210 research outputs found
Testing the Gaussian Copula Hypothesis for Financial Assets Dependences
Using one of the key property of copulas that they remain invariant under an
arbitrary monotonous change of variable, we investigate the null hypothesis
that the dependence between financial assets can be modeled by the Gaussian
copula. We find that most pairs of currencies and pairs of major stocks are
compatible with the Gaussian copula hypothesis, while this hypothesis can be
rejected for the dependence between pairs of commodities (metals).
Notwithstanding the apparent qualification of the Gaussian copula hypothesis
for most of the currencies and the stocks, a non-Gaussian copula, such as the
Student's copula, cannot be rejected if it has sufficiently many ``degrees of
freedom''. As a consequence, it may be very dangerous to embrace blindly the
Gaussian copula hypothesis, especially when the correlation coefficient between
the pair of asset is too high as the tail dependence neglected by the Gaussian
copula can be as large as 0.6, i.e., three out five extreme events which occur
in unison are missed.Comment: Latex document of 43 pages including 14 eps figure
Aromatic Hydrocarbons, Diamonds, and Fullerenes in Interstellar Space: Puzzles to be Solved by Laboratory and Theoretical Astrochemistry
New research is presented, and previous research is reviewed, on the emission
and absorption of interstellar aromatic hydrocarbons. Emission from aromatic
hydrocarbons dominate the mid-infrared emission of many galaxies, including our
own Milky Way galaxy. Only recently have aromatic hydrocarbons been observed in
absorption in the interstellar medium, along lines of sight with high column
densities of interstellar gas and dust. Much work on interstellar aromatics has
been done, with astronomical observations and laboratory and theoretical
astrochemistry. In many cases the predictions of laboratory and theoretical
work are confirmed by astronomical observations, but in other cases clear
discrepancies exist which provide problems to be solved by a combination of
astronomical observations, laboratory studies, and theoretical studies. The
emphasis of this paper will be on current outstanding puzzles concerning
aromatic hydrocarbons which require further laboratory and theoretical
astrochemistry to resolve. This paper will also touch on related topics where
laboratory and theoretical astrochemistry studies are needed to explain
astrophysical observations, such as a possible absorption feature due to
interstellar "diamonds" and the search for fullerenes in space.Comment: Spectrochimica Acta A, Feb. 2001, in press. 33 pages including 11
postscript figures, AASTeX format. Full postscript paper also available at
http://www.astronomy.ohio-state.edu/~sellgren/saa.htm
Dual Screening the Political:Media Events, Social Media, and Citizen Engagement
Dual screening—the complex bundle of practices that involve integrating, and switching across and between, live broadcast media and social medi—is now routine for many citizens during important political media events. But do these practices shape political engagement, and if so, why? We devised a unique research design combining a large-scale Twitter dataset and a custom-built panel survey focusing on the broadcast party leaders’ debates held during the 2014 European Parliament elections in the United Kingdom. We find that relatively active, “lean-forward” practices, such as commenting live on social media as the debate unfolded, and engaging with conversations via Twitter hashtags, have the strongest and most consistent positive associations with political engagement
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Tail Dependence Measure for Examining Financial Extreme Co-movements
Modeling and forecasting extreme co-movements in financial market is important for conducting stress test in risk management. Asymptotic independence and asymptotic dependence behave drastically different in modeling such co-movements. For example, the impact of extreme events is usually overestimated whenever asymptotic dependence is wrongly assumed. On the other hand, the impact is seriously underestimated whenever the data is misspecified as asymptotic independent. Therefore, distinguishing between asymptotic independence/dependence scenarios is very informative for any decision-making and especially in risk management. We investigate the properties of the limiting conditional Kendall’s tau which can be used to detect the presence of asymptotic independence/dependence. We also propose nonparametric estimation for this new measure and derive its asymptotic limit. A simulation study shows good performances of the new measure and its combination with the coefficient of tail dependence proposed by Ledford and Tawn (1996, 1997). Finally, applications to financial and insurance data are provided
Time-dependent density functional study of the electronic spectra of oligoacenes in the charge states -1, 0, +1, and +2
We present a systematic theoretical study of the five smallest oligoacenes
(naphthalene, anthracene, tetracene, pentacene, and hexacene) in their
anionic,neutral, cationic, and dicationic charge states. We used density
functional theory (DFT) to obtain the ground-state optimised geometries, and
time-dependent DFT (TD-DFT) to evaluate the electronic absorption spectra.
Total-energy differences enabled us to evaluate the electron affinities and
first and second ionisation energies, the quasiparticle correction to the
HOMO-LUMO energy gap and an estimate of the excitonic effects in the neutral
molecules. Electronic absorption spectra have been computed by combining two
different implementations of TD-DFT: the frequency-space method to study
general trends as a function of charge-state and molecular size for the
lowest-lying in-plane long-polarised and short-polarised
electronic transitions, and the real-time propagation scheme to obtain the
whole photo-absorption cross-section up to the far-UV. Doubly-ionised PAHs are
found to display strong electronic transitions of character
in the near-IR, visible, and near-UV spectral ranges, like their singly-charged
counterparts. While, as expected, the broad plasmon-like structure with its
maximum at about 17-18 eV is relatively insensitive to the charge-state of the
molecule, a systematic decrease with increasing positive charge of the
absorption cross-section between about 6 and about 12 eV is observed for each
member of the class.Comment: 38 pages, 11 figures, 7 tables, accepted for publication in Chemical
Physic
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Forecasting Using Heterogeneous Panels with Cross-Sectional Dependence
In this paper, we focus on forecasting heterogeneous panels in presence of cross-sectional depen-dence in terms of both spatial error dependence and common factors. We propose two mainapproaches to estimate the factor structure, one using the residuals (“Residuals Based Approach”,RBA) while the second using a panel of some variables (“Auxiliary Variables Approach”, AVA)to extract the factors. Small sample properties of the methods proposed is investigated throughMonte Carlo simulation exercises and used in an application to predict house price inflation inOECD countries
The aromatic infrared bands as seen by ISO-SWS: probing the PAH model
We discuss the Aromatic Infrared Band (AIB) profiles observed by ISO-SWS
towards a number of bright interstellar regions where dense molecular gas is
illuminated by stellar radiation. Our sample spans a broad range of excitation
conditions (exciting radiation fields with effective temperature, Teff, ranging
from 23,000 to 45,000 K). The SWS spectra are decomposed coherently in our
sample into Lorentz profiles and a broadband continuum. We find that the
individual profiles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 microns are well
represented with at most two lorentzians. Furthermore, we show that the
positions and widths of these AIBs are remarkably stable (within a few cm-1).
We then extract the profiles of individual AIBs from the data and compare them
to a model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission which
includes the temperature dependence of the AIB profiles. The present similarity
of the AIB profiles requires that the PAH temperature distribution remains
roughly the same whatever the radiation field hardness. Deriving the
temperature distribution of interstellar PAHs, we show that its hot tail, which
controls the AIB spectrum, sensitively depends on Nmin (the number of C-atoms
in the smallest PAH) and Teff. Comparing the observed profiles of the
individual AIBs to our model results, we can match most of the AIB profiles if
Nmin is increased with Teff. We then discuss our results in the broader context
of ISO observations of fainter interstellar regions where PAHs are expected to
be in neutral form.Comment: 19 pages, 11 figures, accepted for Astronomy and Astrophysic
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