9,754 research outputs found
Continuous time volatility modelling: COGARCH versus Ornstein-Uhlenbeck models
We compare the probabilistic properties of the non-Gaussian Ornstein-Uhlenbeck based stochastic volatility model of Barndorff-Nielsen and Shephard (2001) with those of the COGARCH process. The latter is a continuous time GARCH process introduced by the authors (2004). Many features are shown to be shared by both processes, but differences are pointed out as well. Furthermore, it is shown that the COGARCH process has Pareto like tails under weak regularity conditions
A Continuous Time GARCH Process Driven by a Lévy Process: Stationarity and Second Order Behaviour
We use a discrete time analysis, giving necessary and sufficient conditions for the almost sure convergence of ARCH(1) and GARCH(1,1) discrete time models, tosuggest an extension of the (G)ARCH concept to continuous time processes. Our "COGARCH" (continuous time GARCH) model, based on a single background driving Levy process, is different from, though related to, other continuous time stochastic volatility models that have been proposed. The model generalises the essential features of discrete time GARCH processes, and is amenable to further analysis, possessing useful Markovian and stationarity properties
Optimization of a neutrino factory oscillation experiment
We discuss the optimization of a neutrino factory experiment for neutrino
oscillation physics in terms of muon energy, baselines, and oscillation
channels (gold, silver, platinum). In addition, we study the impact and
requirements for detector technology improvements, and we compare the results
to beta beams. We find that the optimized neutrino factory has two baselines,
one at about 3000 to 5000km, the other at about 7500km (``magic'' baseline).
The threshold and energy resolution of the golden channel detector have the
most promising optimization potential. This, in turn, could be used to lower
the muon energy from about 50GeV to about 20GeV. Furthermore, the inclusion of
electron neutrino appearance with charge identification (platinum channel)
could help for large values of \sin^2 2 \theta_{13}. Though tau neutrino
appearance with charge identification (silver channel) helps, in principle, to
resolve degeneracies for intermediate \sin^2 2 \theta_{13}, we find that
alternative strategies may be more feasible in this parameter range. As far as
matter density uncertainties are concerned, we demonstrate that their impact
can be reduced by the combination of different baselines and channels. Finally,
in comparison to beta beams and other alternative technologies, we clearly can
establish a superior performance for a neutrino factory in the case \sin^2 2
\theta_{13} < 0.01.Comment: 51 pages, 25 figures, 6 tables, references corrected, final version
to appear in Phys. Rev.
Modelling bark beetle disturbances in a large scale forest scenario model to assess climate change impacts and evaluate adaptive management strategies
To study potential consequences of climate-induced changes in the biotic disturbance regime at regional to national scale we integrated a model of Ips typographus (L. Scol. Col.) damages into the large-scale forest scenario model EFISCEN. A two-stage multivariate statistical meta-model was used to upscale stand level damages by bark beetles as simulated in the hybrid forest patch model PICUS v1.41. Comparing EFISCEN simulations including the new bark beetle disturbance module against a 15-year damage time series for Austria showed good agreement at province level (R² between 0.496 and 0.802). A scenario analysis of climate change impacts on bark beetle-induced damages in Austria¿s Norway spruce [Picea abies (L.) Karst.] forests resulted in a strong increase in damages (from 1.33 Mm³ a¿1, period 1990¿2004, to 4.46 Mm³ a¿1, period 2095¿2099). Studying two adaptive management strategies (species change) revealed a considerable time-lag between the start of adaptation measures and a decrease in simulated damages by bark beetle
Universal transport signatures of Majorana fermions in superconductor-Luttinger liquid junctions
One of the most promising proposals for engineering topological
superconductivity and Majorana fermions employs a spin-orbit coupled nanowire
subjected to a magnetic field and proximate to an s-wave superconductor. When
only part of the wire's length contacts to the superconductor, the remaining
conducting portion serves as a natural lead that can be used to probe these
Majorana modes via tunneling. The enhanced role of interactions in one
dimension dictates that this configuration should be viewed as a
superconductor-Luttinger liquid junction. We investigate such junctions between
both helical and spinful Luttinger liquids, and topological as well as
non-topological superconductors. We determine the phase diagram for each case
and show that universal low-energy transport in these systems is governed by
fixed points describing either perfect normal reflection or perfect Andreev
reflection. In addition to capturing (in some instances) the familiar
Majorana-mediated `zero-bias anomaly' in a new framework, we show that
interactions yield dramatic consequences in certain regimes. Indeed, we
establish that strong repulsion removes this conductance anomaly altogether
while strong attraction produces dynamically generated effective Majorana modes
even in a junction with a trivial superconductor. Interactions further lead to
striking signatures in the local density of states and the line-shape of the
conductance peak at finite voltage, and also are essential for establishing
smoking-gun transport signatures of Majorana fermions in spinful Luttinger
liquid junctions.Comment: 25 pages, 6 figures, v
Symmetric Brownian motor
In this paper we present a model of a symmetric Brownian motor (SBM) which
changes the sign of its velocity when the temperature gradient is inverted. The
velocity, external work and efficiency are studied as a function of the
temperatures of the baths and other relevant parameters. The motor shows a
current reversal when another parameter (a phase shift) is varied. Analytical
predictions and results from numerical simulations are performed and agree very
well. Generic properties of this type of motors are discussed.Comment: 8 pages and 10 figure
SPECT and PET myocardial perfusion imaging in Austria, Germany, and Switzerland results of the first joint survey of 2021
PURPOSE
This paper presents the results of the first joint survey on the use of SPECT and PET myocardial perfusion imaging (MPI) and cardiac amyloidosis imaging in Austria, Germany, and Switzerland of the year 2021.
METHODS
A questionnaire was sent in 2022 to centres practicing nuclear medicine.
RESULTS
Data from 14 Austrian (10,710 SPECT), 218 German (133,047 SPECT), and 16 Swiss centres (11,601 MPI (6,879 SPECT, 4722 PET)) were analysed. In Austria and Germany, the PET MPI numbers were close to zero and not considered. Official MPS numbers from 2015 to 2021 from Austria and Germany revealed a decline in Austria by about 40% in the pandemic years 2020 to 2021, but an increase in Germany by 9%. Ambulatory care cardiologists represented the major referral group (56-71%). Mostly, stress tests were performed pharmacologically (58-92%). Contrary to Germany, a 1-day protocol was predominant (58-97%) in Austria and Switzerland. The leading camera systems were SPECT-CT in Austria and Switzerland (57-79%) and multi-head systems in Germany (58%). Switzerland had the highest proportion of SPECT MPI with attenuation correction (84%), followed by Austria (43%), and Germany (33%). Electrocardiogram-gated SPECT MPI showed an overall high penetration of 87-99%. Scoring was most frequently applied in Germany (72%), followed by Austria (64%), and Switzerland (60%). Related to the population, the number of cardiac amyloidosis imaging was highest in Austria, followed by Switzerland and Germany.
CONCLUSIONS
This first joint survey of 2021 shows considerable differences among the countries. The Swiss situation is outstanding due to the wide use of PET MPI. In terms of camera equipment, Switzerland is also leading, followed by Austria and Germany. Despite the differences in procedural issues, the results reveal an overall high standard of MPI imaging
Stationarity and second order behaviour of discrete and continuous time GARCH(1,1) processes
We use a discrete time analysis, giving necessary and sufficient conditions for the almost sure convergence of ARCH(1) and GARCH(1,1) discrete time models, to suggest an extension of the (G)ARCH concept to continuous time processes. The models, based on a single background driving Levy process, are different from, though related to, other continuous time stochastic volatility models that have been proposed. Our models generalise the essential features of discrete time GARCH processes, and are amenable to further analysis, possessing useful Markovian and stationarity properties
Geometric scaling of purely-elastic flow instabilities
We present a combined experimental, numerical and theoretical investigation
of the geometric scaling of the onset of a purely-elastic flow instability in a
serpentine channel. Good qualitative agreement is obtained between experiments,
using dilute solutions of flexible polymers in microfluidic devices, and
two-dimensional numerical simulations using the UCM model. The results are
confirmed by a simple theoretical analysis, based on the dimensionless
criterion proposed by Pakdel-McKinley for onset of a purely-elastic
instability
Lattice-Based zk-SNARKs from Square Span Programs
Zero-knowledge SNARKs (zk-SNARKs) are non-interactive proof systems with short (i.e., independent of the size of the witness) and efficiently verifiable proofs. They elegantly resolve the juxtaposition of individual privacy and public trust, by providing an efficient way of demonstrating knowledge of secret information without actually revealing it. To this day, zk-SNARKs are widely deployed all over the planet and are used to keep alive a system worth billion of euros, namely the cryptocurrency Zcash. However, all current SNARKs implementations rely on so-called pre-quantum assumptions and, for this reason, are not expected to withstand cryptanalitic efforts over the next few decades.
In this work, we introduce a new zk-SNARK that can be instantiated from lattice-based assumptions, and which is thus believed to be post-quantum secure. We provide a generalization in the spirit of Gennaro et al. (Eurocrypt'13) to the SNARK of Danezis et al. (Asiacrypt'14) that is based on Square Span Programs (SSP) and relies on weaker computational assumptions. We focus on designated-verifier proofs and propose a protocol in which a proof consists of just 5 LWE encodings. We provide a concrete choice of parameters, showing that our construction is practically instantiable
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