728 research outputs found
Scarf\u27s Procedure for Integer Programming and a Dual Simplex Algorithm
Herbert Scarf has recently introduced an algorithm for integer programs based on the concept of primitive sets. We show that as the choice variables become continuous, this algorithm converges to a dual simplex algorithm. This result is robust in the sense that even before the limit is reached, the simplex path is contained in the primitive sets which define Scarf’s path to the solution of the integer program
Parallel functional and stoichiometric trait shifts in South American and African forest communities with elevation
The Amazon and Congo basins are the two largest continuous blocks of tropical forest with a central role for global biogeochemical cycles and ecology. However, both biomes differ in structure and species richness and composition. Understanding future directions of the response of both biomes to environmental change is paramount. We used one elevational gradient on both continents to investigate functional and stoichiometric trait shifts of tropical forest in South America and Africa. We measured community-weighted functional canopy traits and canopy and topsoil delta N-15 signatures. We found that the functional forest composition response along both transects was parallel, with a shift towards more nitrogen-conservative species at higher elevations. Moreover, canopy and topsoil delta N-15 signals decreased with increasing altitude, suggesting a more conservative N cycle at higher elevations. This cross-continental study provides empirical indications that both South American and African tropical forest show a parallel response with altitude, driven by nitrogen availability along the elevational gradients, which in turn induces a shift in the functional forest composition. More standardized research, and more research on other elevational gradients is needed to confirm our observations
Synoptic study of the SMC SNRs using XMM-Newton
We present a detailed X-ray spectral analysis of 13 supernova remnants (SNR)
in the Small Magellanic Cloud (SMC). We apply both single-temperature
non-equilibrium ionisation models and models based on the Sedov similarity
solution, where applicable. We also present detailed X-ray images of individual
SNRs, which reveal a range of different morphological features. Eight remnants,
viz DEM S 32, IKT 2, HFPK 419, IKT 6, IKT 16, IKT 18 and IKT 23, are consistent
with being in their Sedov evolutionary phase. IKT 6 and IKT 23 both have a
clear shell like morphology with oxygen-rich X-ray emitting material in the
centre. We draw attention to similarities between these two remnants and the
well studied, oxygen-rich remnant IKT 22 (SNR 0102-72.3) and propose that they
are more evolved versions of IKT 22. IKT 4, IKT 5, DEM S 128 and IKT 5 are
evolved remnants which are in, or in the process of entering, the radiative
cooling stage. We argue that the X-ray emission from these four remnants is
most likely from the ejecta remains of type Ia SNe. Our modeling allow us to
derive estimates for physical parameters, such as densities, ages, masses and
initial explosion energies. Our results indicate that the average SMC hydrogen
density is a factor of ~6 lower as compared to the Large Magellanic Cloud. This
has obvious implications for the evolution and luminosities of the SMC SNRs. We
also estimate the average SMC gas phase abundances for the elements O, Ne, Mg,
Si and Fe.Comment: submitted to A&
Using the Memories of Multiscale Machines to Characterize Complex Systems
A scheme is presented to extract detailed dynamical signatures from
successive measurements of complex systems. Relative entropy based time series
tools are used to quantify the gain in predictive power of increasing past
knowledge. By lossy compression, data is represented by increasingly coarsened
symbolic strings. Each compression resolution is modeled by a machine: a finite
memory transition matrix. Applying the relative entropy tools to each machine's
memory exposes correlations within many time scales. Examples are given for
cardiac arrhythmias and different heart conditions are distinguished.Comment: 4 pages, 2 figure
X-ray spectral imaging and Doppler mapping of Cassiopeia A
A detailed X-ray spectral analysis of Cas A using a deep exposure from the
EPIC-MOS cameras on-board XMM-Newton is presented. Spectral fitting was
performed on a 15x15 grid of 20"x20" pixels using a two component
non-equilibrium ionisation model (NEI) giving maps of ionisation age,
temperature, interstellar column density, abundances and Doppler velocities.
The abundances of Si, S, Ar and Ca are strongly correlated. The abundance
ratios are consistent with the nucleosynthesis yield from the collapse of a 12
Msun progenitor. The abundance ratios Ne/Si, Mg/Si, Fe/Si and Ni/Si are very
variable and distinctly different from S/Si, Ar/Si and Ca/Si, in line with the
current explosive nucleosynthesis models. The ionisation age and temperature of
both NEI components varies considerably over the remnant. Accurate
determination of these parameters yield reliable Doppler velocities for both
components. The data are consistent with a plasma velocity of 2600 km/s at the
shock radius of 153" implying a primary shock velocity of 4000+/-500 km/s. The
Si-K and S-K line emission from the cool component is confined to a relatively
narrow shell with radius 100-150". This component is almost certainly ejecta
material which has been heated by a combination of the reverse shock and
heating of ejecta clumps as they plough through the medium which has been
pre-heated by the primary shock. The Fe-K line emission is expanding faster and
spans a radius range 110-170". The bulk of the Fe emission is confined to two
large clumps and it is likely that these too are the result of ablation from
ejecta bullets rather swept up circumstellar medium.Comment: 10 pages, 11 figures, submitted to Astronomy and Astrophysic
Measuring the cosmic ray acceleration efficiency of a supernova remnant
Cosmic rays are the most energetic particles arriving at earth. Although most
of them are thought to be accelerated by supernova remnants, the details of the
acceleration process and its efficiency are not well determined. Here we show
that the pressure induced by cosmic rays exceeds the thermal pressure behind
the northeast shock of the supernova remnant RCW 86, where the X-ray emission
is dominated by synchrotron radiation from ultra-relativistic electrons. We
determined the cosmic-ray content from the thermal Doppler broadening measured
with optical spectroscopy, combined with a proper-motion study in X- rays. The
measured post-shock proton temperature in combination with the shock velocity
does not agree with standard shock heating, implying that >50% of the
post-shock pressure is produced by cosmic rays.Comment: Published in Science express, 10 pages, 5 figures and 2 table
Test engineering education in Europe: the EuNICE-Test project
The paper deals with a European experience of education in industrial test of ICs and SoCs using remote testing facilities. The project addresses the problem of the shortage in microelectronics engineers aware with the new challenge of testing mixed-signal SoCs far multimedia/telecom market. It aims at providing test training facilities at a European scale in both initial and continuing education contexts. This is done by allowing the academic and industrial partners of the consortium to train engineers using the common test resources center (CRTC) hosted by LIRMM (Laboratoire d'Informatique, de Robotique et de Microelectronique de Montpellier, France). CRTC test tools include up-to-date/high-tech testers that are fully representative of real industrial testers as used on production testfloors. At the end of the project, it is aimed at reaching a cruising speed of about 16 trainees per year per center. Each trainee will have attend at least one one-week training using the remote test facilities of CRTC
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