3,461 research outputs found
A combinatorial description of finite O-sequences and aCM genera
The goal of this paper is to explicitly detect all the arithmetic genera of
arithmetically Cohen-Macaulay projective curves with a given degree . It is
well-known that the arithmetic genus of a curve can be easily deduced
from the -vector of the curve; in the case where is arithmetically
Cohen-Macaulay of degree , must belong to the range of integers
. We develop an algorithmic procedure that
allows one to avoid constructing most of the possible -vectors of . The
essential tools are a combinatorial description of the finite O-sequences of
multiplicity , and a sort of continuity result regarding the generation of
the genera. The efficiency of our method is supported by computational
evidence. As a consequence, we single out the minimal possible
Castelnuovo-Mumford regularity of a curve with Cohen-Macaulay postulation and
given degree and genus.Comment: Final versio
Segments and Hilbert schemes of points
Using results obtained from the study of homogeneous ideals sharing the same
initial ideal with respect to some term order, we prove the singularity of the
point corresponding to a segment ideal with respect to the revlex term order in
the Hilbert scheme of points in . In this context, we look inside
properties of several types of "segment" ideals that we define and compare.
This study led us to focus our attention also to connections between the shape
of generators of Borel ideals and the related Hilbert polynomial, providing an
algorithm for computing all saturated Borel ideals with the given Hilbert
polynomial.Comment: 19 pages, 2 figures. Comments and suggestions are welcome
Photon elastic scattering simulation: validation and improvements to Geant4
Several models for the simulation of photon elastic scattering are
quantitatively evaluated with respect to a large collection of experimental
data retrieved from the literature. They include models based on the form
factor approximation, on S-matrix calculations and on analytical
parameterizations; they exploit publicly available data libraries and
tabulations of theoretical calculations. Some of these models are currently
implemented in general purpose Monte Carlo systems; some have been implemented
and evaluated for the first time in this paper for possible use in Monte Carlo
particle transport. The analysis mainly concerns the energy range between 5 keV
and a few MeV. The validation process identifies the newly implemented model
based on second order S-matrix calculations as the one best reproducing
experimental measurements. The validation results show that, along with
Rayleigh scattering, additional processes, not yet implemented in Geant4 nor in
other major Monte Carlo systems, should be taken into account to realistically
describe photon elastic scattering with matter above 1 MeV. Evaluations of the
computational performance of the various simulation algorithms are reported
along with the analysis of their physics capabilities
Minimal Castelnuovo-Mumford regularity for a given Hilbert polynomial
Let be an algebraically closed field of null characteristic and a
Hilbert polynomial. We look for the minimal Castelnuovo-Mumford regularity
of closed subschemes of projective spaces over with Hilbert
polynomial . Experimental evidences led us to consider the idea that
could be achieved by schemes having a suitable minimal Hilbert
function. We give a constructive proof of this fact. Moreover, we are able to
compute the minimal Castelnuovo-Mumford regularity of
schemes with Hilbert polynomial and given regularity of the
Hilbert function, and also the minimal Castelnuovo-Mumford regularity of
schemes with Hilbert function . These results find applications in the study
of Hilbert schemes. They are obtained by means of minimal Hilbert functions and
of two new constructive methods which are based on the notion of
growth-height-lexicographic Borel set and called ideal graft and extended
lifting.Comment: 21 pages. Comments are welcome. More concise version with a slight
change in the title. A further revised version has been accepted for
publication in Experimental Mathematic
Ionization cross sections for low energy electron transport
Two models for the calculation of ionization cross sections by electron
impact on atoms, the Binary-Encouter-Bethe and the Deutsch-Maerk models, have
been implemented; they are intended to extend and improve Geant4 simulation
capabilities in the energy range below 1 keV. The physics features of the
implementation of the models are described, and their differences with respect
to the original formulations are discussed. Results of the verification with
respect to the original theoretical sources and of extensive validation with
respect to experimental data are reported. The validation process also concerns
the ionization cross sections included in the Evaluated Electron Data Library
used by Geant4 for low energy electron transport. Among the three cross section
options, the Deutsch-Maerk model is identified as the most accurate at
reproducing experimental data over the energy range subject to test.Comment: To be published in IEEE Trans. Nucl. Sci., Dec. 201
Physics-related epistemic uncertainties in proton depth dose simulation
A set of physics models and parameters pertaining to the simulation of proton
energy deposition in matter are evaluated in the energy range up to
approximately 65 MeV, based on their implementations in the Geant4 toolkit. The
analysis assesses several features of the models and the impact of their
associated epistemic uncertainties, i.e. uncertainties due to lack of
knowledge, on the simulation results. Possible systematic effects deriving from
uncertainties of this kind are highlighted; their relevance in relation to the
application environment and different experimental requirements are discussed,
with emphasis on the simulation of radiotherapy set-ups. By documenting
quantitatively the features of a wide set of simulation models and the related
intrinsic uncertainties affecting the simulation results, this analysis
provides guidance regarding the use of the concerned simulation tools in
experimental applications; it also provides indications for further
experimental measurements addressing the sources of such uncertainties.Comment: To be published in IEEE Trans. Nucl. Sc
Auroral Radio Emission from Stars: the case of CU Virginis
CU Virginis is a rapidly rotating Magnetic Chemically Peculiar star with at
present unique characteristics as radio emitter. The most intriguing one is the
presence of intense, 100% circularly polarized radiation ascribed to Cyclotron
Maser. Each time the star rotates, this highly beamed emission points two times
toward the Earth, like a pulsar. We observed CU Vir in April 2010 with the EVLA
in two bands centered at 1450 and 1850 MHz. We covered nearly the whole
rotational period, confirming the presence of the two pulses at a flux density
up to 20 mJy. Dynamical spectra, obtained with unprecedented spectral and
temporal sensitivity, allow us to clearly see the different time delays as a
function of the frequency. We interpret this behaviour as a propagation effect
of the radiation inside the stellar magnetosphere. The emerging scenario
suggests interesting similarities with the auroral radio emission from planets,
in particular with the Auroral Kilometric Radiation (AKR) from Earth, which
originates at few terrestrial radii above the magnetic poles and was only
recently discovered to be highly beamed. We conclude that the magnetospheres of
CU Vir, Earth and other planets, maybe also exoplanets, could have similar
geometrical and physical characteristics in the regions where the cyclotron
maser is generated. In addition, the pulses are perfect "markers" of the
rotation period. This has given us for the first time the possibility to
measure with extraordinary accuracy the spin down of a star on or near the main
sequence.Comment: 18 pages, 4 figures, Accepted to APJ Letter, EVLA special issu
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