29,185 research outputs found
Generalizing Refinement Operators to Learn Prenex Conjunctive Normal Forms
Inductive Logic Programming considers almost exclusively universally quantied theories. To add expressiveness, prenex conjunctive normal forms (PCNF) with existential variables should also be considered. ILP mostly uses learning with refinement operators. To extend refinement operators to PCNF, we should first do so with substitutions. However, applying a classic substitution to a PCNF with existential variables, one often obtains a generalization rather than a specialization. In this article we define substitutions that specialize a given PCNF and a weakly complete downward refinement operator. Moreover, we analyze the complexities of this operator in different types of languages and search spaces. In this way we lay a foundation for learning systems on PCNF. Based on this operator, we have implemented a simple learning system PCL on some type of PCNF.learning;PCNF;completeness;refinement;substitutions
Hard X-ray timing and spectral properties of PSR B0540-69
We report the hard X-ray properties of the young Crab-like LMC pulsar PSR
B0540-69, using archival RXTE PCA (2 - 60 keV) and RXTE HEXTE (15 - 250 keV)
data. Making use of the very long effective exposure of 684 ks, we derived a
very detailed master pulse profile for energies 2 - 20 keV. We confirm the
broad single-pulse shape with a dip in the middle and with a significant fine
structure to the left of the dip. For the first time pulse profiles in the 10 -
50 keV energy interval are shown. Remarkably, the coarse pulse shape is stable
from the optical up to X-ray energies analogous to the case of the Crab pulsar
(PSR B0531+21). The profiles can be described with two Gaussians with a phase
separation of ~0.2; the distribution of the ratios between the two components
from the optical to the X-ray range is consistent with being flat. Therefore we
cannot conclude that the profile consists of two distinct components. We also
derived a new total pulsed spectrum in the ~0.01 - 50 keV range in a consistent
analysis including also archival ROSAT PSPC (0.01 - 2.5 keV) data. This
spectrum cannot be described by a single power-law, but requires an additional
energy dependent term. The bending of the spectrum around 10 keV resembles that
of the Crab pulsar spectrum. Although model calculations using Outer Gap
scenarios could probably explain the high-energy characteristics of PSR
B0540-69 as they successfully do for the Crab, our measurements do not entirely
agree with the latest calculations by Zhang & Cheng (2000). The small
discrepancies are likely to be caused by uncertainties in the pulsar's
geometry.Comment: 7 pages and 4 figures. Accepted for publication in Astronomy &
Astrophysic
Covariant Hamiltonian Field Theory
A consistent, local coordinate formulation of covariant Hamiltonian field
theory is presented. Whereas the covariant canonical field equations are
equivalent to the Euler-Lagrange field equations, the covariant canonical
transformation theory offers more general means for defining mappings that
preserve the form of the field equations than the usual Lagrangian description.
It is proved that Poisson brackets, Lagrange brackets, and canonical 2-forms
exist that are invariant under canonical transformations of the fields. The
technique to derive transformation rules for the fields from generating
functions is demonstrated by means of various examples. In particular, it is
shown that the infinitesimal canonical transformation furnishes the most
general form of Noether's theorem. We furthermore specify the generating
function of an infinitesimal space-time step that conforms to the field
equations.Comment: 93 pages, no figure
On the Nature of Intrinsic Absorption in Reddened Seyfert 1 Galaxies
We discuss the origin of the ``dusty lukewarm absorber'', which we previously
identified in the reddened Seyfert 1 galaxies NGC 3227 and Akn 564. This
absorber is characterized by saturated UV absorption lines (C IV, N V) near the
systemic velocity of the host galaxy, and is likely responsible for reddening
both the continuum and the emission lines (including those from the narrow-line
region) from these Seyferts. From a large sample of Seyfert 1 galaxies, we find
that continuum reddening (as measured by UV color) tends to increase with
inclination of the host galaxy. Furthermore, reddened, inclined Seyfert
galaxies observed at moderate to high spectral resolution all show evidence for
dusty lukewarm absorbers. We suggest that these absorbers lie in the plane of
the host galaxy at distances > 100 pc from the nucleus, and are physically
distinct from the majority of intrinsic absorbers that are outflowing from the
nucleus.Comment: 14 pages, including 2 figures, accepted for publication in the
Astrophysical Journal (Letters
Asymptotics of relative heat traces and determinants on open surfaces of finite area
The goal of this paper is to prove that on surfaces with asymptotically cusp
ends the relative determinant of pairs of Laplace operators is well defined. We
consider a surface with cusps (M,g) and a metric h on the surface that is a
conformal transformation of the initial metric g. We prove the existence of the
relative determinant of the pair under suitable
conditions on the conformal factor. The core of the paper is the proof of the
existence of an asymptotic expansion of the relative heat trace for small
times. We find the decay of the conformal factor at infinity for which this
asymptotic expansion exists and the relative determinant is defined. Following
the paper by B. Osgood, R. Phillips and P. Sarnak about extremal of
determinants on compact surfaces, we prove Polyakov's formula for the relative
determinant and discuss the extremal problem inside a conformal class. We
discuss necessary conditions for the existence of a maximizer.Comment: This is the final version of the article before it gets published. 51
page
Failing to ignore: Paradoxical neural effects of perceptual load on early attentional selection in normal aging
We examined visual selective attention under perceptual load - simultaneous presentation of task-relevant and -irrelevant information - in healthy young and older adult human participants to determine whether age differences are observable at early stages of selection in the visual cortices. Participants viewed 50/50 superimposed face/place images and judged whether the faces were male or female, rendering places perceptible but task-irrelevant. Each stimulus was repeated, allowing us to index dynamic stimulus-driven competition from places. Consistent with intact early selection in young adults, we observed no adaptation to unattended places in parahippocampal place area (PPA) and significant adaptation to attended faces in fusiform face area (FFA). Older adults, however, exhibited both PPA adaptation to places and weak FFA adaptation to faces. We also probed participants\u27 associative recognition for face-place pairs post-task. Older adults with better place recognition memory scores were found to exhibit both the largest magnitudes of PPA adaptation and the smallest magnitudes of FFA adaptation on the attention task. In a control study, we removed the competing perceptual information to decrease perceptual load. These data revealed that the initial age-related impairments in selective attention were not due to a general decline in visual cortical selectivity; both young and older adults exhibited robust FFA adaptation and neither group exhibited PPA adaptation to repeated faces. Accordingly, distracting information does not merely interfere with attended input in older adults, but is co-encoded along with the contents of attended input, to the extent that this information can subsequently be recovered from recognition memory. Copyright © 2010 the authors
Evidence for Duality of Conifold from Fundamental String
We study the spectrum of BPS D5-D3-F1 states in type IIB theory, which are
proposed to be dual to D4-D2-D0 states on the resolved conifold in type IIA
theory. We evaluate the BPS partition functions for all values of the moduli
parameter in the type IIB side, and find them completely agree with the results
in the type IIA side which was obtained by using Kontsevich-Soibelman's
wall-crossing formula. Our result is a quite strong evidence for string
dualities on the conifold.Comment: 24 pages, 13 figures, v2: typos corrected, v3: explanations about
wall-crossing improved and figures adde
Light-Front Approach for Heavy Pentaquark Transitions
Assuming the two diquark structure for the pentaquark state as advocated in
the Jaffe-Wilczek model, there exist exotic parity-even anti-sextet and
parity-odd triplet heavy pentaquark baryons. The theoretical estimate of
charmed and bottom pentaquark masses is quite controversial and it is not clear
whether the ground-state heavy pentaquark lies above or below the strong-decay
threshold. We study the weak transitions of heavy pentaquark states using the
light-front quark model. In the heavy quark limit, heavy-to-heavy pentaquark
transition form factors can be expressed in terms of three Isgur-Wise
functions: two of them are found to be normalized to unity at zero recoil,
while the third one is equal to 1/2 at the maximum momentum transfer, in
accordance with the prediction of the large-Nc approach or the quark model.
Therefore, the light-front model calculations are consistent with the
requirement of heavy quark symmetry. Numerical results for form factors and
Isgur-Wise functions are presented. Decay rates of the weak decays Theta_b+ to
Theta_c0 pi+ (rho+), Theta_c0 to Theta+ pi- (rho-), Sigma'_{5b}+ to
Sigma'_{5c}0 pi+ (rho+) and Sigma'_{5c}0 to N_8+ pi- (rho-) with Theta_Q,
Sigma'_{5Q} and N_8 being the heavy anti-sextet, heavy triplet and light
octet pentaquarks, respectively, are obtained. For weakly decaying Theta_b+ and
Theta_c0, the branching ratios of Theta_b+ to Theta_c0 pi+, Theta_c0 to Theta+
pi- are estimated to be at the level of 10^{-3} and a few percents,
respectively.Comment: 33 pages, 3 figures, version to be published in Phys. Rev.
Wetting transitions of Ne
We report studies of the wetting behavior of Ne on very weakly attractive
surfaces, carried out with the Grand Canonical Monte Carlo method. The Ne-Ne
interaction was taken to be of Lennard-Jones form, while the Ne-surface
interaction was derived from an ab initio calculation of Chizmeshya et al.
Nonwetting behavior was found for Li, Rb, and Cs in the temperature regime
explored (i.e., T < 42 K). Drying behavior was manifested in a depleted fluid
density near the Cs surface. In contrast, for the case of Mg (a more attractive
potential) a prewetting transition was found near T= 28 K. This temperature was
found to shift slightly when a corrugated potential was used instead of a
uniform potential. The isotherm shape and the density profiles did not differ
qualitatively between these cases.Comment: 22 pages, 12 figures, submitted to Phys. Rev.
Light-Front Approach for Pentaquark Strong Decays
Assuming the two diquark structure for the pentaquark state as advocated in
the Jaffe-Wilczek model, we study the strong decays of light and heavy
parity-even pentaquark states using the light-front quark model in conjunction
with the spectator approximation. The narrowness of the Theta width is ascribed
to the p-wave configuration of the diquark pair. Taking the Theta width as a
benchmark, we estimate the rates of the strong decays Xi_{3/2}-- to Xi- pi-,
Sigma- K-, Sigma_{5c}0 to D_s- p, D_{s0}*- p and Xi_{5c}0 to D_s- Sigma+,
D_{s0}^{*-} Sigma+ with Sigma_{5c} Xi_{5c} being antisextet charmed pentaquarks
and D_{s0}* a scalar strange charmed meson. The ratio of Gamma(P_c to Baryon
D_{s0}*)/Gamma(P_c to Baryon D_s) is very useful for verifying the parity of
the antisextet charmed pentaquark P_c. It is expected to be of order unity for
an even parity P_c and much less than one for an odd parity pentaquark.Comment: 24 pages, 2 figure
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