389 research outputs found
Book Reviews
The Variational Auto-Encoder (VAE) is one of the most used unsupervised
machine learning models. But although the default choice of a Gaussian
distribution for both the prior and posterior represents a mathematically
convenient distribution often leading to competitive results, we show that this
parameterization fails to model data with a latent hyperspherical structure. To
address this issue we propose using a von Mises-Fisher (vMF) distribution
instead, leading to a hyperspherical latent space. Through a series of
experiments we show how such a hyperspherical VAE, or -VAE, is
more suitable for capturing data with a hyperspherical latent structure, while
outperforming a normal, -VAE, in low dimensions on other data
types.Comment: GitHub repository: http://github.com/nicola-decao/s-vae-tf, Blogpost:
https://nicola-decao.github.io/s-va
Increasing Expressivity of a Hyperspherical VAE
Learning suitable latent representations for observed, high-dimensional data
is an important research topic underlying many recent advances in machine
learning. While traditionally the Gaussian normal distribution has been the
go-to latent parameterization, recently a variety of works have successfully
proposed the use of manifold-valued latents. In one such work (Davidson et al.,
2018), the authors empirically show the potential benefits of using a
hyperspherical von Mises-Fisher (vMF) distribution in low dimensionality.
However, due to the unique distributional form of the vMF, expressivity in
higher dimensional space is limited as a result of its scalar concentration
parameter leading to a 'hyperspherical bottleneck'. In this work we propose to
extend the usability of hyperspherical parameterizations to higher dimensions
using a product-space instead, showing improved results on a selection of image
datasets.Comment: NeurIPS 2019, in Workshop on Bayesian Deep Learnin
Extra-dimensional cosmology with domain-wall branes
We show how to define a consistent braneworld cosmology in a model in which
the brane is constructed as a field-theoretic domain wall of finite thickness.
The Friedmann, Robertson-Walker metric is recovered in the region of the brane,
but, remarkably, with scale factor that depends on particle energy and on
particle species, constituting a breakdown of the weak equivalence principle on
sufficiently small scales. This unusual effect comes from the extended nature
of particles confined to a domain-wall brane, and the fact that they feel an
"average" of the bulk spacetime. We demonstrate how to recover the standard
results of brane cosmology in the infinitely-thin brane limit, and comment on
how our results have the potential to place bounds on parameters such as the
thickness of domain-wall braneworlds.Comment: 23 pages; v2 has additional references and reflects journal versio
The Off-diagonal Goldberger-Treiman Relation and Its Discrepancy
We study the off-diagonal Goldberger-Treiman relation (ODGTR) and its
discrepancy (ODGTD) in the N, Delta, pi sector through O(p^2) using heavy
baryon chiral perturbation theory. To this order, the ODGTD and axial vector N
to Delta transition radius are determined solely by low energy constants. Loop
corrections appear at O(p^4). For low-energy constants of natural size, the
ODGTD would represent a ~ 2% correction to the ODGTR. We discuss the
implications of the ODGTR and ODGTD for lattice and quark model calculations of
the transition form factors and for parity-violating electroexcitation of the
Delta.Comment: 11 pages, 1 eps figur
Reaction and Axial Vector Coupling
The reaction is studied in the region of low
to investigate the effect of deuteron structure and width of the
resonance on the differential cross section. The results are used to extract
the axial vector coupling from the experimental data on
this reaction. The possibility to determine this coupling from electroweak
interaction experiments with high intensity electron accelerators is discussed.Comment: 14 pages, REVTEX, 5 figure
Intertemporal excess burden, bequest motives, and the budget deficit
The author aims to empirically determine the significant factors that affect the levels of budget deficits of central governments across time and across countries. He empirically tests two prominent theories of budget deficits-the Barro (1979) tax-smoothing approach, and the still-untested theory of negative bequest motives advocated by Cukierman and Meltzer (1989). The author uses econometric techniques including fixed-effects (both country and time) panel regressions spanning 87 countries over the period 1975 to 1992, and the Griliches treatment of missing data. The author finds relatively stronger statistical support for the tax-smoothing approach among developing countries but not in industrial countries. The existence of empirical evidence supporting the theory of negative bequest motives is indeterminate. The author also conducted post-regression analyses to assess the proportion of observed differences in budget deficits the factors were actually able to explain. These reveal that both theories are generally weak in accounting for inter-temporal changes in budget deficit shares for both industrial and developing countries. The theories performed significantly better in accounting for cross-section differences. The author has many contributions to the literature. First, he analyzes the question of what determines the size of central government budget deficits using cross-country time series data leading into the 1990s. Second, he provides empirical tests of the still-untested Cukierman-Meltzer (1989) negative bequest motive theory of budget deficits. By using the panel data, the author attempts to determine the factors that influence not only the inter-temporal differences in budget deficits but also those factors that lead to cross-country differences. Last but not least, he provides some preliminary evidence that poverty reduction is necessary for long-term government budget deficit reduction.Public Sector Economics&Finance,Environmental Economics&Policies,Economic Theory&Research,Banks&Banking Reform,Municipal Financial Management,Public Sector Economics&Finance,Economic Theory&Research,Economic Stabilization,Banks&Banking Reform,National Governance
Sungas : Opportunities and challenges for solar thermos chemical fuels
Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.Displacing petroleum-derived fuels with renewable solar
fuels offers an opportunity to harness the earth’s most abundant
energy resource, to reduce anthropogenic emissions of
greenhouse gases, and to meet an expanding global demand for
fuel. This paper presents near-term and forward looking paths
to produce solar fuels using concentrated solar energy as the
source of process heat to drive thermochemical processes.
Solar gasification of biomass is presented as an important
stepping stone toward the goal of thermochemical metal oxide
redox cycles to split water and carbon dioxide.dc201
Spherical and deformed isomers in Pb-188
Several isomers in Pb-188 have been identified using pulsed beams, the recoil-shadow technique, and the Er-164(Si-28,4n) Pb-188 reaction. Two of the isomers feed the 10(+) state of the yrast sequence and are suggested to be the 11(-) and 12(+) states from oblate and spherical configurations, respectively. The 12(+) isomer is fed weakly by another isomer with a relatively long lifetime, but it has not been characterized. A fourth isomer with a lifetime of about 1.2 mu s leads via a complicated path to the 8(+) and lower spin yrast states. It is a candidate for the K-pi = 8(-), two-quasineutron state which occurs systematically in N = 106 prolate-deformed nuclei, supporting the assumption that the intruding collective well is prolate
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
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