4,204 research outputs found
On wild extensions of a p-adic field
In this paper we consider the problem of classifying the isomorphism classes
of extensions of degree pk of a p-adic field, restricting to the case of
extensions without intermediate fields. We establish a correspondence between
the isomorphism classes of these extensions and some Kummer extensions of a
suitable field F containing K. We then describe such classes in terms of the
representations of Gal(F/K). Finally, for k = 2 and for each possible Galois
group G, we count the number of isomorphism classes of the extensions whose
normal closure has a Galois group isomorphic to G. As a byproduct, we get the
total number of isomorphism classes
From continuous improvement to collaborative innovation : the next challenge in supply chain management
This paper considers the growing importance of inter-company collaboration, and develops the concept of intra-company continuous improvement through to what may be termed collaborative innovation between members of an extended manufacturing enterprise (EME). The importance of ICTs to such company networks is considered but research has shown that no amount of technology can overcome a lack of trust and ineffective goal setting between key partners involved in the cross-company projects. Different governance models may also impact on the success or otherwise of the network. This paper provides an overview of the main topics considered in this Special Issu
A QR based approach for the nonlinear eigenvalue problem
We describe a fast and numerically robust approach based on the structured QR eigenvalue algorithm for computing approximations of the eigenvalues of a holomorphic matrix-valued function inside the unit circle. Numerical experiments confirm the effectiveness of the proposed method
Gershgorin disks for multiple eigenvalues of non-negative matrices
Gershgorin's famous circle theorem states that all eigenvalues of a square
matrix lie in disks (called Gershgorin disks) around the diagonal elements.
Here we show that if the matrix entries are non-negative and an eigenvalue has
geometric multiplicity at least two, then this eigenvalue lies in a smaller
disk. The proof uses geometric rearrangement inequalities on sums of higher
dimensional real vectors which is another new result of this paper
In Search of Differential Inhibitors of Aldose Reductase
Aldose reductase, classified within the aldo-keto reductase family as AKR1B1, is an NADPH dependent enzyme that catalyzes the reduction of hydrophilic as well as hydrophobic aldehydes. AKR1B1 is the first enzyme of the so-called polyol pathway that allows the conversion of glucose into sorbitol, which in turn is oxidized to fructose by sorbitol dehydrogenase. The activation of the polyol pathway in hyperglycemic conditions is generally accepted as the event that is responsible for a series of long-term complications of diabetes such as retinopathy, cataract, nephropathy and neuropathy. The role of AKR1B1 in the onset of diabetic complications has made this enzyme the target for the development of molecules capable of inhibiting its activity. Virtually all synthesized compounds have so far failed as drugs for the treatment of diabetic complications. This failure may be partly due to the ability of AKR1B1 to reduce alkenals and alkanals, produced in oxidative stress conditions, thus acting as a detoxifying agent. In recent years we have proposed an alternative approach to the inhibition of AKR1B1, suggesting the possibility of a differential inhibition of the enzyme through molecules able to preferentially inhibit the reduction of either hydrophilic or hydrophobic substrates. The rationale and examples of this new generation of aldose reductase differential inhibitors (ARDIs) are presented
Giant Anisotropic Magneto-Resistance in ferromagnetic atomic contacts
Magneto-resistance is a physical effect of great fundamental and industrial
interest since it is the basis for the magnetic field sensors used in computer
read-heads and Magnetic Random Access Memories. As device dimensions are
reduced, some important physical length scales for magnetism and electrical
transport will soon be attained. Ultimately, there is a strong need to know if
the physical phenomena responsible for magneto-resistance still hold at the
atomic scale. Here, we show that the anisotropy of magneto-resistance is
greatly enhanced in atomic size constrictions. We explain this physical effect
by a change in the electronic density of states in the junction when the
magnetization is rotated, as supported by our ab-initio calculations. This
stems from the "spin-orbit coupling" mechanism linking the shape of the
orbitals with the spin direction. This sensitively affects the conductance of
atomic contacts which is determined by the overlap of the valence orbitals.Comment: latex AAMR.tex, 6 files, 5 figures, 4 pages
(http://www-drecam.cea.fr/spec/articles/S06/011
First-principles study of lattice instabilities in the ferromagnetic martensite NiMnGa
The phonon dispersion relations and elastic constants for ferromagnetic
NiMnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
, the TA tranverse acoustic branch along and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on . Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C=(CC)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding modulation. The computed relation between the
ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ( = 1) and martensitic ( =
0.94) phases of NiMnGa.Comment: submitted to Phys. Rev.
Spontaneous polarization and piezoelectric constants of III-V nitrides
The spontaneous polarization, dynamical Born charges, and piezoelectric
constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using
the Berry phase approach to polarization in solids. The piezoelectric constants
are found to be up 10 times larger than in conventional III-V's and II-VI's,
and comparable to those of ZnO. Further properties at variance with those of
conventional III-V compounds are the sign of the piezoelectric constants
(positive as in II-VI's) and the very large spontaneous polarization.Comment: RevTeX 4 pages, improved upon revie
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
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