3,891 research outputs found
Low-energy electron transport with the method of discrete ordinates
The one-dimensional discrete ordinates code ANISN was adapted to transport low energy (a few MeV) electrons. Calculated results obtained with ANISN were compared with experimental data for transmitted electron energy and angular distribution data for electrons normally incident on aluminum slabs of various thicknesses. The calculated and experimental results are in good agreement for a thin slab (0.2 of the electron range), but not for the thicker slabs (0.6 of the electron range). Calculated results obtained with ANISN were also compared with results obtained using Monte Carlo methods
Value at Risk models with long memory features and their economic performance
We study alternative dynamics for Value at Risk (VaR) that incorporate a slow moving component and information on recent aggregate returns in established quantile (auto) regression models. These models are compared on their economic performance, and also on metrics of first-order importance such as violation ratios. By better economic performance, we mean that changes in the VaR forecasts should have a lower variance to reduce transaction costs and should lead to lower exceedance sizes without raising the average level of the VaR. We find that, in combination with a targeted estimation strategy, our proposed models lead to improved performance in both statistical and economic terms
Two are better than one: Volatility forecasting using multiplicative component GARCHâMIDAS models
We examine the properties and forecast performance of multiplicative volatility
specifications that belong to the class of generalized autoregressive conditional
heteroskedasticityâmixed-data sampling (GARCH-MIDAS) models suggested in
Engle, Ghysels, and Sohn (Review of Economics and Statistics, 2013, 95, 776â797).
In those models volatility is decomposed into a short-term GARCH component
and a long-term component that is driven by an explanatory variable. We derive
the kurtosis of returns, the autocorrelation function of squared returns, and
the R2 of a MincerâZarnowitz regression and evaluate the QMLE and forecast
performance of these models in a Monte Carlo simulation. For S&P 500 data,
we compare the forecast performance of GARCH-MIDAS models with a wide
range of competitor models such as HAR (heterogeneous autoregression), realized GARCH, HEAVY (high-frequency-based volatility) and Markov-switching
GARCH. Our results show that the GARCH-MIDAS based on housing starts
as an explanatory variable significantly outperforms all competitor models at
forecast horizons of 2 and 3 months ahead
Lorentzian spin foam amplitudes: graphical calculus and asymptotics
The amplitude for the 4-simplex in a spin foam model for quantum gravity is
defined using a graphical calculus for the unitary representations of the
Lorentz group. The asymptotics of this amplitude are studied in the limit when
the representation parameters are large, for various cases of boundary data. It
is shown that for boundary data corresponding to a Lorentzian simplex, the
asymptotic formula has two terms, with phase plus or minus the Lorentzian
signature Regge action for the 4-simplex geometry, multiplied by an Immirzi
parameter. Other cases of boundary data are also considered, including a
surprising contribution from Euclidean signature metrics.Comment: 30 pages. v2: references now appear. v3: presentation greatly
improved (particularly diagrammatic calculus). Definition of "Regge state"
now the same as in previous work; signs change in final formula as a result.
v4: two references adde
The merit of high-frequency data in portfolio allocation
This paper addresses the open debate about the usefulness of high-frequency (HF) data in large-scale portfolio allocation. Daily covariances are estimated based on HF data of the S&P 500 universe employing a blocked realized kernel estimator. We propose forecasting covariance matrices using a multi-scale spectral decomposition where volatilities, correlation eigenvalues and eigenvectors evolve on different frequencies. In an extensive out-of-sample forecasting study, we show that the proposed approach yields less risky and more diversified portfolio allocations as prevailing methods employing daily data. These performance gains hold over longer horizons than previous studies have shown
Viral expression and molecular profiling in liver tissue versus microdissected hepatocytes in hepatitis B virus - associated hepatocellular carcinoma.
Background: The molecular mechanisms whereby hepatitis B virus (HBV) induces hepatocellular carcinoma (HCC)
remain elusive. We used genomic and molecular techniques to investigate host-virus interactions by studying multiple areas of the same liver from patients with HCC.
Methods: We compared the gene signature of whole liver tissue (WLT) versus laser capture-microdissected (LCM)
hepatocytes along with the intrahepatic expression of HBV. Gene expression profiling was performed on up to 17 WLT specimens obtained at various distances from the tumor center from individual livers of 11 patients with HCC and on selected LCM samples. HBV markers in liver and serum were determined by real-time polymerase chain reaction (PCR)and confocal immunofluorescence.
Results: Analysis of 5 areas of the liver showed a sharp change in gene expression between the immediate perilesional area and tumor periphery that correlated with a significant decrease in the intrahepatic expression of HB surface antigen (HBsAg). The tumor was characterized by a large preponderance of down-regulated genes, mostly involved in the
metabolism of lipids and fatty acids, glucose, amino acids and drugs, with down-regulation of pathways involved in the activation of PXR/RXR and PPARα/RXRα nuclear receptors, comprising PGC-1α and FOXO1, two key regulators
critically involved not only in the metabolic functions of the liver but also in the life cycle of HBV, acting as essential transcription factors for viral gene expression. These findings were confirmed by gene expression of microdissected
hepatocytes. Moreover, LCM of malignant hepatocytes also revealed up-regulation of unique genes associated with
cancer and signaling Pathways, including two novel HCC-associated cancer testis antigen genes, NUF2 and TTK.
Conclusions: Integrated gene expression profiling of whole liver tissue with that of microdissected hepatocytes
demonstrated that HBV-associated HCC is characterized by a metabolism switch-off and by a significant reduction in
HBsAg. LCM proved to be a critical tool to validate gene signatures associated with HCC and to identify genes that may play a role in hepatocarcinogenesis, opening new perspectives for the discovery of novel diagnostic markers and therapeutic targets
Spin foams with timelike surfaces
Spin foams of 4d gravity were recently extended from complexes with purely
spacelike surfaces to complexes that also contain timelike surfaces. In this
article, we express the associated partition function in terms of vertex
amplitudes and integrals over coherent states. The coherent states are
characterized by unit 3--vectors which represent normals to surfaces and lie
either in the 2--sphere or the 2d hyperboloids. In the case of timelike
surfaces, a new type of coherent state is used and the associated completeness
relation is derived. It is also shown that the quantum simplicity constraints
can be deduced by three different methods: by weak imposition of the
constraints, by restriction of coherent state bases and by the master
constraint.Comment: 22 pages, no figures; v2: remarks on operator formalism added in
discussion; correction: the spin 1/2 irrep of the discrete series does not
appear in the Plancherel decompositio
Topological Graph Polynomials in Colored Group Field Theory
In this paper we analyze the open Feynman graphs of the Colored Group Field
Theory introduced in [arXiv:0907.2582]. We define the boundary graph
\cG_{\partial} of an open graph \cG and prove it is a cellular complex.
Using this structure we generalize the topological (Bollobas-Riordan) Tutte
polynomials associated to (ribbon) graphs to topological polynomials adapted to
Colored Group Field Theory graphs in arbitrary dimension
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