5,308 research outputs found
The one-round Voronoi game replayed
We consider the one-round Voronoi game, where player one (``White'', called
``Wilma'') places a set of n points in a rectangular area of aspect ratio r
<=1, followed by the second player (``Black'', called ``Barney''), who places
the same number of points. Each player wins the fraction of the board closest
to one of his points, and the goal is to win more than half of the total area.
This problem has been studied by Cheong et al., who showed that for large
enough and r=1, Barney has a strategy that guarantees a fraction of 1/2+a,
for some small fixed a.
We resolve a number of open problems raised by that paper. In particular, we
give a precise characterization of the outcome of the game for optimal play: We
show that Barney has a winning strategy for n>2 and r>sqrt{2}/n, and for n=2
and r>sqrt{3}/2. Wilma wins in all remaining cases, i.e., for n>=3 and
r<=sqrt{2}/n, for n=2 and r<=sqrt{3}/2, and for n=1. We also discuss complexity
aspects of the game on more general boards, by proving that for a polygon with
holes, it is NP-hard to maximize the area Barney can win against a given set of
points by Wilma.Comment: 14 pages, 6 figures, Latex; revised for journal version, to appear in
Computational Geometry: Theory and Applications. Extended abstract version
appeared in Workshop on Algorithms and Data Structures, Springer Lecture
Notes in Computer Science, vol.2748, 2003, pp. 150-16
Local impurity effects in superconducting graphene
We study the effect of impurities in superconducting graphene and discuss
their influence on the local electronic properties. In particular, we consider
the case of magnetic and non-magnetic impurities being either strongly
localized or acting as a potential averaged over one unit cell. The spin
dependent local density of states is calculated and possibilities for
visualizing impurities by means of scanning tunneling experiments is pointed
out. A possibility of identifying magnetic scatters even by non spin-polarized
scanning tunneling spectroscopy is explained.Comment: 4 pages, 4 figure
Dynamical singlets and correlation-assisted Peierls transition in VO2
A theory of the metal-insulator transition in vanadium dioxide from the
high-temperature rutile to the low- temperature monoclinic phase is proposed on
the basis of cluster dynamical mean field theory, in conjunction with the
density functional scheme. The interplay of strong electronic Coulomb
interactions and structural distortions, in particular the dimerization of
vanadium atoms in the low temperature phase, plays a crucial role. We find that
VO2 is not a conventional Mott insulator, but that the formation of dynamical
V-V singlet pairs due to strong Coulomb correlations is necessary to trigger
the opening of a Peierls gap.Comment: 5 page
Electronic structure and spectral properties of Am, Cm and Bk: Charge density self-consistent LDA+HIA calculations in FP-LAPW basis
We provide a straightforward and numerically efficient procedure to perform
local density approximation + Hubbard I (LDA+HIA) calculations, including
self-consistency over the charge density, within the full potential linearized
augmented plane wave (FP-LAPW) method. This implementation is all-electron,
includes spin-orbit interaction, and makes no shape approximations for the
charge density. The method is applied to calculate selected heavy actinides in
the paramagnetic phase. The electronic structure and spectral properties of Am
and Cm metals obtained are in agreement with previous dynamical mean-field
theory (LDA+DMFT) calculations and with available experimental data. We point
out that the charge density self-consistent LDA+HIA calculations predict the
charge on Bk to exceed the atomic integer value by 0.22.Comment: 8 pages, 1 figur
Thermodynamic consistency of the charge response in dynamical mean-field based approaches
We consider the thermodynamic consistency of the charge response function in
the (extended) Hubbard model. In DMFT, thermodynamic consistency is preserved.
We prove that the static, homogeneous DMFT susceptibility is consistent as long
as vertex corrections obtained from the two-particle impurity correlation
function are included. In presence of a nonlocal interaction, the problem may
be treated within extended DMFT (EDMFT), or its diagrammatic extension, the
dual boson approach. We show that here, maintaining thermodynamic consistency
requires knowledge of three- and four-particle impurity correlation functions,
which are typically neglected. Nevertheless, the dual boson approximation to
the response is remarkably close to consistency. This holds even when
two-particle vertex corrections are neglected. EDMFT is consistent only in the
strongly correlated regime and near half-filling, where the physics is
predominantly local.Comment: 11 pages (incl. appendix), 4 figure
Formal Design of Asynchronous Fault Detection and Identification Components using Temporal Epistemic Logic
Autonomous critical systems, such as satellites and space rovers, must be
able to detect the occurrence of faults in order to ensure correct operation.
This task is carried out by Fault Detection and Identification (FDI)
components, that are embedded in those systems and are in charge of detecting
faults in an automated and timely manner by reading data from sensors and
triggering predefined alarms. The design of effective FDI components is an
extremely hard problem, also due to the lack of a complete theoretical
foundation, and of precise specification and validation techniques. In this
paper, we present the first formal approach to the design of FDI components for
discrete event systems, both in a synchronous and asynchronous setting. We
propose a logical language for the specification of FDI requirements that
accounts for a wide class of practical cases, and includes novel aspects such
as maximality and trace-diagnosability. The language is equipped with a clear
semantics based on temporal epistemic logic, and is proved to enjoy suitable
properties. We discuss how to validate the requirements and how to verify that
a given FDI component satisfies them. We propose an algorithm for the synthesis
of correct-by-construction FDI components, and report on the applicability of
the design approach on an industrial case-study coming from aerospace.Comment: 33 pages, 20 figure
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Engaging Students in MIS Course through the Creation of e-Businesses: A Self Determination Theory Analysis
We describe an entrepreneurial approach for teaching a Management Information Systems (MIS) course. The course builds on the psychological needs for autonomy, competence and relatedness to encourage student motivation and engagement. Students are required to create new electronic businesses and to build prototypes of their electronic web fronts. Students are also required to use the concepts taught in the course and to analyze the ventures’ strategies, related network externalities, as well as business processes and data analytics. The course was taught since 2009 to five classes with more than two hundred students. Using student evaluation questionnaires and four detailed interviews, we find that more than half of the students are enthusiastic about the new approach, feeling satisfied and even proud of their projects. However, a minority of students found the course over-complicated and even boring. We found also that students who expressed autonomy orientation engaged with the course, while students who expressed impersonal or controlled orientation did not. We believe that this analysis should help instructors in aligning new teaching opportunities created by IT to fit students’ orientations and needs
Trends and guidelines in online privacy policy
Online privacy policies (OPP) are important mechanisms for informing online consumers about the level of information privacy protection afforded when visiting web sites. To date, societal mechanisms and technologies have been the focus of attempts to improve the quality and effectiveness of OPPs. We present findings from a longitudinal, empirical study of online privacy policies. Our research found that although online privacy policies have improved in quality and effectiveness since 2000, they still fall well short of the level of privacy assurance desired by consumers. This study analyses trends in OPPs over the two years of the study, identifying areas of deficiency and improvements, and offering a solution in the form of a detailed set of guidelines for organisational online privacy policy. Our study adds to existing theory in this area and, more immediately, will assist businesses concerned about the effect of privacy issues on consumer web usage.<br /
Electron energy spectrum of the spin-liquid state in a frustrated Hubbard model
Non-local correlation effects in the half-filled Hubbard model on an
isotropic triangular lattice are studied within a spin polarized extension of
the dual fermion approach. A competition between the antiferromagnetic
non-collinear and the spin liquid states is strongly enhanced by an
incorporation of a k-dependent self-energy beyond the local dynamical
mean-field theory. The dual fermion correc- tions drastically decrease the
energy of a spin liquid state while leaving the non-collinear magnetic states
almost non-affected. This makes the spin liquid to become a preferable state in
a certain interval of interaction strength of an order of the magnitude of a
bandwidth. The spectral function of the spin-liquid Mott insulator is
determined by a formation of local singlets which results in the energy gap of
about twice larger than that of the 120 degrees antiferromagnetic Neel state.Comment: 6 pages, 4 figure
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