1,477 research outputs found
On the origin of <i>Halipeurus heraldicus</i> on Round Island petrels: cophylogenetic relationships between petrels and their chewing lice
Lice phylogenetic relationships have often been used to elucidate host relationships and vice versa. In this study, we investigate the louse genus Halipeurus which parasitizes bird hosts in the families Procellariidae, Hydrobatidae and Pelecanoididae. The presence of two lice species on Pterodroma arminjoniana in different breeding grounds (Halipeurus heraldicus on Round Island, off Mauritius in the Indian Ocean and Halipeurus kermadecensis on Trindade Island in the Atlantic Ocean) has led to some confusion in the distribution of Pt. arminjoniana and its close relatives Pt. heraldica and Pt. neglecta. By using a cophylogenetic approach that incorporates uncertainties in phylogenetic reconstructions, we show significant overall coevolution between Halipeurus lice and their hosts. However, the study also indicates that the presence of H. heraldicus on Pt. arminjoniana and Pt. neglecta on Round Island and on Pt. heraldica on Gambier Island are the result of a host switch whereas H. kermadecensis is the ancestral parasite of Pt. arminjoniana. This suggests that H. kermadecensis was lost during or after colonisation of Round Island by Pt. arminjoniana. We conclude that cophylogenetic analyses are central to inferring the evolutionary history and biogeographical patterns of hosts and their parasites
Pedestrian flows in bounded domains with obstacles
In this paper we systematically apply the mathematical structures by
time-evolving measures developed in a previous work to the macroscopic modeling
of pedestrian flows. We propose a discrete-time Eulerian model, in which the
space occupancy by pedestrians is described via a sequence of Radon positive
measures generated by a push-forward recursive relation. We assume that two
fundamental aspects of pedestrian behavior rule the dynamics of the system: On
the one hand, the will to reach specific targets, which determines the main
direction of motion of the walkers; on the other hand, the tendency to avoid
crowding, which introduces interactions among the individuals. The resulting
model is able to reproduce several experimental evidences of pedestrian flows
pointed out in the specialized literature, being at the same time much easier
to handle, from both the analytical and the numerical point of view, than other
models relying on nonlinear hyperbolic conservation laws. This makes it
suitable to address two-dimensional applications of practical interest, chiefly
the motion of pedestrians in complex domains scattered with obstacles.Comment: 25 pages, 9 figure
Scott correction for large atoms and molecules in a self-generated magnetic field
We consider a large neutral molecule with total nuclear charge in
non-relativistic quantum mechanics with a self-generated classical
electromagnetic field. To ensure stability, we assume that Z\al^2\le \kappa_0
for a sufficiently small , where \al denotes the fine structure
constant. We show that, in the simultaneous limit , \al\to 0 such
that \kappa =Z\al^2 is fixed, the ground state energy of the system is given
by a two term expansion . The leading
term is given by the non-magnetic Thomas-Fermi theory. Our result shows that
the magnetic field affects only the second (so-called Scott) term in the
expansion
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Diode and final-focus simulations for DARHT
Beam dynamics calculations for the injector and final-focus region of a 4 kA, 20 MeV linear induction accelerator are presented. The injector is a low-emittance 4 MeV thermionic or photocathode diode designed to produce four 70 ns pulses over 2 {micro}sec. Due to the long total pule length, the authors have kept the field stress to < 200 kV/cm over the cathode electrode, and to {approx} 50 kV/cm on the radial insulator stacks. The normalized edge emittance produced by the diode is only {approx} 0.019 cm-rad. In the final-focus region, the authors have modeled the effect of ion emission from the target. The intense electric field of the beam at the 1-mm-diameter focal spot produces substantial ion velocities, and, if the space-charge-limited current density can be supplied, significant focal spot degradation may occur due to ion space-charge. Calculations for the existing Integrated Test Stand, which has a larger focal spot, show that the effect should be observable for H{sup +} and C{sup +} ion species. The effect is lessened if there is insufficient ion density on the target to supply the space-charge-limited current density, or if the ion charge-to-mass ratio is sufficiently small
Time-evolving measures and macroscopic modeling of pedestrian flow
This paper deals with the early results of a new model of pedestrian flow,
conceived within a measure-theoretical framework. The modeling approach
consists in a discrete-time Eulerian macroscopic representation of the system
via a family of measures which, pushed forward by some motion mappings, provide
an estimate of the space occupancy by pedestrians at successive time steps.
From the modeling point of view, this setting is particularly suitable to
treat nonlocal interactions among pedestrians, obstacles, and wall boundary
conditions. In addition, analysis and numerical approximation of the resulting
mathematical structures, which is the main target of this work, follow more
easily and straightforwardly than in case of standard hyperbolic conservation
laws, also used in the specialized literature by some Authors to address
analogous problems.Comment: 27 pages, 6 figures -- Accepted for publication in Arch. Ration.
Mech. Anal., 201
Particle transport in the scrape-off layer and its relationship to discharge density limit in Alcator C-Mod
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