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On State-Space Reduction in Multi-Strain Pathogen Models, with an Application to Antigenic Drift in Influenza A
Sergey Kryazhimskiy ,Ulf Dieckmann,Simon A Levin,Jonathan Dushoff
PLOS Computational Biology , 2007, DOI: 10.1371/journal.pcbi.0030159
Abstract: Many pathogens exist in phenotypically distinct strains that interact with each other through competition for hosts. General models that describe such multi-strain systems are extremely difficult to analyze because their state spaces are enormously large. Reduced models have been proposed, but so far all of them necessarily allow for coinfections and require that immunity be mediated solely by reduced infectivity, a potentially problematic assumption. Here, we suggest a new state-space reduction approach that allows immunity to be mediated by either reduced infectivity or reduced susceptibility and that can naturally be used for models with or without coinfections. Our approach utilizes the general framework of status-based models. The cornerstone of our method is the introduction of immunity variables, which describe multi-strain systems more naturally than the traditional tracking of susceptible and infected hosts. Models expressed in this way can be approximated in a natural way by a truncation method that is akin to moment closure, allowing us to sharply reduce the size of the state space, and thus to consider models with many strains in a tractable manner. Applying our method to the phenomenon of antigenic drift in influenza A, we propose a potentially general mechanism that could constrain viral evolution to a one-dimensional manifold in a two-dimensional trait space. Our framework broadens the class of multi-strain systems that can be adequately described by reduced models. It permits computational, and even analytical, investigation and thus serves as a useful tool for understanding the evolution and ecology of multi-strain pathogens.
A description of parasites from Mountain Wolf Snakes, Lycodon ruhstrati ruhstrati (Serpentes: Colubridae), from two localities in western Taiwan
Gerrut Norval,Stephen R. Goldberg,Charles R. Bursey,Simon Dieckmann
Asian Journal of Conservation Biology , 2012,
Abstract:
The formation of relativistic plasma structures and their potential role in the generation of cosmic ray electrons
M. E. Dieckmann
Nonlinear Processes in Geophysics (NPG) , 2008,
Abstract: Recent particle-in-cell (PIC) simulation studies have addressed particle acceleration and magnetic field generation in relativistic astrophysical flows by plasma phase space structures. We discuss the astrophysical environments such as the jets of compact objects, and we give an overview of the global PIC simulations of shocks. These reveal several types of phase space structures, which are relevant for the energy dissipation. These structures are typically coupled in shocks, but we choose to consider them here in an isolated form. Three structures are reviewed. (1) Simulations of interpenetrating or colliding plasma clouds can trigger filamentation instabilities, while simulations of thermally anisotropic plasmas observe the Weibel instability. Both transform a spatially uniform plasma into current filaments. These filament structures cause the growth of the magnetic fields. (2) The development of a modified two-stream instability is discussed. It saturates first by the formation of electron phase space holes. The relativistic electron clouds modulate the ion beam and a secondary, spatially localized electrostatic instability grows, which saturates by forming a relativistic ion phase space hole. It accelerates electrons to ultra-relativistic speeds. (3) A simulation is also revised, in which two clouds of an electron-ion plasma collide at the speed 0.9c. The inequal densities of both clouds and a magnetic field that is oblique to the collision velocity vector result in waves with a mixed electrostatic and electromagnetic polarity. The waves give rise to growing corkscrew distributions in the electrons and ions that establish an equipartition between the electron, the ion and the magnetic energy. The filament-, phase space hole- and corkscrew structures are discussed with respect to electron acceleration and magnetic field generation.
Textos apócrifos determinantes de repertorios plásticos cristianos = Textos apócrifos determinantes de repertorios plásticos cristianos
Patricia Grau-Dieckmann
Acta Scientiarum : Education , 2011,
Abstract: As primeiras imagens crist s apareceram nas paredes das catacumbas e nas superfícies dos sarcófagos em sítios fúnebres. A sua principal finalidade foi transmitir aos fiéis a possibilidade definitiva de salva o eterna. Recordavam também a história sagrada e estimulavam a piedade crist , além de embelezar os locais fúnebres. A emergência gradual de vários textos religiosos, com conteúdos diversos e controversos, estimulou decis es sobre quais escritos eram can nicos e quais apócrifos. Ao mesmo tempo, a tolerancia do Império Romano ao Cristianismo causou uma prolifera o de lugares onde podiam ser colocadas essas imagens. Muitos temas originais apareceram, mas outros já existentes foram modificados e adaptados, fortalecendo o repertório iconográfico as partir do segundo século, principalmente sob a influência dos livros apócrifos. As imagens referentes à vida de Maria e de Jesus, mais tarde condenadas pela Igreja, foram designadas e moldadas sob a aprova o eclesiástica, ocupando lugares de destaque e de visibilidade em encontros religiosos. Las primitivas imágenes cristianas se manifestaron dentro de los lugares de enterramiento en los muros catacumbarios y en las caras de los sarcófagos. Su intencionalidad central fue transmitir a los fieles la posibilidad real de la salvación de sus almas. También rememoraban la historia sacra e incitaban a la piedad, a la vez que embellecían los antros funerarios. La progresiva aparición de numerosos textos religiosos —de diversa índole y de contenido muchas veces polémico— provocó que tempranamente se determinara cuáles escritos serían considerados canónicos y cuáles apócrifos. Simultáneamente, la tolerancia de la religión cristiana en el Imperio Romano permitió la multiplicación de soportes en los que se podían ubicar las imágenes. Surgieron temáticas originales a la vez que se modificaron y adaptaron algunas ya existentes, vigorizando el repertorio iconográfico generado a partir de fines del siglo II como resultado, en gran medida, de la influencia de textos apócrifos. Muchas de las imágenes sobre las vidas de María y de Jesús, pese a que su sustento escrito fue oportunamente anatemizado por la Iglesia, fueron dise adas y plasmadas bajo el amparo de las autoridades eclesiásticas, ocupando lugares de exposición y visibilidad preponderantes en los espacios sacros oficiales.
The filamentation instability driven by warm electron beams: Statistics and electric field generation
M E Dieckmann
Physics , 2009, DOI: 10.1088/0741-3335/51/12/124042
Abstract: The filamentation instability of counterpropagating symmetric beams of electrons is examined with 1D and 2D particle-in-cell (PIC) simulations, which are oriented orthogonally to the beam velocity vector. The beams are uniform, warm and their relative speed is mildly relativistic. The dynamics of the filaments is examined in 2D and it is confirmed that their characteristic size increases linearly in time. Currents orthogonal to the beam velocity vector are driven through the magnetic and electric fields in the simulation plane. The fields are tied to the filament boundaries and the scale size of the flow-aligned and the perpendicular currents are thus equal. It is confirmed that the electrostatic and the magnetic forces are equally important, when the filamentation instability saturates in 1D. Their balance is apparently the saturation mechanism of the filamentation instability for our initial conditions. The electric force is relatively weaker but not negligible in the 2D simulation, where the electron temperature is set higher to reduce the computational cost. The magnetic pressure gradient is the principal source of the electrostatic field, when and after the instability saturates in the 1D simulation and in the 2D simulation.
Electric field generation by the electron beam filamentation instability: Filament size effects
M E Dieckmann,A Bret
Physics , 2009, DOI: 10.1088/0031-8949/81/01/015502
Abstract: The filamentation instability (FI) of counter-propagating beams of electrons is modelled with a particle-in-cell simulation in one spatial dimension and with a high statistical plasma representation. The simulation direction is orthogonal to the beam velocity vector. Both electron beams have initially equal densities, temperatures and moduli of their nonrelativistic mean velocities. The FI is electromagnetic in this case. A previous study of a small filament demonstrated, that the magnetic pressure gradient force (MPGF) results in a nonlinearly driven electrostatic field. The probably small contribution of the thermal pressure gradient to the force balance implied, that the electrostatic field performed undamped oscillations around a background electric field. Here we consider larger filaments, which reach a stronger electrostatic potential when they saturate. The electron heating is enhanced and electrostatic electron phase space holes form. The competition of several smaller filaments, which grow simultaneously with the large filament, also perturbs the balance between the electrostatic and magnetic fields. The oscillations are damped but the final electric field amplitude is still determined by the MPGF.
How large can the electron to proton mass ratio be in Particle-In-Cell simulations of unstable systems?
A. Bret,M. E. Dieckmann
Physics , 2010, DOI: 10.1063/1.3357336
Abstract: Particle-in-cell (PIC) simulations are widely used as a tool to investigate instabilities that develop between a collisionless plasma and beams of charged particles. However, even on contemporary supercomputers, it is not always possible to resolve the ion dynamics in more than one spatial dimension with such simulations. The ion mass is thus reduced below 1836 electron masses, which can affect the plasma dynamics during the initial exponential growth phase of the instability and during the subsequent nonlinear saturation. The goal of this article is to assess how far the electron to ion mass ratio can be increased, without changing qualitatively the physics. It is first demonstrated that there can be no exact similarity law, which balances a change of the mass ratio with that of another plasma parameter, leaving the physics unchanged. Restricting then the analysis to the linear phase, a criterion allowing to define a maximum ratio is explicated in terms of the hierarchy of the linear unstable modes. The criterion is applied to the case of a relativistic electron beam crossing an unmagnetized electron-ion plasma.
Thrombosis of abdominal aorta during cisplatin-based chemotherapy of testicular seminoma - a case report
Klaus-Peter Dieckmann, Ralf Gehrckens
BMC Cancer , 2009, DOI: 10.1186/1471-2407-9-459
Abstract: A 49 year old man with advanced seminoma underwent two courses of chemotherapy according to the PEB regimen. Upon restaging, two thrombotic deposits were noted in the descending part of the thoracic aorta and in the infrarenal abdominal aorta, respectively. Although thrombotic plaques caused aortic occlusion of about 30%, no clinical signs of malperfusion of limbs were registered. The patient was placed on anticoagulant therapy. Six months after completion of chemotherapy, thrombotic deposits had completely resolved. In the absence of other predisposing factors, it must be assumed that cisplatin-based chemotherapy represented a strong stimulus for arterial thrombosis in the aorta.This is the first case of endo-aortic thrombosis during chemotherapy for testicular germ cell cancer. Providers of chemotherapy must be aware of arterial thrombosis even in young patients with testicular cancer.Cis-platin based chemotherapy is the cornerstone in the management of metastatic testicular germ cell tumour (TGCT) [1]. The regimen consisting of cisplatin, etoposide and bleomycin (PEB) is usually well tolerated, yet, the toxicity profile of this procedure is significant [2]. Basically, there are acute (immediate) and chronic (late) untoward effects of the PEB protocol. Second malignancies and cardiovascular events constitute well recognized long-term hazards of this chemotherapy [3]. With regard to late cardiovascular problems, cisplatin is thought to initiate degenerative processes of vessel walls, thus causing occlusive vascular disease in the long run. All types of arteries may be involved, and accordingly, there is sound evidence for an excess of myocardial infarctions, arterial hypertension, and cerebral strokes occurring in TGCT patients during long-term follow-up. Only recently it became apparent that cardiovascular complications secondary to cisplatin-based chemotherapy may also occur early during the application of systemic therapy or immediately thereafter [4]. Such comp
La reina testigo: Jeanne d'Evreux en su libro de horas
Manzi,Ofelia; Grau-Dieckmann,Patricia;
Temas medievales , 2004,
Abstract: among the pictorial documents produced during the lower middle ages, some of the most important manifestations are to be found within the genre known as livres d′heures. one of its most interesting examples -not only due to its exceptional artistic level, but also because of its high degree of conservation and its great originality- is the tiny livre d'heures which belonged to jeanne d'evreux, king charles iv's third wife whose responsibility it was to materialize the last chance of survival of the capetian dynasty. in the manuscript there appear certain historical personalities who may be key to an interpretation. two pages belonging respectively to the heures of the virgin and those of st. louis, containing miniatures which are a sort of an exchange between past and present in which present day history is integrated into the biblical history, illustrate the value of pictures and the many possibilities by which they can convey different meanings.
UNA VISIóN CRíTICA DEL ENFOQUE DE LOS HEURíSTICOS RáPIDOS Y FRUGALES
GARCíA-RETAMERO,ROCIO; DIECKMANN,ANJA;
Revista Latinoamericana de Psicología , 2006,
Abstract: gigerenzer, todd, and the abc research group (1999; see also todd gigerenzer, 2000) suggested that we use fast and frugal heuristics to make decisions in our everyday life. indeed, there is accumulating evidence for the use of fast and frugal heuristics in a wide range of experiments. some of the fast and frugal heuristics, however, have been criticized. for instance, it is not specified how we select the information that these heuristics need to be effective. in this article, we focus on fast and frugal heuristics for two-alternative forced-choice tasks. more specifically, we present an overview of the empirical research and the main criticisms they received so far. we then explore to what extent recent research overcomes these shortcomings and which questions remain to be addressed by future research.
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