441 research outputs found
Comparison of Perron and Floquet eigenvalues in age structured cell division cycle models
We study the growth rate of a cell population that follows an age-structured
PDE with time-periodic coefficients. Our motivation comes from the comparison
between experimental tumor growth curves in mice endowed with intact or
disrupted circadian clocks, known to exert their influence on the cell division
cycle. We compare the growth rate of the model controlled by a time-periodic
control on its coefficients with the growth rate of stationary models of the
same nature, but with averaged coefficients. We firstly derive a delay
differential equation which allows us to prove several inequalities and
equalities on the growth rates. We also discuss about the necessity to take
into account the structure of the cell division cycle for chronotherapy
modeling. Numerical simulations illustrate the results.Comment: 26 page
Peningkatan Sifat Mekanik Paduan Aluminium A356.2 Dengan Penambahan Manganese (Mn) Dan Perlakuan Panas T6
Aluminum Alloy A356.2 is one of aluminum alloys that is suitable to be used as a car wheel - rim material. This is because it has some benefits such as lightweight material, corrosion resistance material, interesting color but its mechanical properties do not meet criteria of JIS H 5202. For that reason, to meet the standard its mechanical properties need to be improved. Mechanical properties of this alloy can be improved using many ways; one of these is by changing its chemical composition. In this research, Mn element is added to alloy A356.2 containing Mn 0.05%w originally. Mn addition to the alloy is commenced from the amount of 0.2%w, 0.4; 0.6; 0.8; 1.0; 1.2; 1.4 and finally 1.6%w. After this, some tests are conducted to these new alloys including tensile test, hardness test, impact test and micro - structural test. Test results show that the change of material mechanical properties occurs due to Mn addition and heat treatment T6 to these alloys. The optimum mechanical properties are obtained with the addition of 1.2%w Mn to the alloy. In this condition, the alloy has Ultimate Tensile Stength of 31.58 kg/mm2, elongation of 7.54%, hardness test of 90.74 HVN and impact test of 5.88 J/cm2 and these results fulfill JIS H 5202 standard
Pitfalls on evaluating pair exchange interactions for modelling molecule-based magnetism
Molecule-based magnetism is a solid-state property that results from the microscopic interaction between magnetic centres or radicals. The observed magnetic response is due to unpaired electrons whose coupling leads to a particular magnetic topology. Therefore, to understand the magnetic response of a given molecule-based magnet and reproduce the available experimental magnetic properties by means of statistical mechanics, one has to be able to determine the value of the JAB magnetic exchange coupling between radicals. The calculation of JAB is thus a key point for modelling molecule-based magnetism. In this Perspectives article, we will build upon our experience in modelling molecular magnetism to point out some pitfalls on evaluating JAB couplings. Special attention must be paid to the cluster models used to evaluate JAB, which should account for cooperative effects among JAB interactions and also consider the environment (counterions, hydrogen bonding) of the two radicals whose interaction has to be evaluated. It will be also necessary to assess whether a DFT-based or a wavefunction-based method is best to study a given radical. Finally, in addition to model and method, the JAB couplings have to be able to adapt to changes in the magnetic topology due to thermal fluctuations. Therefore, it is most important to appraise in which systems molecular dynamics simulations would be required. Given the large number of issues one must tackle when choosing the correct model and method to evaluate JAB interactions for modelling magnetic properties in molecule-based materials, the “human factor” is a must to cross-examine and challenge computations before trusting any result
Global observational diagnosis of soil moisture control on the land surface energy balance
An understanding of where and how strongly the surface energy budget is constrained by soil
moisture is hindered by a lack of large-scale observations, and this contributes to uncertainty in climate
models. Here we present a new approach combining satellite observations of land surface temperature and
rainfall.We derive a Relative Warming Rate (RWR) diagnostic, which is a measure of how rapidly the land warms
relative to the overlying atmosphere during 10 day dry spells. In our dry spell composites, 73% of the land
surface between 60°S and 60°N warms faster than the atmosphere, indicating water-stressed conditions, and
increases in sensible heat. Higher RWRs are found for shorter vegetation and bare soil than for tall, deep-rooted
vegetation, due to differences in aerodynamic and hydrological properties. We show how the variation of RWR
with antecedent rainfall helps to identify different evaporative regimes in the major nonpolar climate zones
Migration paths saturations in meta-epidemic systems
In this paper we consider a simple two-patch model in which a population
affected by a disease can freely move. We assume that the capacity of the
interconnected paths is limited, and thereby influencing the migration rates.
Possible habitat disruptions due to human activities or natural events are
accounted for. The demographic assumptions prevent the ecosystem to be wiped
out, and the disease remains endemic in both populated patches at a stable
equilibrium, but possibly also with an oscillatory behavior in the case of
unidirectional migrations. Interestingly, if infected cannot migrate, it is
possible that one patch becomes disease-free. This fact could be exploited to
keep disease-free at least part of the population
Compilation and validation of SAR and optical data products for a complete and global map of inland/ocean water tailored to the climate modeling community
Accurate maps of surface water extent are of paramount importance for water management, satellite data processing and climate modeling. Several maps of water bodies based on remote sensing data have been released during the last decade. Nonetheless, none has a truly (90°N/90°S) global coverage while being thoroughly validated. This paper describes a global, spatially-complete (void-free) and accurate mask of inland/ocean water for the 2000–2012 period, built in the framework of the European Space Agency (ESA) Climate Change Initiative (CCI). This map results from the synergistic combination of multiple individual SAR and optical water body and auxiliary datasets. A key aspect of this work is the original and rigorous stratified random sampling designed for the quality assessment of binary classifications where one class is marginally distributed. Input and consolidated products were assessed qualitatively and quantitatively against a reference validation database of 2110 samples spread throughout the globe. Using all samples, overall accuracy was always very high among all products, between 98% and 100%. The CCI global map of open water bodies provided the best water class representation (F-score of 89%) compared to its constitutive inputs. When focusing on the challenging areas for water bodies’ mapping, such as shorelines, lakes and river banks, all products yielded substantially lower accuracy figures with overall accuracies ranging between 74% and 89%. The inland water area of the CCI global map of open water bodies was estimated to be 3.17 million km2 ± 0.24 million km2. The dataset is freely available through the ESA CCI Land Cover viewer
Effects of rapid prey evolution on predator-prey cycles
We study the qualitative properties of population cycles in a predator-prey
system where genetic variability allows contemporary rapid evolution of the
prey. Previous numerical studies have found that prey evolution in response to
changing predation risk can have major quantitative and qualitative effects on
predator-prey cycles, including: (i) large increases in cycle period, (ii)
changes in phase relations (so that predator and prey are cycling exactly out
of phase, rather than the classical quarter-period phase lag), and (iii)
"cryptic" cycles in which total prey density remains nearly constant while
predator density and prey traits cycle. Here we focus on a chemostat model
motivated by our experimental system [Fussmann et al. 2000,Yoshida et al. 2003]
with algae (prey) and rotifers (predators), in which the prey exhibit rapid
evolution in their level of defense against predation. We show that the effects
of rapid prey evolution are robust and general, and furthermore that they occur
in a specific but biologically relevant region of parameter space: when traits
that greatly reduce predation risk are relatively cheap (in terms of reductions
in other fitness components), when there is coexistence between the two prey
types and the predator, and when the interaction between predators and
undefended prey alone would produce cycles. Because defense has been shown to
be inexpensive, even cost-free, in a number of systems [Andersson and Levin
1999, Gagneux et al. 2006,Yoshida et al. 2004], our discoveries may well be
reproduced in other model systems, and in nature. Finally, some of our key
results are extended to a general model in which functional forms for the
predation rate and prey birth rate are not specified.Comment: 35 pages, 8 figure
Analysis of symmetries in models of multi-strain infections
In mathematical studies of the dynamics of multi-strain diseases caused by antigenically diverse pathogens, there is a substantial interest in analytical insights. Using the example of a generic model of multi-strain diseases with cross-immunity between strains, we show that a significant understanding of the stability of steady states and possible dynamical behaviours can be achieved when the symmetry of interactions between strains is taken into account. Techniques of equivariant bifurcation theory allow one to identify the type of possible symmetry-breaking Hopf bifurcation, as well as to classify different periodic solutions in terms of their spatial and temporal symmetries. The approach is also illustrated on other models of multi-strain diseases, where the same methodology provides a systematic understanding of bifurcation scenarios and periodic behaviours. The results of the analysis are quite generic, and have wider implications for understanding the dynamics of a large class of models of multi-strain diseases
The meaning and importance of dignified care: Findings from a survey of health and social care professionals
This article is available through the Brunel Open Access Publishing Fund. Copyright © 2013 Cairns et al.; licensee BioMed Central Ltd.There are well established national and local policies championing the need to provide dignity in care for older people. We have evidence as to what older people and their relatives understand by the term 'dignified care' but less insight into the perspectives of staff regarding their understanding of this key policy objective.This research was supported by the Dunhill Medical Trust [grant number: R93/1108]
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