398 research outputs found
Maximum weight cycle packing in directed graphs, with application to kidney exchange programs
Centralized matching programs have been established in several countries to organize kidney exchanges between incompatible patient-donor pairs. At the heart of these programs are algorithms to solve kidney exchange problems, which can be modelled as cycle packing problems in a directed graph, involving cycles of length 2, 3, or even longer. Usually, the goal is to maximize the number of transplants, but sometimes the total benefit is maximized by considering the differences between suitable kidneys. These problems correspond to computing cycle packings of maximum size or maximum weight in directed graphs. Here we prove the APX-completeness of the problem of finding a maximum size exchange involving only 2-cycles and 3-cycles. We also present an approximation algorithm and an exact algorithm for the problem of finding a maximum weight exchange involving cycles of bounded length. The exact algorithm has been used to provide optimal solutions to real kidney exchange problems arising from the National Matching Scheme for Paired Donation run by NHS Blood and Transplant, and we describe practical experience based on this collaboration
Geometric approach to chaos in the classical dynamics of abelian lattice gauge theory
A Riemannian geometrization of dynamics is used to study chaoticity in the
classical Hamiltonian dynamics of a U(1) lattice gauge theory. This approach
allows one to obtain analytical estimates of the largest Lyapunov exponent in
terms of time averages of geometric quantities. These estimates are compared
with the results of numerical simulations, and turn out to be very close to the
values extrapolated for very large lattice sizes even when the geometric
quantities are computed using small lattices. The scaling of the Lyapunov
exponent with the energy density is found to be well described by a quadratic
power law.Comment: REVTeX, 9 pages, 4 PostScript figures include
Fluid dynamical equations and transport coefficients of relativistic gases with non-extensive statistics
We derive equations for fluid dynamics from a non-extensive Boltzmann
transport equation consistent with Tsallis' non-extensive entropy formula. We
evaluate transport coefficients employing the relaxation time approximation and
investigate non-extensive effects in leading order dissipative phenomena at
relativistic energies, like heat conductivity, shear and bulk viscosity.Comment: 9 pages, 5 figures. Some small corrections in the text and in the
first figure caption; accepted for publication in Physical Review
Tuning the electronic structure of graphene by ion irradiation
Mechanically exfoliated graphene layers deposited on SiO2 substrate were
irradiated with Ar+ ions in order to experimentally study the effect of atomic
scale defects and disorder on the low-energy electronic structure of graphene.
The irradiated samples were investigated by scanning tunneling microscopy and
spectroscopy measurements, which reveal that defect sites, besides acting as
scattering centers for electrons through local modification of the on-site
potential, also induce disorder in the hopping amplitudes. The most important
consequence of the induced disorder is the substantial reduction in the Fermi
velocity, revealed by bias-dependent imaging of electron-density oscillations
observed near defect sites
Strange hyperon and antihyperon production from quark and string-rope matter
Hyperon and antihyperon production is investigated using two microscopical
models: {\bf (1)} the fast hadronization of quark matter as given by the ALCOR
model; {\bf (2)} string formation and fragmentation as in the HIJING/B model.
We calculate the particle numbers and momentum distributions for Pb+Pb
collisions at CERN SPS energies in order to compare the two models with each
other and with the available experimental data. We show that these two
theoretical approaches give similar yields for the hyperons, but strongly
differ for antihyperons.Comment: 11 pages, Latex, 3 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
Relativistic hydrodynamics with strangeness production
The relativistic hydrodynamic approach is used to describe production of
strangeness and/or heavy quarks in ultrarelativistic heavy ion reactions.
Production processes are important ingredients of dissipative effects in the
hadronic liquid. Beyond viscosity also chemo- and thermo-diffusion processes
are considered. This also allows to specify chemical and thermal freeze-out
conditions.Comment: v.2 with minor editorial corrections, 7 pages, talk given on the
SQM2007 conference, Levoca, June 24-29, 2007. To appear in the proceceeding:
Journal of Physics
How much is your diet? (Estimation about prices of “traditional Hungarian”, diabetic, low energy diets, and related life-style expenses)
Nutrition and lifestyle-related diseases are some of the leading morbidities among the Hungarian population. People who want to lose weight often complain that healthy diet is expensive.
Our aim was to quantify the costs of three different types of diet for a three-day period. We compared “traditional Hungarian”, low energy, and diabetic diets, considering both energy content and expenses related to lifestyle.
According to our estimation: diabetic (including medication) and ”traditional” Hungarian diets were the most expensive. Low energy diet proved to be the most cost-effective despite the extra expenditures of higher physical activity
Different sensing mechanisms in single wire and mat carbon nanotubes chemical sensors
Chemical sensing properties of single wire and mat form sensor structures
fabricated from the same carbon nanotube (CNT) materials have been compared.
Sensing properties of CNT sensors were evaluated upon electrical response in
the presence of five vapours as acetone, acetic acid, ethanol, toluene, and
water. Diverse behaviour of single wire CNT sensors was found, while the mat
structures showed similar response for all the applied vapours. This indicates
that the sensing mechanism of random CNT networks cannot be interpreted as a
simple summation of the constituting individual CNT effects, but is associated
to another robust phenomenon, localized presumably at CNT-CNT junctions, must
be supposed.Comment: 12 pages, 5 figures,Applied Physics A: Materials Science and
Processing 201
Nonextensive thermal sources of cosmic rays?
The energy spectrum of cosmic rays (CR) exhibits power-like behavior with a
very characteristic "knee" structure. We consider a possibility that such a
spectrum could be generated by some specific nonstatistical temperature
fluctuations in the source of CR with the "knee" structure reflecting an abrupt
change of the pattern of such fluctuations. This would result in a generalized
nonextensive statistical model for the production of CR. The possible physical
mechanisms leading to these effects are discussed together with the resulting
chemical composition of the CR, which follows the experimentally observed
abundance of nuclei.Comment: 16 pages, 3 figures, rewritten and updated version, to be published
in Centr. Eur. J. Phy
Gluon production, cooling and entropy in nuclear collisions
We study the cooling (heating) of a glue-parton gas due to production
(destruction) of particles and determine the associated production of entropy.
We incorporate sharing of the system energy among a changing number of
particles. We find that the entropy of an evolving glue-parton gas changes in
an insignificant range once the initial high temperature state has been formed,
despite a great change in particle number and temperature.Comment: Replaced for bad printing on US paper. 7 pages, LaTeX, 4 postscript
figure
- …