3,349 research outputs found
A paradox in community detection
Recent research has shown that virtually all algorithms aimed at the
identification of communities in networks are affected by the same main
limitation: the impossibility to detect communities, even when these are
well-defined, if the average value of the difference between internal and
external node degrees does not exceed a strictly positive value, in literature
known as detectability threshold. Here, we counterintuitively show that the
value of this threshold is inversely proportional to the intrinsic quality of
communities: the detection of well-defined modules is thus more difficult than
the identification of ill-defined communities.Comment: 5 pages, 3 figure
Reachability problems for PAMs
Piecewise affine maps (PAMs) are frequently used as a reference model to show
the openness of the reachability questions in other systems. The reachability
problem for one-dimentional PAM is still open even if we define it with only
two intervals. As the main contribution of this paper we introduce new
techniques for solving reachability problems based on p-adic norms and weights
as well as showing decidability for two classes of maps. Then we show the
connections between topological properties for PAM's orbits, reachability
problems and representation of numbers in a rational base system. Finally we
show a particular instance where the uniform distribution of the original orbit
may not remain uniform or even dense after making regular shifts and taking a
fractional part in that sequence.Comment: 16 page
Mechanical Unfolding of a Simple Model Protein Goes Beyond the Reach of One-Dimensional Descriptions
We study the mechanical unfolding of a simple model protein. The Langevin
dynamics results are analyzed using Markov-model methods which allow to
describe completely the configurational space of the system. Using transition
path theory we also provide a quantitative description of the unfolding
pathways followed by the system. Our study shows a complex dynamical scenario.
In particular, we see that the usual one-dimensional picture: free-energy vs
end-to-end distance representation, gives a misleading description of the
process. Unfolding can occur following different pathways and configurations
which seem to play a central role in one-dimensional pictures are not the
intermediate states of the unfolding dynamics.Comment: 10 pages, 6 figure
Restriction on the energy and luminosity of e+e- storage rings due to beamstrahlung
The role of beamstrahlung in high-energy e+e- storage-ring colliders (SRCs)
is examined. Particle loss due to the emission of single energetic
beamstrahlung photons is shown to impose a fundamental limit on SRC
luminosities at energies 2E_0 >~ 140 GeV for head-on collisions and 2E_0 >~ 40
GeV for crab-waist collisions. With beamstrahlung taken into account, we
explore the viability of SRCs in the E_0=240-500 GeV range, which is of
interest in the precision study of the Higgs boson. At 2E_0=240 GeV, SRCs are
found to be competitive with linear colliders; however, at 2E_0=400-500 GeV,
the attainable SRC luminosity would be a factor 15-25 smaller than desired.Comment: Latex, 5 pages. v2 differs only by minor changes is abstract and
introduction, one reference is added. v3 corresponds to the paper published
in PR
ORIGAMIX, a CdTe-based spectro-imager development for nuclear applications
The Astrophysics Division of CEA Saclay has a long history in the development
of CdTe based pixelated detection planes for X and gamma-ray astronomy, with
time-resolved imaging and spectrometric capabilities. The last generation,
named Caliste HD, is an all-in-one modular instrument that fulfills
requirements for space applications. Its full-custom front-end electronics is
designed to work over a large energy range from 2 keV to 1 MeV with excellent
spectroscopic performances, in particular between 10 and 100 keV (0.56 keV FWHM
and 0.67 keV FWHM at 13.9 and 59.5 keV). In the frame of the ORIGAMIX project,
a consortium based on research laboratories and industrials has been settled in
order to develop a new generation of gamma camera. The aim is to develop a
system based on the Caliste architecture for post-accidental interventions or
homeland security, but integrating new properties (advanced spectrometry,
hybrid working mode) and suitable for industry. A first prototype was designed
and tested to acquire feedback for further developments. In this study, we
particularly focused on spectrometric performances with high energies and high
fluxes. Therefore, our device was exposed to energies up to 700 keV (133Ba,
137Cs) and we measured the evolution of energy resolution (0.96 keV at 80 keV,
2.18 keV at 356 keV, 3.33 keV at 662 keV). Detection efficiency decreases after
150 keV, as Compton effect becomes dominant. However, CALISTE is also designed
to handle multiple events, enabling Compton scattering reconstruction, which
can drastically improve detection efficiencies and dynamic range for higher
energies up to 1408 keV (22Na, 60Co, 152Eu) within a 1-mm thick detector. In
particular, such spectrometric performances obtained with 152Eu and 60Co were
never measured before with this kind of detector.Comment: Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment. Available
online 9 January 2015, ISSN 0168-9002
(http://www.sciencedirect.com/science/article/pii/S0168900215000133).
Keywords: CdTe; X-ray; Gamma-ray; Spectrometry; Charge-sharing; Astrophysics
Instrumentation; Nuclear Instrumentation; Gamma-ray camera
The Identity Correspondence Problem and its Applications
In this paper we study several closely related fundamental problems for words
and matrices. First, we introduce the Identity Correspondence Problem (ICP):
whether a finite set of pairs of words (over a group alphabet) can generate an
identity pair by a sequence of concatenations. We prove that ICP is undecidable
by a reduction of Post's Correspondence Problem via several new encoding
techniques.
In the second part of the paper we use ICP to answer a long standing open
problem concerning matrix semigroups: "Is it decidable for a finitely generated
semigroup S of square integral matrices whether or not the identity matrix
belongs to S?". We show that the problem is undecidable starting from dimension
four even when the number of matrices in the generator is 48. From this fact,
we can immediately derive that the fundamental problem of whether a finite set
of matrices generates a group is also undecidable. We also answer several
question for matrices over different number fields. Apart from the application
to matrix problems, we believe that the Identity Correspondence Problem will
also be useful in identifying new areas of undecidable problems in abstract
algebra, computational questions in logic and combinatorics on words.Comment: We have made some proofs clearer and fixed an important typo from the
published journal version of this article, see footnote 3 on page 1
Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles
Using the framework of stochastic thermodynamics, we present an experimental
study of a doublet of magnetic colloidal particles which is manipulated by a
time-dependent magnetic field. Due to hydrodynamic interactions, each bead
experiences a state-dependent friction, which we characterize using a
hydrodynamic model. In this work, we compare two estimates of the dissipation
in this system: the first one is energy based since it relies on the measured
interaction potential, while the second one is information based since it uses
only the information content of the trajectories. While the latter only offers
a lower bound of the former, we find it to be simple to implement and of
general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5
figure
Three-photon detachment of electrons from the fluorine negative ion
Absolute three-photon detachment cross sections are calculated for the
fluorine negative ion within the lowest-order perturbation theory. The Dyson
equation of the atomic many-body theory is used to obtain the ground-state 2p
wavefunction with correct asymptotic behaviour, corresponding to the true
(experimental) binding energy. We show that in accordance with the adiabatic
theory (Gribakin and Kuchiev 1997 {Phys. Rev. A} {\bf 55} 3760) this is crucial
for obtaining absolute values of the multiphoton cross sections. Comparisons
with other calculations and experimental data are presented.Comment: 10 pages, two figures, Latex, IOP styl
The Stellar Composition of the Star Formation Region CMa R1. II. Spectroscopic and Photometric Observations of 9 Young Stars
We present new high and low resolution spectroscopic and photometric data of
nine members of the young association CMa R1. All the stars have circumstellar
dust at some distance as could be expected from their association with
reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and
LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars
with discs. Our photometric and spectroscopic observations on these stars
reveal new characteristics of their variability. We present first
interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha
stars in terms of a partially eclipsing binary, a magnetic activity cycle and
circumstellar dust variations, respectively. The remaining five stars show no
clear indications of Halpha emission in their spectra, although their spectral
types and ages are comparable with those of HD 52721 and HD 53367. This
indicates that the presence of a disc around a star in CMa R1 may depend on the
environment of the star. In particular we find that all Halpha emission stars
are located at or outside the arc-shaped border of the H II region, which
suggests that the stars inside the arc have lost their discs through
evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc)
supernova, about 1 Myr ago.Comment: 17 pages, 13 figures, accepted by MNRA
A New Technique for Determining the Properties of a Narrow -channel Resonance at a Muon Collider
We explore an alternative to the usual procedure of scanning for determining
the properties of a narrow -channel resonance. By varying the beam energy
resolution while sitting on the resonance peak, the width and branching ratios
of the resonance can be determined. The statistical accuracy achieved is
superior to that of the usual scan procedure in the case of a light SM-like
Higgs boson with \mh>130\gev or for the lightest pseudogoldstone boson of a
strong electroweak breaking model if \mpzero>150\gev.Comment: 4 pages, 1 figur
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