3,634 research outputs found
Considerations of high altitude emissions
The status of the Federal Aviation Administration's High Altitude Pollution Program is described which was instituted to develop the detailed quantitative information needed to judge whether or not regulatory action to limit such exhaust emissions would be necessary. The complexities of this question and the nature and magnitude of uncertainties still present in our scientific understanding of the potential interactions between aircraft exhaust emissions and stratospheric ozone and climate are reviewed. The direction and scope of future Federal and international activities are described
PASS-GLM: polynomial approximate sufficient statistics for scalable Bayesian GLM inference
Generalized linear models (GLMs) -- such as logistic regression, Poisson
regression, and robust regression -- provide interpretable models for diverse
data types. Probabilistic approaches, particularly Bayesian ones, allow
coherent estimates of uncertainty, incorporation of prior information, and
sharing of power across experiments via hierarchical models. In practice,
however, the approximate Bayesian methods necessary for inference have either
failed to scale to large data sets or failed to provide theoretical guarantees
on the quality of inference. We propose a new approach based on constructing
polynomial approximate sufficient statistics for GLMs (PASS-GLM). We
demonstrate that our method admits a simple algorithm as well as trivial
streaming and distributed extensions that do not compound error across
computations. We provide theoretical guarantees on the quality of point (MAP)
estimates, the approximate posterior, and posterior mean and uncertainty
estimates. We validate our approach empirically in the case of logistic
regression using a quadratic approximation and show competitive performance
with stochastic gradient descent, MCMC, and the Laplace approximation in terms
of speed and multiple measures of accuracy -- including on an advertising data
set with 40 million data points and 20,000 covariates.Comment: In Proceedings of the 31st Annual Conference on Neural Information
Processing Systems (NIPS 2017). v3: corrected typos in Appendix
Theory of the electronic structure of dilute bismide and bismide-nitride alloys of GaAs: Tight-binding and k.p models
The addition of dilute concentrations of bismuth (Bi) into GaAs to form
GaBiAs alloys results in a large reduction of the band gap energy Eg
accompanied by a significant increase of the spin-orbit-splitting energy
(delta_SO), leading to an Eg < delta_SO regime for ~10% Bi composition which is
technologically relevant for the design of highly efficient photonic devices.
The quaternary alloy GaBiNAs offers further flexibility for band gap tuning,
because both nitrogen and bismuth can independently induce band gap reduction.
This work reports sp3s* tight binding and 14-band k.p models for the study of
the electronic structure of GaBiAs and GaBiNAs alloys. Our results are in good
agreement with the available experimental data.Comment: 2 pages, 1 figur
Truncated Random Measures
Completely random measures (CRMs) and their normalizations are a rich source
of Bayesian nonparametric priors. Examples include the beta, gamma, and
Dirichlet processes. In this paper we detail two major classes of sequential
CRM representations---series representations and superposition
representations---within which we organize both novel and existing sequential
representations that can be used for simulation and posterior inference. These
two classes and their constituent representations subsume existing ones that
have previously been developed in an ad hoc manner for specific processes.
Since a complete infinite-dimensional CRM cannot be used explicitly for
computation, sequential representations are often truncated for tractability.
We provide truncation error analyses for each type of sequential
representation, as well as their normalized versions, thereby generalizing and
improving upon existing truncation error bounds in the literature. We analyze
the computational complexity of the sequential representations, which in
conjunction with our error bounds allows us to directly compare representations
and discuss their relative efficiency. We include numerous applications of our
theoretical results to commonly-used (normalized) CRMs, demonstrating that our
results enable a straightforward representation and analysis of CRMs that has
not previously been available in a Bayesian nonparametric context.Comment: To appear in Bernoulli; 58 pages, 3 figure
Theory and design of InGaAsBi mid-infrared semiconductor lasers: type-I quantum wells for emission beyond 3 m on InP substrates
We present a theoretical analysis and optimisation of the properties and
performance of mid-infrared semiconductor lasers based on the dilute bismide
alloy InGaAsBi, grown on conventional (001) InP
substrates. The ability to independently vary the epitaxial strain and emission
wavelength in this quaternary alloy provides significant scope for band
structure engineering. Our calculations demonstrate that structures based on
compressively strained InGaAsBi quantum wells (QWs)
can readily achieve emission wavelengths in the 3 -- 5 m range, and that
these QWs have large type-I band offsets. As such, these structures have the
potential to overcome a number of limitations commonly associated with this
application-rich but technologically challenging wavelength range. By
considering structures having (i) fixed QW thickness and variable strain, and
(ii) fixed strain and variable QW thickness, we quantify key trends in the
properties and performance as functions of the alloy composition, structural
properties, and emission wavelength, and on this basis identify routes towards
the realisation of optimised devices for practical applications. Our analysis
suggests that simple laser structures -- incorporating
InGaAsBi QWs and unstrained ternary
InGaAs barriers -- which are compatible with established
epitaxial growth, provide a route to realising InP-based mid-infrared diode
lasers.Comment: Submitted versio
Novel applications of fibre tapers
In this talk we will discuss new applications for fibre tapers ranging from femtosecond pulse manipulation to optical clock generation in micro-coil resonators. Fundamental to these interactions is the enhanced nonlinearity arising from the smaller cores making compact optical devices possible
'What you look like is such a big factor': interviews with adolescent girls about the appearance culture in an all-girls' school
High school is a key venue for the development and expression of body image concerns in adolescent girls. Researchers have begun to investigate the role of school-based āappearance culturesā in magnifying the body image concerns of students. To date, however, no research has examined girlsā experience as participants within these cultures, and thus the opportunity to learn how girls account for the development and maintenance of these cultures has been missed. In interviews with nine girls attending an all-girlsā school, the existence of a strong āappearance cultureā in the school was identified as a major influence on the body image concerns of students. Girls talked about the ways in which appearance-focused conversations, dieting, and weight monitoring occurred as part of the everyday interaction with friends and peers at school. They also identified many ways in which their school attempted to address body image concerns, although these attempts were often portrayed as ineffective, if not counter-productive. These findings suggest that attempts to address the body image concerns of students will need to be sensitive to the lived reality of appearance cultures within schools
Masses of Nearby Supermassive Black Holes with Very-Long Baseline Interferometry
Dynamical mass measurements to date have allowed determinations of the mass M
and the distance D of a number of nearby supermassive black holes. In the case
of Sgr A*, these measurements are limited by a strong correlation between the
mass and distance scaling roughly as M ~ D^2. Future very-long baseline
interferometric (VLBI) observations will image a bright and narrow ring
surrounding the shadow of a supermassive black hole, if its accretion flow is
optically thin. In this paper, we explore the prospects of reducing the
correlation between mass and distance with the combination of dynamical
measurements and VLBI imaging of the ring of Sgr A*. We estimate the signal to
noise ratio of near-future VLBI arrays that consist of five to six stations,
and we simulate measurements of the mass and distance of Sgr A* using the
expected size of the ring image and existing stellar ephemerides. We
demonstrate that, in this best-case scenario, VLBI observations at 1 mm can
improve the error on the mass by a factor of about two compared to the results
from the monitoring of stellar orbits alone. We identify the additional sources
of uncertainty that such imaging observations have to take into account. In
addition, we calculate the angular diameters of the bright rings of other
nearby supermassive black holes and identify the optimal targets besides Sgr A*
that could be imaged by a ground-based VLBI array or future space-VLBI missions
allowing for refined mass measurements.Comment: 8 pages, 4 figures, 2 tables, refereed version, accepted for
publication in Ap
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