SOC in a population model with global control

We study a plant population model introduced recently by J. Wallinga [OIKOS {\bf 74}, 377 (1995)]. It is similar to the contact process (`simple epidemic', `directed percolation'), but instead of using an infection or recovery rate as control parameter, the population size is controlled directly and globally by removing excess plants. We show that the model is very closely related to directed percolation (DP). Anomalous scaling laws appear in the limit of large populations, small densities, and long times. These laws, associated critical exponents, and even some non-universal parameters, can be related to those of DP. As in invasion percolation and in other models where the r\^oles of control and order parameters are interchanged, the critical value $p_c$ of the wetting probability $p$ is obtained in the scaling limit as singular point in the distribution of infection rates. We show that a mean field type approximation leads to a model studied by Y.C. Zhang et al. [J. Stat. Phys. {\bf 58}, 849 (1990)]. Finally, we verify the claim of Wallinga that family extinction in a marginally surviving population is governed by DP scaling laws, and speculate on applications to human mitochondrial DNA.Comment: 19 pages, with 10 ps-figured include

Monitoring and mitigation of the sound effects of hydrocarbon exploration activities on marine mammal populations

Offshore Exploration and Production (E&P) activities, such as seismic surveys and drilling, generate sound that can affect marine mammals in different ways. These effects range from permanent or temporary auditory impacts to disturbance or behavioral changes, and communication masking. Depending on the intensity and duration of these effects, and without implementation of appropriate mitigation measures, this can result in population-level consequences. The overarching objective of this study was to advance the protection of marine mammals during the implementation of E&P activities through the following themes: (1) enhancement of the state of knowledge of risk management, (2) efficacy of mitigation, (3) advanced monitoring technology, (4) implementation of advanced industry monitoring and mitigation measures and (5) measurement of heretofore unassessed E&P activities. In this study several marine mammal monitoring and mitigation programs associated with E&P projects are presented to further advance these themes. Topics being addressed include the use of autonomous camera systems for aerial monitoring of a narwhal population, long-term photo-identification studies of western gray whales to better understand site fidelity to their summer feeding grounds, mitigation of gray whales’ behavioral responses to a seismic survey near these feeding grounds and use of Passive Acoustic Monitoring to characterize seismic pulses and drilling activity as well as marine mammal presence in remote arctic areas. A synthesis of the main findings is provided that includes identification of future research needs. Conclusions and specific recommendations are made that will contribute to our ability to assess and mitigate risks of E&P sound to marine mammals

DETERMINANTES DOS RATINGS CORPORATIVOS NA INDÚSTRIA PETROLÍFERA: O CASO DA REPSOL-YPF

The financing patterns of corporations have changed over the last fifteen years. Bond financing has superseded bank loans. In addition, the deeper integration of worldwide financial markets has diffused risk assessment mechanisms, which were previously restricted to industrialized countries. In this context, credit ratings issued by rating agencies such as Standard & Poor’s and Moody’s play an important role in the procurement of financing. A rating is referred to as “… an opinion about an issuer’s future capability, legal responsibility, and willingness to meet the payment of the principal and interest of a specific bond…” (Moody’s 1999a, p.5).This paper aims to verify whether or not it is possible to predict corporate ratings based on a set of financial indicators. We study the case of Repsol-YPF, a world-renowned Argentine oil company. The ordered logit model was used as estimation method, as it yields better results than the more commonly used least squares and probit models. From a small set of financial indicators, the most relevant appear to be earnings (EBITDA) and short term debt over total debt (STD/TD). This small number of financial indicators was able to reasonably predict Repsol-YPF corporate ratings, suggesting than ratings may not have information in addition to that publicly available.El patrón de financiamiento de empresas cambió en los últimos 15 años. Préstamos bancarios cediendo lugar a los títulos de deuda. A la vez, la profundidad de la integración del mercado financiero mundial trajo la difusión de mecanismos de evaluación de riesgo antes restrictos a los países desarrollados. En este contexto, los ratings de crédito emitidos por agencias de clasificación, como Standard & Poor'S y Moody's, pasaron a tener un papel importante en la obtención de la financiación. La clasificación se refiere la “...una opinión sobre la capacidad futura, la responsabilidad jurídica, y la intención de un emitente de efectuar, dentro del plazo, pagos del principal e intereses de un título específico de renta fija...” (MOODY'S, 1999la, p.5).El objetivo de este artículo es mostrar se es posible prever los ratings corporativos o de empresas a partir de un grupo de indicadores financieros. Los indicadores pertenecen la Repsol-YPF, empresa petrolífera argentina de notoriedad internacional. El método de estimación usado en modelos econométricos fue el ordered logit, que mostró mejores resultados que los modelos de mínimos cuadrados y probit. De un conjunto pequeño de indicadores financieros, los más relevantes parecen ser el EBITDA, Deuda de Corto Plazo sobre Deuda Total. Se concluyó que un pequeño número de indicadores financieros pudo prever los ratings corporativos de la Repsol-YPF, sugiriendo que ratings no traen informaciones adicionales a aquella ya disponible en demostrativos contables y financieros.TEXTO (PDF) EM INGLÊSO padrão de financiamento de empresas mudou nos últimos 15 anos. Empréstimos bancários cedendo lugar aos títulos de dívida. Ao mesmo tempo, o aprofundamento da integração do mercado financeiro mundial trouxe a difusão de mecanismos de avaliação de risco antes restritos aos países desenvolvidos. Neste contexto, os ratings de crédito emitidos por agências de classificação, como a Standard & Poor´s e Moody´s, passaram a ter um papel importante na obtenção do financiamento. O rating refere-se a “...uma opinião sobre a capacidade futura, a responsabilidade jurídica, e a vontade de um emitente de efetuar, dentro do prazo, pagamentos do principal e juros de um título específico de renda fixa...” (MOODY’S, 1999a, p.5).O objetivo deste artigo é mostrar se é possível prever os ratings corporativos ou de empresas a partir de um grupo de indicadores financeiros. Os indicadores pertencem a Repsol-YPF, empresa petrolífera argentina de notoriedade internacional. O método de estimação usado em modelos econométricos foi o ordered logit, que mostrou melhores resultados que os modelos de mínimos quadrados e probit. De um conjunto pequeno de indicadores financeiros, os mais relevantes parecem ser o EBITDA, Dívida de Curto Prazo sobre Dívida Total. Concluiu-se que um pequeno número de indicadores financeiros pôde prever os ratings corporativos da Repsol-YPF, sugerindo que ratings não trazem informações adicionais àquela já disponível em demonstrativos contábeis e financeiros

A IMPORTÂNCIA DOS FUNDAMENTOS NOS RATINGS SOBERANOS BRASILEIROS, 1994-2002

The international financial crises of the 1990s rose doubts on the usefulness of sovereign ratings. The present paper has two aims: identify whether sovereign ratings can be predicted using a small set of macroeconomic fundamentals; and test whether sovereign spreads can be predicted by sovereign ratings and/or fundamentals. In the first case, a good adjustment of an ordered logit model can be obtained using the following indicators: Debt/Exports, Public Sector Net Debt, Gov´t Deficit and Current Account. In the second case, when fundamentals (mostly public sector financial conditions) are included, sovereign ratings do not help predict sovereign spreads.

T Cell Migration in Three-dimensional Extracellular Matrix: Guidance by Polarity and Sensations

The locomotion of T lymphocytes within 3-D extracellular matrix (ECM) is a highly dynamic and flexible process following the principles of ameboid movement. Ameboid motility is characterized by a polarized yet simple cell shape allowing high speed, rapid directional oscillations, and low affinity interactions to the substrate that are coupled to a low degree of cytoskeletal organization lacking discrete focal contacts. At the onset of T cell migration, a default program, here described as migration-associated polarization, is initiated, resulting in the polar redistribution of cell surface receptors and cytoskeletal elements. Polarization involves protein cycling either to the leading edge (i.e. LFA-1, CD45RO, chemokine receptors, focal adhesion kinase), to a central polarizing compartment (MTOC, PKC, MARCKS), or into the uropod (CD44, CD43, ICAM- and –3, β1 integrins). The function of such compartment formation may be important in chemotactic response, scanning of encountered cells, and a flexible and adaptive interaction with the ECM itself. Due to the simple shape and a diffusely organized cytoskeleton, the interactions to the surrounding extracellular matrix are rapid and reversible and appear to allow a broad spectrum of molecular migration strategies. These range from (1) adhesive and haptokinetic following i.e. chemokine-induced motility across 2-D surfaces to (2) largely integrin-independent migration predominantly guided by shape change and morphological flexibility, as seen in 3-D type I collagen matrices. Their prominent capacity to rapidly adapt to a given structural environment coupled to contact guidance mechanisms set T cell locomotion apart from slow, focal contact-dependent and more adhesive migration strategies established by fibroblast-like cells and cell clusters. It is therefore likely that, within the tissues, besides chemotactic or haptotactic gradients, the preformed matrix structure has an important impact on T cell trafficking and positioning in health and disease

Pairing-based algorithms for jacobians of genus 2 curves with maximal endomorphism ring

Using Galois cohomology, Schmoyer characterizes cryptographic non-trivial self-pairings of the $\ell$-Tate pairing in terms of the action of the Frobenius on the $\ell$-torsion of the Jacobian of a genus 2 curve. We apply similar techniques to study the non-degeneracy of the $\ell$-Tate pairing restrained to subgroups of the $\ell$-torsion which are maximal isotropic with respect to the Weil pairing. First, we deduce a criterion to verify whether the jacobian of a genus 2 curve has maximal endomorphism ring. Secondly, we derive a method to construct horizontal $(\ell,\ell)$-isogenies starting from a jacobian with maximal endomorphism ring

Collective dynamics and pair-distribution function of active Brownian ellipsoids

While the collective dynamics of spherical active Brownian particles is relatively well understood by now, the much more complex dynamics of nonspherical active particles still raises interesting open questions. Previous work has shown that the dynamics of rod-like or ellipsoidal active particles can differ significantly from that of spherical ones. Here, we obtain the full state diagram of active Brownian ellipsoids depending on the P\'eclet number and packing density via computer simulations. The system is found to exhibit a rich state behavior that includes cluster formation, local polar order, polar flocks, and disordered states. Moreover, we obtain numerical results and an analytical representation for the pair-distribution function of active ellipsoids. This function provides useful quantitative insights into the collective behavior of active particles with lower symmetry and has potential applications in the development of predictive theoretical models.Comment: 30 pages, 8 figures, 14 table