6,688 research outputs found
Catching homologies by geometric entropy
A geometric entropy is defined as the Riemannian volume of the parameter
space of a statistical manifold associated with a given network. As such it can
be a good candidate for measuring networks complexity. Here we investigate its
ability to single out topological features of networks proceeding in a
bottom-up manner: first we consider small size networks by analytical methods
and then large size networks by numerical techniques. Two different classes of
networks, the random graphs and the scale--free networks, are investigated
computing their Betti numbers and then showing the capability of geometric
entropy of detecting homologies.Comment: 12 pages, 2 Figure
G-quartet biomolecular nanowires
We present a first-principle investigation of quadruple helix nanowires,
consisting of stacked planar hydrogen-bonded guanine tetramers. Our results
show that long wires form and are stable in potassium-rich conditions. We
present their electronic bandstructure and discuss the interpretation in terms
of effective wide-bandgap semiconductors. The microscopic structural and
electronic properties of the guanine quadruple helices make them suitable
candidates for molecular nanoelectronics.Comment: 7 pages, 3 figures, to be published in Applied Physics Letters (2002
Can we see naked singularities?
We study singularities which can form in a spherically symmetric
gravitational collapse of a general matter field obeying weak energy condition.
We show that no energy can reach an outside observer from a null naked
singularity. That means they will not be a serious threat to the Cosmic
Censorship Conjecture (CCC). For the timelike naked singularities, where only
the central shell gets singular, the redshift is always finite and they can in
principle, carry energy to a faraway observer. Hence for proving or disproving
CCC the study of timelike naked singularities will be more important. Our
results are very general and are independent of initial data and the form of
the matter.Comment: 10 page
The Formation of non-Keplerian Rings of Matter about Compact Stars
The formation of energetic rings of matter in a Kerr spacetime with an
outward pointing acceleration field does not appear to have previously been
noted as a relativistic effect. In this paper we show that such rings are a
gravimagneto effect with no Newtonian analog, and that they do not occur in the
static limit. The energy efficiency of these rings can, depending of the
strength of the acceleration field, be much greater than that of Keplerian
disks. The rings rotate in a direction opposite to that of compact star about
which they form. The size and energy efficiency of the rings depend on the
fundamental parameters of the spacetime as well as the strength the
acceleration field.Comment: 19 pages, 7 figures, 1 diagram. Figures are included in the text
using the "graphicx" package. If you do not have this package you can use
epsfig, or another package as long as you alter the tex file appropriately.
Alternatively you could print the figures out seperatel
Factors invoved in onset and recovery from postnatal depression
Background: A wide variety of psychosocial variables have been implicated in the onset and recovery from postnatal depression. A number of these factors were examined on a representative sample of pregnant Maltese women attending St Luke's Hospital. Method: A random sample of 239 pregnant women were interviewed at booking using a detailed sociodemographic history, the Revised Version of the Clinical Interview Schedule (CIS-R) and Maltese translation of the Edinburgh Postnatal Depression Scale (EPDS). The CIS-R was again administered over the phone at 36 weeks and the EPDS sent by post. At eight weeks postpartum, the CIS-R, modified version of the Social Maladjustment Schedule and the EPDS were again administered to 95.8% of women. Results: Onset of depression in the postpartum was not predicted by depressive symptomatology during pregnancy, marital status, level of education, planning of pregnancy, woman's, partner's or family's reaction to pregnancy and marital adjustment. However being primiparous and employed during pregnancy were significantly associated with postnatal depression. In contrast, depressed women who remained depressed postnatally had a past personal psychiatry history and significantly higher scores on the CISR recorded at 36 weeks gestation. Limitation: The number of women examined in the study yielded a small number of depressed women, for which the results are limited in value. The sample is that of Maltese women booking in at the antenatal clinic, thus excluding women who present late, close to delivery date. The follow up period was limited to eight weeks postpartum only, excluding those who develop depressive episodes after 8 weeks. Conclusions: Some cases of postpartum depression may be traced back to pregnancy while others do not seem to be predictable during pregnancy further contributing evidence to the view that the prenatal period may be a separate entity from the postnatal period with regards to depressive illness.peer-reviewe
A TDDFT study of the excited states of DNA bases and their assemblies
We present a detailed study of the optical absorption spectra of DNA bases
and base pairs, carried out by means of time dependent density functional
theory. The spectra for the isolated bases are compared to available
theoretical and experimental data and used to assess the accuracy of the method
and the quality of the exchange-correlation functional: Our approach turns out
to be a reliable tool to describe the response of the nucleobases. Furthermore,
we analyze in detail the impact of hydrogen bonding and -stacking in the
calculated spectra for both Watson-Crick base pairs and Watson-Crick stacked
assemblies. We show that the reduction of the UV absorption intensity
(hypochromicity) for light polarized along the base-pair plane depends strongly
on the type of interaction. For light polarized perpendicular to the basal
plane, the hypochromicity effect is reduced, but another characteristic is
found, namely a blue shift of the optical spectrum of the base-assembly
compared to that of the isolated bases. The use of optical tools as
fingerprints for the characterization of the structure (and type of
interaction) is extensively discussed.Comment: 31 pages, 8 figure
Conditions for the cosmological viability of the most general scalar-tensor theories and their applications to extended Galileon dark energy models
In the Horndeski's most general scalar-tensor theories with second-order
field equations, we derive the conditions for the avoidance of ghosts and
Laplacian instabilities associated with scalar, tensor, and vector
perturbations in the presence of two perfect fluids on the flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) background. Our general results are
useful for the construction of theoretically consistent models of dark energy.
We apply our formulas to extended Galileon models in which a tracker solution
with an equation of state smaller than -1 is present. We clarify the allowed
parameter space in which the ghosts and Laplacian instabilities are absent and
we numerically confirm that such models are indeed cosmologically viable.Comment: 18 pages, 6 figure
Variational approach to gravitational theories with two independent connections
A new variational approach for general relativity and modified theories of
gravity is presented. In addition to the metric tensor, two independent affine
connections enter the action as dynamical variables. In the matter action the
dependence upon one of the connections is left completely unspecified. When the
variation is applied to the Einstein-Hilbert action the Einstein field
equations are recovered. However when applied to and Scalar-Tensor
theories, it yields gravitational field equations which differ from their
equivalents obtained with a metric or Palatini variation and reduce to the
former ones only when no connections appear in the matter action.Comment: 11 pages, no figure
Cosmological constraints on extended Galileon models
The extended Galileon models possess tracker solutions with de Sitter
attractors along which the dark energy equation of state is constant during the
matter-dominated epoch, i.e. w_DE = -1-s, where s is a positive constant. Even
with this phantom equation of state there are viable parameter spaces in which
the ghosts and Laplacian instabilities are absent. Using the observational data
of the supernovae type Ia, the cosmic microwave background (CMB), and baryon
acoustic oscillations, we place constraints on the tracker solutions at the
background level and find that the parameter s is constrained to be s=0.034
(-0.034,+0.327) (95% CL) in the flat Universe. In order to break the degeneracy
between the models we also study the evolution of cosmological density
perturbations relevant to the large-scale structure (LSS) and the
Integrated-Sachs-Wolfe (ISW) effect in CMB. We show that, depending on the
model parameters, the LSS and the ISW effect is either positively or negatively
correlated. It is then possible to constrain viable parameter spaces further
from the observational data of the ISW-LSS cross-correlation as well as from
the matter power spectrum.Comment: 17 pages, 9 figures, uses RevTeX4-
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