343 research outputs found
Weakly-nonlocal Symplectic Structures, Whitham method, and weakly-nonlocal Symplectic Structures of Hydrodynamic Type
We consider the special type of the field-theoretical Symplectic structures
called weakly nonlocal. The structures of this type are in particular very
common for the integrable systems like KdV or NLS. We introduce here the
special class of the weakly nonlocal Symplectic structures which we call the
weakly nonlocal Symplectic structures of Hydrodynamic Type. We investigate then
the connection of such structures with the Whitham averaging method and propose
the procedure of "averaging" of the weakly nonlocal Symplectic structures. The
averaging procedure gives the weakly nonlocal Symplectic Structure of
Hydrodynamic Type for the corresponding Whitham system. The procedure gives
also the "action variables" corresponding to the wave numbers of -phase
solutions of initial system which give the additional conservation laws for the
Whitham system.Comment: 64 pages, Late
An update on the Hirsch conjecture
The Hirsch conjecture was posed in 1957 in a letter from Warren M. Hirsch to
George Dantzig. It states that the graph of a d-dimensional polytope with n
facets cannot have diameter greater than n - d.
Despite being one of the most fundamental, basic and old problems in polytope
theory, what we know is quite scarce. Most notably, no polynomial upper bound
is known for the diameters that are conjectured to be linear. In contrast, very
few polytopes are known where the bound is attained. This paper collects
known results and remarks both on the positive and on the negative side of the
conjecture. Some proofs are included, but only those that we hope are
accessible to a general mathematical audience without introducing too many
technicalities.Comment: 28 pages, 6 figures. Many proofs have been taken out from version 2
and put into the appendix arXiv:0912.423
New and simple algorithms for stable flow problems
Stable flows generalize the well-known concept of stable matchings to markets
in which transactions may involve several agents, forwarding flow from one to
another. An instance of the problem consists of a capacitated directed network,
in which vertices express their preferences over their incident edges. A
network flow is stable if there is no group of vertices that all could benefit
from rerouting the flow along a walk.
Fleiner established that a stable flow always exists by reducing it to the
stable allocation problem. We present an augmenting-path algorithm for
computing a stable flow, the first algorithm that achieves polynomial running
time for this problem without using stable allocation as a black-box
subroutine. We further consider the problem of finding a stable flow such that
the flow value on every edge is within a given interval. For this problem, we
present an elegant graph transformation and based on this, we devise a simple
and fast algorithm, which also can be used to find a solution to the stable
marriage problem with forced and forbidden edges.
Finally, we study the stable multicommodity flow model introduced by
Kir\'{a}ly and Pap. The original model is highly involved and allows for
commodity-dependent preference lists at the vertices and commodity-specific
edge capacities. We present several graph-based reductions that show
equivalence to a significantly simpler model. We further show that it is
NP-complete to decide whether an integral solution exists
From Fe3O4/NiO bilayers to NiFe2O4-like thin films through Ni interdiffusion
Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni ions out of NiO into FeO ultrathin films, resulting in off-stoichiometric nickel ferrite–like thin layers. We synthesized epitaxial FeO bilayers on Nb-doped SrTiO(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400C, 600C, and 800C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFeO. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni and Fe ions and the Fe ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films
A reconstruction of the initial conditions of the Universe by optimal mass transportation
Reconstructing the density fluctuations in the early Universe that evolved
into the distribution of galaxies we see today is a challenge of modern
cosmology [ref.]. An accurate reconstruction would allow us to test
cosmological models by simulating the evolution starting from the reconstructed
state and comparing it to the observations. Several reconstruction techniques
have been proposed [8 refs.], but they all suffer from lack of uniqueness
because the velocities of galaxies are usually not known. Here we show that
reconstruction can be reduced to a well-determined problem of optimisation, and
present a specific algorithm that provides excellent agreement when tested
against data from N-body simulations. By applying our algorithm to the new
redshift surveys now under way [ref.], we will be able to recover reliably the
properties of the primeval fluctuation field of the local Universe and to
determine accurately the peculiar velocities (deviations from the Hubble
expansion) and the true positions of many more galaxies than is feasible by any
other method.
A version of the paper with higher-quality figures is available at
http://www.obs-nice.fr/etc7/nature.pdfComment: Latex, 4 pages, 3 figure
Electrical resistivity, magnetism and electronic structure of the intermetallic 3d/4f Laves phase compounds ErNi2Mnx
The non-stoichiometric intermetallic compounds RENi2Mnx (RE = rare earth) with
the cubic MgCu2-type structure display a large variety of magnetic properties which
is due to a complex interplay between the degrees of freedom of the 3d and
4f electrons and their interactions. We performed a comprehensive study of the
electrical resistivity, magnetic properties and the electronic structure of ErNi2Mnx
(x =0, 0.25, 0.5, 0.75, 1, 1.25) compounds by employing a suitable set of complementary
experimental approaches. We find an increase in electrical resistance compared
to ErNi2 upon Mn doping, the residual resistivity ratio decreases with increasing
manganese content. The Curie temperature exhibits a sharp increase to around 50 K
for Mn concentrations x 0.5, whereas the saturation magnetization decreases
with growing Mn content x 0.5. Valence band X-ray photoelectron spectroscopy
reveals an increasing intensity of Mn 3d states near Fermi energy in dependence
of Mn concentration and Curie temperature. Resonant photoelectron spectroscopy
of ErNi2Mn0.75 reveals that the photoemission decay channels dominate the valence
band spectra across the Er N5 and Mn L3 X-ray absorption maxima, whereas the
L3VV Auger dictates the resonant valence band spectra close to and at the Ni L3
X-ray absorption edge
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