605 research outputs found
Stable Marriage with Multi-Modal Preferences
We introduce a generalized version of the famous Stable Marriage problem, now
based on multi-modal preference lists. The central twist herein is to allow
each agent to rank its potentially matching counterparts based on more than one
"evaluation mode" (e.g., more than one criterion); thus, each agent is equipped
with multiple preference lists, each ranking the counterparts in a possibly
different way. We introduce and study three natural concepts of stability,
investigate their mutual relations and focus on computational complexity
aspects with respect to computing stable matchings in these new scenarios.
Mostly encountering computational hardness (NP-hardness), we can also spot few
islands of tractability and make a surprising connection to the \textsc{Graph
Isomorphism} problem
Taming the magnetoresistance anomaly in graphite
At low temperatures, graphite presents a magnetoresistance anomaly which
manifests as a transition to a high-resistance state (HRS) above a certain
critical magnetic field . Such HRS is currently attributed
to a c-axis charge-density-wave taking place only when the lowest Landau level
is populated. By controlling the charge carrier concentration of a gated sample
through its charge neutrality level (CNL), we were able to experimentally
modulate the HRS in graphite for the first time. We demonstrate that the HRS is
triggered both when electrons and holes are the majority carriers but is
attenuated near the CNL. Taking screening into account, our results indicate
that the HRS possess a strong in-plane component and can occur below the
quantum limit, being at odds with the current understanding of the phenomenon.
We also report the effect of sample thickness on the HRS
Electron-hole coexistence in disordered graphene probed by high-field magneto-transport
We report on magneto-transport measurement in disordered graphene under
pulsed magnetic field of up to 57T. For large electron or hole doping, the
system displays the expected anomalous Integer Quantum Hall Effect (IQHE)
specific to graphene up to filling factor . In the close vicinity of the
charge neutrality point, the system breaks up into co-existing puddles of holes
and electrons, leading to a vanishing Hall and finite longitudinal resistance
with no hint of divergence at very high magnetic field. Large resistance
fluctuations are observed near the Dirac point. They are interpreted as the the
natural consequence of the presence of electron and hole puddles. The magnetic
field at which the amplitude of the fluctuations are the largest is directly
linked to the mean size of the puddles
Search for neutrinos from transient sources with the ANTARES telescope and optical follow-up observations
The ANTARES telescope has the opportunity to detect transient neutrino
sources, such as gamma-ray bursts, core-collapse supernovae, flares of active
nuclei... To enhance the sensitivity to these sources, we have developed a new
detection method based on the optical follow-up of "golden" neutrino events
such as neutrino doublets coincident in time and space or single neutrinos of
very high energy. The ANTARES Collaboration has therefore implemented a very
fast on-line reconstruction with a good angular resolution. These
characteristics allow to trigger an optical telescope network; since February
2009. ANTARES is sending alert trigger one or two times per month to the two 25
cm robotic telescope of TAROT. This follow-up of such special events would not
only give access to the nature of the sources but also improves the sensitivity
for transient neutrino sources.Comment: 3 pages, 3 figures, Proceedings of the 31st ICRC, Lodz, Polan, July
200
Integer Quantum Hall Effect in Trilayer Graphene
The Integer Quantum Hall Effect (IQHE) is a distinctive phase of
two-dimensional electronic systems subjected to a perpendicular magnetic field.
Thus far, the IQHE has been observed in semiconductor heterostructures and in
mono- and bi-layer graphene. Here we report on the IQHE in a new system:
trilayer graphene. Experimental data are compared with self-consistent Hartree
calculations of the Landau levels for the gated trilayer. The plateau structure
in the Hall resistivity determines the stacking order (ABA versus ABC). We find
that the IQHE in ABC trilayer graphene is similar to that in the monolayer,
except for the absence of a plateau at filling factor v=2. At very low filling
factor, the Hall resistance vanishes due to the presence of mixed electron and
hole carriers induced by disorder.Comment: 5 pages, 4 figure
Approximability of Connected Factors
Finding a d-regular spanning subgraph (or d-factor) of a graph is easy by
Tutte's reduction to the matching problem. By the same reduction, it is easy to
find a minimal or maximal d-factor of a graph. However, if we require that the
d-factor is connected, these problems become NP-hard - finding a minimal
connected 2-factor is just the traveling salesman problem (TSP).
Given a complete graph with edge weights that satisfy the triangle
inequality, we consider the problem of finding a minimal connected -factor.
We give a 3-approximation for all and improve this to an
(r+1)-approximation for even d, where r is the approximation ratio of the TSP.
This yields a 2.5-approximation for even d. The same algorithm yields an
(r+1)-approximation for the directed version of the problem, where r is the
approximation ratio of the asymmetric TSP. We also show that none of these
minimization problems can be approximated better than the corresponding TSP.
Finally, for the decision problem of deciding whether a given graph contains
a connected d-factor, we extend known hardness results.Comment: To appear in the proceedings of WAOA 201
A Survey on Approximation Mechanism Design without Money for Facility Games
In a facility game one or more facilities are placed in a metric space to
serve a set of selfish agents whose addresses are their private information. In
a classical facility game, each agent wants to be as close to a facility as
possible, and the cost of an agent can be defined as the distance between her
location and the closest facility. In an obnoxious facility game, each agent
wants to be far away from all facilities, and her utility is the distance from
her location to the facility set. The objective of each agent is to minimize
her cost or maximize her utility. An agent may lie if, by doing so, more
benefit can be obtained. We are interested in social choice mechanisms that do
not utilize payments. The game designer aims at a mechanism that is
strategy-proof, in the sense that any agent cannot benefit by misreporting her
address, or, even better, group strategy-proof, in the sense that any coalition
of agents cannot all benefit by lying. Meanwhile, it is desirable to have the
mechanism to be approximately optimal with respect to a chosen objective
function. Several models for such approximation mechanism design without money
for facility games have been proposed. In this paper we briefly review these
models and related results for both deterministic and randomized mechanisms,
and meanwhile we present a general framework for approximation mechanism design
without money for facility games
Reoptimization of Some Maximum Weight Induced Hereditary Subgraph Problems
The reoptimization issue studied in this paper can be described as follows: given an instance I of some problem Î , an optimal solution OPT for Î in I and an instance IâČ resulting from a local perturbation of I that consists of insertions or removals of a small number of data, we wish to use OPT in order to solve Î in I', either optimally or by guaranteeing an approximation ratio better than that guaranteed by an ex nihilo computation and with running time better than that needed for such a computation. We use this setting in order to study weighted versions of several representatives of a broad class of problems known in the literature as maximum induced hereditary subgraph problems. The main problems studied are max independent set, max k-colorable subgraph and max split subgraph under vertex insertions and deletion
Inter-network regions of the Sun at millimetre wavelengths
The continuum intensity at wavelengths around 1 mm provides an excellent way
to probe the solar chromosphere. Future high-resolution millimetre arrays, such
as the Atacama Large Millimeter Array (ALMA), will thus produce valuable input
for the ongoing controversy on the thermal structure and the dynamics of this
layer. Synthetic brightness temperature maps are calculated on basis of
three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. While the
millimetre continuum at 0.3mm originates mainly from the upper photosphere, the
longer wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and also from
disk-centre towards the solar limb. The average intensity contribution
functions are usually rather broad and in some cases they are even
double-peaked as there are contributions from hot shock waves and cool
post-shock regions in the model chromosphere. Taking into account the
deviations from ionisation equilibrium for hydrogen gives a less strong
variation of the electron density and with it of the optical depth. The result
is a narrower formation height range. The average brightness temperature
increases with wavelength and towards the limb. The relative contrast depends
on wavelength in the same way as the average intensity but decreases towards
the limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in formation
height and the variation of temperature fluctuations with height in the model
atmospheres.Comment: 15 pages, 10 figures, accepted for publication in A&A (15.05.07
- âŠ