1,086 research outputs found
Branching on multi-aggregated variables
open5siopenGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, DomenicoGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, Domenic
The subdivision of large simplicial cones in Normaliz
Normaliz is an open-source software for the computation of lattice points in
rational polyhedra, or, in a different language, the solutions of linear
diophantine systems. The two main computational goals are (i) finding a system
of generators of the set of lattice points and (ii) counting elements
degree-wise in a generating function, the Hilbert Series. In the homogeneous
case, in which the polyhedron is a cone, the set of generators is the Hilbert
basis of the intersection of the cone and the lattice, an affine monoid.
We will present some improvements to the Normaliz algorithm by subdividing
simplicial cones with huge volumes. In the first approach the subdivision
points are found by integer programming techniques. For this purpose we
interface to the integer programming solver SCIP to our software. In the second
approach we try to find good subdivision points in an approximating overcone
that is faster to compute.Comment: To appear in the proceedings of the ICMS 2016, published by Springer
as Volume 9725 of Lecture Notes in Computer Science (LNCS
Self-Similar Evolution of Cosmic-Ray Modified Shocks: The Cosmic-Ray Spectrum
We use kinetic simulations of diffusive shock acceleration (DSA) to study the
time-dependent evolution of plane, quasi-parallel, cosmic-ray (CR) modified
shocks. Thermal leakage injection of low energy CRs and finite Alfv\'en wave
propagation and dissipation are included. Bohm diffusion as well as the
diffusion with the power-law momentum dependence are modeled. As long as the
acceleration time scale to relativistic energies is much shorter than the
dynamical evolution time scale of the shocks, the precursor and subshock
transition approach the time-asymptotic state, which depends on the shock sonic
and Alfv\'enic Mach numbers and the CR injection efficiency. For the diffusion
models we employ, the shock precursor structure evolves in an approximately
self-similar fashion, depending only on the similarity variable, x/(u_s t).
During this self-similar stage, the CR distribution at the subshock maintains a
characteristic form as it evolves: the sum of two power-laws with the slopes
determined by the subshock and total compression ratios with an exponential
cutoff at the highest accelerated momentum, p_{max}(t). Based on the results of
the DSA simulations spanning a range of Mach numbers, we suggest functional
forms for the shock structure parameters, from which the aforementioned form of
CR spectrum can be constructed. These analytic forms may represent approximate
solutions to the DSA problem for astrophysical shocks during the self-similar
evolutionary stage as well as during the steady-state stage if p_{max} is
fixed.Comment: 38 pages, 12 figures, ApJ accepte
The European Union's Ambient and Assisted Living Joint Programme: An evaluation of its impact on population health and well-being
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Potential for classical biological control of the potato bug Closterotomus norwegicus (Hemiptera: Miridae): description, parasitism and host specificity of Peristenus closterotomae sp. n. (Hymenoptera: Braconidae)
The potato bug, Closterotomus norwegicus (Gmelin) (Hemiptera: Miridae) is an introduced pest of lucerne, white clover and lotus seed crops in New Zealand and a key pest of pistachios in California, USA. Efforts were made to identify potential biological control agents of C. norwegicus in Europe. A total of eight parasitoids, including six primary parasitoids from the genus Peristenus (Hymenoptera: Braconidae) and two hyperparasitoids from the genus Mesochorus (Hymenoptera: Ichneumonidae), were reared from C. norwegicus nymphs collected in various habitats in northern Germany. With a proportion of more than 85% of all C. norwegicus parasitoids, Peristenus closterotomae (Hymenoptera: Braconidae), a new species, was the most dominant parasitoid, whereas other parasitoid species only occurred sporadically. Peristenus closterotomae did not fit in the keys to any described species and is described as new to science. Parasitism caused by P. closterotomae was on average 24% (maximum 77%). To assess the host specificity of parasitoids associated with C. norwegicus, the parasitoid complexes of various Miridae occurring simultaneously with C. norwegicus were studied. Peristenus closterotomae was frequently reared from Calocoris affinis (Herrich-Schaeffer), and a few specimens were reared from Calocoris roseomaculatus (De Geer) and the meadow plant bug, Leptopterna dolobrata (Linnaeus) (all Hemiptera: Miridae). The remaining primary parasitoids associated with C. norwegicus were found to be dominant in hosts other than C. norwegicus. Whether nymphal parasitoids may potentially be used in a classical biological control initiative against the potato bug in countries where it is introduced and considered to be a pest is discusse
Fermi gamma-ray `bubbles' from stochastic acceleration of electrons
Gamma-ray data from Fermi-LAT reveal a bi-lobular structure extending up to
50 degrees above and below the galactic centre, which presumably originated in
some form of energy release there less than a few million years ago. It has
been argued that the gamma-rays arise from hadronic interactions of high energy
cosmic rays which are advected out by a strong wind, or from inverse-Compton
scattering of relativistic electrons accelerated at plasma shocks present in
the bubbles. We explore the alternative possibility that the relativistic
electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave
turbulence through the entire volume of the bubbles. The observed gamma-ray
spectral shape is then explained naturally by the resulting hard electron
spectrum and inverse Compton losses. Rather than a constant volume emissivity
as in other models, we predict a nearly constant surface brightness, and
reproduce the observed sharp edges of the bubbles.Comment: 4 pages, 4 figures; REVTeX4-1; discussion amended and one figure
added; to appear in PR
Constraints On the Diffusive Shock Acceleration From the Nonthermal X-ray Thin Shells In SN1006 NE Rim
Characteristic scale lengths of nonthermal X-rays from the SN1006 NE rim,
which are observed by Chandra, are interpreted in the context of the diffusive
shock acceleration on the assumption that the observed spatial profile of
nonthermal X-rays corresponds to that of accelerated electrons with energies of
a few tens of TeV. To explain the observed scale lengths, we construct two
simple models with a test particle approximation, where the maximum energy of
accelerated electrons is determined by the age of SN1006 (age-limited model) or
the energy loss (energy loss-limited model), and constrain the magnetic field
configuration and the diffusion coefficients of accelerated electrons. When the
magnetic field is nearly parallel to the shock normal, the magnetic field
should be in the range of 20-85 micro Gauss and highly turbulent both in
upstream and downstream, which means that the mean free path of accelerated
electrons is on the order of their gyro-radius (Bohm limit). This situation can
be realized both in the age-limited and energy loss-limited model. On the other
hand, when the magnetic field is nearly perpendicular to the shock normal,
which can exist only in the age-limited case, the magnetic field is several
micro Gauss in the upstream and 14-20 micro Gauss in the downstream, and the
upstream magnetic field is less turbulent than the downstream.Comment: 9 pages, 4 figures, accepted for publication in A&
Effect of enhanced pCO2 levels on the production of dissolved organic carbon and transparent exopolymer particles in short-term bioassay experiments
It has been proposed that increasing levels of pCO(2) in the surface ocean will lead to more partitioning of the organic carbon fixed by marine primary production into the dissolved rather than the particulate fraction. This process may result in enhanced accumulation of dissolved organic carbon (DOC) in the surface ocean and/or concurrent accumulation of transparent exopolymer particles (TEPs), with important implications for the functioning of the marine carbon cycle. We investigated this in shipboard bioassay experiments that considered the effect of four different pCO(2) scenarios (ambient, 550, 750 and 1000 mu atm) on unamended natural phytoplankton communities from a range of locations in the northwest European shelf seas. The environmental settings, in terms of nutrient availability, phytoplankton community structure and growth conditions, varied considerably between locations. We did not observe any strong or consistent effect of pCO(2) on DOC production. There was a significant but highly variable effect of pCO(2) on the production of TEPs. In three of the five experiments, variation of TEP production between pCO(2) treatments was caused by the effect of pCO(2) on phytoplankton growth rather than a direct effect on TEP production. In one of the five experiments, there was evidence of enhanced TEP production at high pCO(2) (twice as much production over the 96 h incubation period in the 750 mu atm treatment compared with the ambient treatment) independent of indirect effects, as hypothesised by previous studies. Our results suggest that the environmental setting of experiments (community structure, nutrient availability and occurrence of phytoplankton growth) is a key factor determining the TEP response to pCO(2) perturbations
Filaments in the southern giant lobe of Centaurus A : Constraints on nature and origin from modelling and GMRT observations
Date of acceptance: 22/05/2014We present results from imaging of the radio filaments in the southern giant lobe of CentaurusA using data from Giant Metrewave Radio Telescope observations at 325 and 235 MHz, and outcomes from filament modelling. The observations reveal a rich filamentary structure, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to the jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressuredwith respect to the global lobe plasma showing no evidence of largescale efficient Fermi I-type particle acceleration, and persist for ~2-3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318-444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabilities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic magnetohydrodynamical turbulence or from a moderately recent jet activity of the parent AGN, or an interplay of both.Peer reviewe
Status of neutrino astronomy
Astrophysical neutrinos can be produced in proton interactions of charged
cosmic rays with ambient photon or baryonic fields. Cosmic rays are observed in
balloon, satellite and air shower experiments every day, from below 1e9 eV up
to macroscopic energies of 1e21 eV. The observation of different photon fields
has been done ever since, today with detections ranging from radio wavelengths
up to very high-energy photons in the TeV range. The leading question for
neutrino astronomers is now which sources provide a combination of efficient
proton acceleration with sufficiently high photon fields or baryonic targets at
the same time in order to produce a neutrino flux that is high enough to exceed
the background of atmospheric neutrinos. There are only two confirmed
astrophysical neutrino sources up to today: the sun and SuperNova 1987A emit
and emitted neutrinos at MeV energies. The aim of large underground Cherenkov
telescopes like IceCube and KM3NeT is the detection of neutrinos at energies
above 100 GeV. In this paper, recent developments of neutrino flux modeling for
the most promising extragalactic sources, gamma ray bursts and active galactic
nuclei, are presented.Comment: Talk given at Neutrino 2008, Christchurch (New Zealand) 6 pages, 4
figures, 1 tabl
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