864 research outputs found
Deterministic continutation of stochastic metastable equilibria via Lyapunov equations and ellipsoids
Numerical continuation methods for deterministic dynamical systems have been
one of the most successful tools in applied dynamical systems theory.
Continuation techniques have been employed in all branches of the natural
sciences as well as in engineering to analyze ordinary, partial and delay
differential equations. Here we show that the deterministic continuation
algorithm for equilibrium points can be extended to track information about
metastable equilibrium points of stochastic differential equations (SDEs). We
stress that we do not develop a new technical tool but that we combine results
and methods from probability theory, dynamical systems, numerical analysis,
optimization and control theory into an algorithm that augments classical
equilibrium continuation methods. In particular, we use ellipsoids defining
regions of high concentration of sample paths. It is shown that these
ellipsoids and the distances between them can be efficiently calculated using
iterative methods that take advantage of the numerical continuation framework.
We apply our method to a bistable neural competition model and a classical
predator-prey system. Furthermore, we show how global assumptions on the flow
can be incorporated - if they are available - by relating numerical
continuation, Kramers' formula and Rayleigh iteration.Comment: 29 pages, 7 figures [Fig.7 reduced in quality due to arXiv size
restrictions]; v2 - added Section 9 on Kramers' formula, additional
computations, corrected typos, improved explanation
Climate change, society issues and sustainable agriculture
Despite its prediction 100 years ago by scientists studying CO2, man-made climate change has been officially recognised only in 2007 by the Nobel prize committee. Climate changes since the industrial revolution have already deeply impacted ecosystems. I report major impacts of climate change on waters, terrestrial ecosystems, agriculture, and economy in Europe. The lesson of the climate change story is that humans do not learn from scientists until it really hurts. Furthermore, all society issues cannot be solved anymore using the old, painkiller approach because all issues are now huge, linked, global and fast-developing. In that respect, actual society structures are probably outdated. Here, agronomists are the most advanced scientists to solve society issues because they master the study of complex systems, from the molecule to the global scale. Now more than ever agriculture is a central point to which all society issues are bound. Indeed, humans eat food
On the nonlocal viscosity kernel of mixtures
In this report we investigate the multiscale hydrodynamical response of a
liquid as a function of mixture composition. This is done via a series of
molecular dynamics simulations where the wave vector dependent viscosity kernel
is computed for three mixtures each with 7-15 different compositions. We
observe that the nonlocal viscosity kernel is dependent on composition for
simple atomic mixtures for all the wave vectors studied here, however, for a
model polymer melt mixture the kernel is independent of composition for large
wave vectors. The deviation from ideal mixing is also studied. Here it is shown
that a Lennard-Jones mixture follows the ideal mixing rule surprisingly well
for a large range of wave vectors, whereas for both the Kob-Andersen mixture
and the polymer melt large deviations are found. Furthermore, for the polymer
melt the deviation is wave vector dependent such that there exists a critical
length scale at which the ideal mixing goes from under-estimating to
over-estimating the viscosity
First-principles study on field evaporation for silicon atom on Si(001) surface
The simulations of field-evaporation processes for silicon atoms on various
Si(001) surfaces are implemented using the first-principles calculations based
on the real-space finite-difference method. We find that the atoms which locate
on atomically flat Si(001) surfaces and at step edges are easily removed by
applying external electric field, and the threshold value of the external
electric field for evaporation of atoms on atomically flat Si(001) surfaces,
which is predicted between 3.0 and 3.5 V/\AA, is in agreement with the
experimental data of 3.8 V/\AA. In this situation, the local field around an
evaporating atom does not play a crucial role. This result is instead
interpreted in terms of the bond strength between an evaporating atom and
surface.Comment: 5 pages and 4 figure
Theorien der Chemie
In der folgenden Reihe von Vorlesungen werde ich Ehre haben, Ihnen einen Ueberblick über die Entwicklung der Theorie in der Chemie zu geben.Material perteneciente a la Biblioteca del Departamento de Física de la Universidad Nacional de La Plata digitalizado en SEDICI.Biblioteca del Departamento de Físic
Lord Rutherford of Nelson, His 1908 Nobel Prize in Chemistry and Why He Didn't Get a Second Prize
"I have dealt with many different transformations with various periods of
time, but the quickest that I have met was my own transformation in one moment
from a physicist to a chemist."
Ernest Rutherford (Nobel Banquet, 1908)
This article is about how Ernest Rutherford (1871-1937) got the 1908 Nobel
Prize in Chemistry and why he did not get a second Prize for his subsequent
outstanding discoveries in physics, specially the discovery of the atomic
nucleus and the proton. Who were those who nominated him and who did he
nominate for the Nobel Prizes.
In order to put the Prize issue into its proper context, I will briefly
describe Rutherford's whereabouts.
Rutherford, an exceptionally gifted scientist who revolutionized chemistry
and physics, was moulded in the finest classical tradition. What were his
opinions on some scientific issues such as Einstein's photon, uncertainty
relations and the future prospects for atomic energy? What would he have said
about the "Theory of Everything"?Comment: Extended version of an invited talk presented at the neutrino
conference "Neutrino 2008", Christchurch, New Zealand, 25-31 May 200
Theorien der Chemie
In der folgenden Reihe von Vorlesungen werde ich Ehre haben, Ihnen einen Ueberblick über die Entwicklung der Theorie in der Chemie zu geben.Material perteneciente a la Biblioteca del Departamento de Física de la Universidad Nacional de La Plata digitalizado en SEDICI.Biblioteca del Departamento de Físic
Asymmetries in the Value of Existence
According to asymmetric comparativism, it is worse for a person to exist with a miserable life than not to exist, but it is not better for a person to exist with a happy life than not to exist. My aim in this paper is to explain how asymmetric comparativism could possibly be true. My account of asymmetric comparativism begins with a different asymmetry, regarding the (dis)value of early death. I offer an account of this early death asymmetry, appealing to the idea of conditional goods, and generalize it to explain how asymmetric comparativism could possibly be true. I also address the objection that asymmetric comparativism has unacceptably antinatalist implications
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