) were calculated. A relationship on the form T = a + b erC was fitted for each species. In the three species, winter rest release seems to be controlled by two opposite quantitative effects of the temperature, which interrelate according to an inverse exponential relation. Besides, the bud-burst date was found to be stable between years characterised by different winter thermal trends. These three Elm species have small chilling requirements. U. minor has chilling requirements lower than those of the other two species, in agreement with the more southern distribution.Durante las primaveras de los años 2000, 2001 y 2002 se registró en campo, en seis localidades de cinco países europeos, la fecha de apertura de yemas en un número de clones pertenecientes a Ulmus minor, Ulmus glabra y Ulmus laevis. Simultáneamente se obtuvieron datos meteorológicos en estaciones próximas a las parcelas. Se calculó los grados día (T) y el requerimiento de frío (C). Para cada especie se estableció una relación de la forma T = a + b erC. En las tres especies, la interrupción de la quiescencia parece estar controlada por dos efectos termales cuantitativos opuestos, relacionados por una exponencial inversa. Se ha encontrados que la fecha de apertura de yemas ha sido estable incluso en años con características térmicas invernales considerablemente diferentes. Los requerimientos de frío son pequeños en las tres especies de olmo. U. minor requiere fríos menores que las otras dos especies, lo que es consistente con su distribución más meridional

Silicon-hydroxyapatite bioactive coatings (Si-HA) from diatomaceous earth and silica. Study of adhesion and proliferation of osteoblast-like cells

The aim of this study consisted on investigating the influence of silicon substituted hydroxyapatite (Si–HA) coatings over the human osteoblast-like cell line (SaOS-2) behaviour. Diatomaceous earth and silica, together with commercial hydroxyapatite were respectively the silicon and HA sources used to produce the Si–HA coatings. HA coatings with 0 wt% of silicon were used as control of the experiment. Pulsed laser deposition (PLD) was the selected technique to deposit the coatings. The Si–HA thin films were characterized by Fourier Transformed Infrared Spectroscopy (FTIR) demonstrating the efficient transfer of Si to the HA structure. The in vitro cell culture was established to assess the cell attachment, proliferation and osteoblastic activity respectively by, Scanning Electron Microscopy (SEM), DNA and alkaline phosphatase (ALP) quantification. The SEM analysis demonstrated a similar adhesion behaviour of the cells on the tested materials and the maintenance of the typical osteoblastic morphology along the time of culture. The Si–HA coatings did not evidence any type of cytotoxic behaviour when compared with HA coatings. Moreover, both the proliferation rate and osteoblastic activity results showed a slightly better performance on the Si–HA coatings from diatoms than on the Si–HA from silica.This work was supported by the UE-Interreg IIIA (SP1.P151/03) Proteus project and Xunta de Galicia ( Projects: 2006/12 and PGIDITO5PXIC30301PN)

Runaway Events Dominate the Heavy Tail of Citation Distributions

Statistical distributions with heavy tails are ubiquitous in natural and social phenomena. Since the entries in heavy tail have disproportional significance, the knowledge of its exact shape is very important. Citations of scientific papers form one of the best-known heavy tail distributions. Even in this case there is a considerable debate whether citation distribution follows the log-normal or power-law fit. The goal of our study is to solve this debate by measuring citation distribution for a very large and homogeneous data. We measured citation distribution for 418,438 Physics papers published in 1980-1989 and cited by 2008. While the log-normal fit deviates too strong from the data, the discrete power-law function with the exponent $\gamma=3.15$ does better and fits 99.955% of the data. However, the extreme tail of the distribution deviates upward even from the power-law fit and exhibits a dramatic "runaway" behavior. The onset of the runaway regime is revealed macroscopically as the paper garners 1000-1500 citations, however the microscopic measurements of autocorrelation in citation rates are able to predict this behavior in advance.Comment: 6 pages, 5 Figure

A reverse engineering approach to the suppression of citation biases reveals universal properties of citation distributions

The large amount of information contained in bibliographic databases has recently boosted the use of citations, and other indicators based on citation numbers, as tools for the quantitative assessment of scientific research. Citations counts are often interpreted as proxies for the scientific influence of papers, journals, scholars, and institutions. However, a rigorous and scientifically grounded methodology for a correct use of citation counts is still missing. In particular, cross-disciplinary comparisons in terms of raw citation counts systematically favors scientific disciplines with higher citation and publication rates. Here we perform an exhaustive study of the citation patterns of millions of papers, and derive a simple transformation of citation counts able to suppress the disproportionate citation counts among scientific domains. We find that the transformation is well described by a power-law function, and that the parameter values of the transformation are typical features of each scientific discipline. Universal properties of citation patterns descend therefore from the fact that citation distributions for papers in a specific field are all part of the same family of univariate distributions.Comment: 9 pages, 6 figures. Supporting information files available at http://filrad.homelinux.or