1,100 research outputs found
A Bichromatic Incidence Bound and an Application
We prove a new, tight upper bound on the number of incidences between points
and hyperplanes in Euclidean d-space. Given n points, of which k are colored
red, there are O_d(m^{2/3}k^{2/3}n^{(d-2)/3} + kn^{d-2} + m) incidences between
the k red points and m hyperplanes spanned by all n points provided that m =
\Omega(n^{d-2}). For the monochromatic case k = n, this was proved by Agarwal
and Aronov.
We use this incidence bound to prove that a set of n points, no more than n-k
of which lie on any plane or two lines, spans \Omega(nk^2) planes. We also
provide an infinite family of counterexamples to a conjecture of Purdy's on the
number of hyperplanes spanned by a set of points in dimensions higher than 3,
and present new conjectures not subject to the counterexample.Comment: 12 page
Assessment of patient-derived tumour xenografts (PDXs) as a discovery tool for cancer epigenomics
Background: The use of tumour xenografts is a well-established research tool in cancer genomics but has not yet
been comprehensively evaluated for cancer epigenomics.
Methods: In this study, we assessed the suitability of patient-derived tumour xenografts (PDXs) for methylome analysis
using Infinium 450 K Beadchips and MeDIP-seq.
Results: Controlled for confounding host (mouse) sequences, comparison of primary PDXs and matching patient
tumours in a rare (osteosarcoma) and common (colon) cancer revealed that an average 2.7% of the assayed CpG sites
undergo major (Δβ ≥ 0.51) methylation changes in a cancer-specific manner as a result of the xenografting procedure.
No significant subsequent methylation changes were observed after a second round of xenografting between primary
and secondary PDXs. Based on computational simulation using publically available methylation data, we additionally
show that future studies comparing two groups of PDXs should use 15 or more samples in each group to minimise
the impact of xenografting-associated changes in methylation on comparison results.
Conclusions: Our results from rare and common cancers indicate that PDXs are a suitable discovery tool for cancer
epigenomics and we provide guidance on how to overcome the observed limitations
Entropic Interactions in Suspensions of Semi-Flexible Rods: Short-Range Effects of Flexibility
We compute the entropic interactions between two colloidal spheres immersed
in a dilute suspension of semi-flexible rods. Our model treats the
semi-flexible rod as a bent rod at fixed angle, set by the rod contour and
persistence lengths. The entropic forces arising from this additional
rotational degree of freedom are captured quantitatively by the model, and
account for observations at short range in a recent experiment. Global fits to
the interaction potential data suggest the persistence length of fd-virus is
about two to three times smaller than the commonly used value of .Comment: 4 pages, 5 figures, submitted to PRE rapid communication
Lifetime distributions in the methods of non-equilibrium statistical operator and superstatistics
A family of non-equilibrium statistical operators is introduced which differ
by the system age distribution over which the quasi-equilibrium (relevant)
distribution is averaged. To describe the nonequilibrium states of a system we
introduce a new thermodynamic parameter - the lifetime of a system.
Superstatistics, introduced in works of Beck and Cohen [Physica A \textbf{322},
(2003), 267] as fluctuating quantities of intensive thermodynamical parameters,
are obtained from the statistical distribution of lifetime (random time to the
system degeneracy) considered as a thermodynamical parameter. It is suggested
to set the mixing distribution of the fluctuating parameter in the
superstatistics theory in the form of the piecewise continuous functions. The
distribution of lifetime in such systems has different form on the different
stages of evolution of the system. The account of the past stages of the
evolution of a system can have a substantial impact on the non-equilibrium
behaviour of the system in a present time moment.Comment: 18 page
Dark matter halos and the anisotropy of ultra-high energy cosmic rays
Several explanations for the existence of Ultra High Energy Cosmic Rays
invoke the idea that they originate from the decay of massive particles created
in the reheating following inflation. It has been suggested that the decay
products can explain the observed isotropic flux of cosmic rays. We have
calculated the anisotropy expected for various models of the dark matter
distribution and find that at present data are too sparse above eV to discriminate between different models. However we show that with
data from three years of operation of the southern section of the Pierre Auger
Observatory significant progress in testing the proposals will be made.Comment: 21 pages, 6 figures (ps), Astroparticle Physics (accepted for
publication
Gauge fixing and the Hamiltonian for cylindrical spacetimes
We introduce a complete gauge fixing for cylindrical spacetimes in vacuo
that, in principle, do not contain the axis of symmetry. By cylindrically
symmetric we understand spacetimes that possess two commuting spacelike Killing
vectors, one of them rotational and the other one translational. The result of
our gauge fixing is a constraint-free model whose phase space has four
field-like degrees of freedom and that depends on three constant parameters.
Two of these constants determine the global angular momentum and the linear
momentum in the axis direction, while the third parameter is related with the
behavior of the metric around the axis. We derive the explicit expression of
the metric in terms of the physical degrees of freedom, calculate the reduced
equations of motion and obtain the Hamiltonian that generates the reduced
dynamics. We also find upper and lower bounds for this reduced Hamiltonian that
provides the energy per unit length contained in the system. In addition, we
show that the reduced formalism constructed is well defined and consistent at
least when the linear momentum in the axis direction vanishes. Furthermore, in
that case we prove that there exists an infinite number of solutions in which
all physical fields are constant both in the surroundings of the axis and at
sufficiently large distances from it. If the global angular momentum is
different from zero, the isometry group of these solutions is generally not
orthogonally transitive. Such solutions generalize the metric of a spinning
cosmic string in the region where no closed timelike curves are present.Comment: 12 pages, accepted for publication in Physical Review
The clustering of ultra-high energy cosmic rays and their sources
The sky distribution of cosmic rays with energies above the 'GZK cutoff'
holds important clues to their origin. The AGASA data, although consistent with
isotropy, shows evidence for small-angle clustering, and it has been argued
that such clusters are aligned with BL Lacertae objects, implicating these as
sources. It has also been suggested that clusters can arise if the cosmic rays
come from the decays of very massive relic particles in the Galactic halo, due
to the expected clumping of cold dark matter. We examine these claims and show
that both are in fact not justified.Comment: 13 pages, 8 figures, version in press at Phys. Rev.
Comment on "Critique of q-entropy for thermal statistics" by M. Nauenberg
It was recently published by M. Nauenberg [1] a quite long list of objections
about the physical validity for thermal statistics of the theory sometimes
referred to in the literature as {\it nonextensive statistical mechanics}. This
generalization of Boltzmann-Gibbs (BG) statistical mechanics is based on the
following expression for the entropy:
S_q= k\frac{1- \sum_{i=1}^Wp_i^q}{q-1} (q \in {\cal R}; S_1=S_{BG} \equiv
-k\sum_{i=1}^W p_i \ln p_i) .
The author of [1] already presented orally the essence of his arguments in
1993 during a scientific meeting in Buenos Aires. I am replying now
simultaneously to the just cited paper, as well as to the 1993 objections
(essentially, the violation of "fundamental thermodynamic concepts", as stated
in the Abstract of [1]).Comment: 7 pages including 2 figures. This is a reply to M. Nauenberg, Phys.
Rev. E 67, 036114 (2003
Charge-Symmetry Violation in Pion Scattering from Three-Body Nuclei
We discuss the experimental and theoretical status of charge-symmetry
violation (CSV) in the elastic scattering of pi+ and pi- on 3H and 3He.
Analysis of the experimental data for the ratios r1, r2, and R at Tpi = 142,
180, 220, and 256 MeV provides evidence for the presence of CSV. We describe
pion scattering from the three-nucleon system in terms of single- and
double-scattering amplitudes. External and internal Coulomb interactions as
well as the Delta-mass splitting are taken into account as sources of CSV.
Reasonable agreement between our theoretical calculations and the experimental
data is obtained for Tpi = 180, 220, and 256 MeV. For these energies, it is
found that the Delta-mass splitting and the internal Coulomb interaction are
the most important contributions for CSV in the three-nucleon system. The CSV
effects are rather sensitive to the choice of pion-nuclear scattering
mechanisms, but at the same time, our theoretical predictions are much less
sensitive to the choice of the nuclear wave function. It is found, however,
that data for r2 and R at Tpi = 142 MeV do not agree with the predictions of
our model, which may indicate that there are additional mechanisms for CSV
which are important only at lower energies.Comment: 26 pages of RevTeX, 16 postscript figure
Nonextensivity of the cyclic Lattice Lotka Volterra model
We numerically show that the Lattice Lotka-Volterra model, when realized on a
square lattice support, gives rise to a {\it finite} production, per unit time,
of the nonextensive entropy . This finiteness only occurs for for the growth mode
(growing droplet), and for for the one (growing stripe). This
strong evidence of nonextensivity is consistent with the spontaneous emergence
of local domains of identical particles with fractal boundaries and competing
interactions. Such direct evidence is for the first time exhibited for a
many-body system which, at the mean field level, is conservative.Comment: Latex, 6 pages, 5 figure
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