2,375 research outputs found
The localization sequence for the algebraic K-theory of topological K-theory
We prove a conjecture of Rognes by establishing a localization cofiber
sequence of spectra, K(Z) to K(ku) to K(KU) to Sigma K(Z), for the algebraic
K-theory of topological K-theory. We deduce the existence of this sequence as a
consequence of a devissage theorem identifying the K-theory of the Waldhausen
category of Postnikov towers of modules over a connective A-infinity ring
spectrum R with the Quillen K-theory of the abelian category of finitely
generated pi_0(R)-modules.Comment: Updated final version. Small change in definition of S' construction
and correction to the proof of 2.
NASCAP simulation of PIX 2 experiments
The latest version of the NASCAP/LEO digital computer code used to simulate the PIX 2 experiment is discussed. NASCAP is a finite-element code and previous versions were restricted to a single fixed mesh size. As a consequence the resolution was dictated by the largest physical dimension to be modeled. The latest version of NASCAP/LEO can subdivide selected regions. This permitted the modeling of the overall Delta launch vehicle in the primary computational grid at a coarse resolution, with subdivided regions at finer resolution being used to pick up the details of the experiment module configuration. Langmuir probe data from the flight were used to estimate the space plasma density and temperature and the Delta ground potential relative to the space plasma. This information is needed for input to NASCAP. Because of the uncertainty or variability in the values of these parameters, it was necessary to explore a range around the nominal value in order to determine the variation in current collection. The flight data from PIX 2 were also compared with the results of the NASCAP simulation
The debris disk - terrestrial planet connection
The eccentric orbits of the known extrasolar giant planets provide evidence
that most planet-forming environments undergo violent dynamical instabilities.
Here, we numerically simulate the impact of giant planet instabilities on
planetary systems as a whole. We find that populations of inner rocky and outer
icy bodies are both shaped by the giant planet dynamics and are naturally
correlated. Strong instabilities -- those with very eccentric surviving giant
planets -- completely clear out their inner and outer regions. In contrast,
systems with stable or low-mass giant planets form terrestrial planets in their
inner regions and outer icy bodies produce dust that is observable as debris
disks at mid-infrared wavelengths. Fifteen to twenty percent of old stars are
observed to have bright debris disks (at wavelengths of ~70 microns) and we
predict that these signpost dynamically calm environments that should contain
terrestrial planets.Comment: Contribution to proceedings of IAU 276: Astrophysics of Planetary
System
Required Sample Size to Detect Mediation in 3-Level Implementation Studies
Background: Statistical tests of mediation are important for advancing implementation science; however, little research has examined the sample sizes needed to detect mediation in 3-level designs (e.g., organization, provider, patient) that are common in implementation research. Using a generalizable Monte Carlo simulation method, this paper examines the sample sizes required to detect mediation in 3-level designs under a range of conditions plausible for implementation studies.
Method: Statistical power was estimated for 17,496 3-level mediation designs in which the independent variable (X) resided at the highest cluster level (e.g., organization), the mediator (M) resided at the intermediate nested level (e.g., provider), and the outcome (Y) resided at the lowest nested level (e.g., patient). Designs varied by sample size per level, intraclass correlation coefficients of M and Y, effect sizes of the two paths constituting the indirect (mediation) effect (i.e., X→M and M→Y), and size of the direct effect. Power estimates were generated for all designs using two statistical models—conventional linear multilevel modeling of manifest variables (MVM) and multilevel structural equation modeling (MSEM)—for both 1- and 2-sided hypothesis tests.
Results: For 2-sided tests, statistical power to detect mediation was sufficient (≥0.8) in only 463 designs (2.6%) estimated using MVM and 228 designs (1.3%) estimated using MSEM; the minimum number of highest-level units needed to achieve adequate power was 40; the minimum total sample size was 900 observations. For 1-sided tests, 808 designs (4.6%) estimated using MVM and 369 designs (2.1%) estimated using MSEM had adequate power; the minimum number of highest-level units was 20; the minimum total sample was 600. At least one large effect size for either the X→M or M→Y path was necessary to achieve adequate power across all conditions.
Conclusions: While our analysis has important limitations, results suggest many of the 3-level mediation designs that can realistically be conducted in implementation research lack statistical power to detect mediation of highest-level independent variables unless effect sizes are large and 40 or more highest-level units are enrolled. We suggest strategies to increase statistical power for multilevel mediation designs and innovations to improve the feasibility of mediation tests in implementation research
A model structure for coloured operads in symmetric spectra
We describe a model structure for coloured operads with values in the
category of symmetric spectra (with the positive model structure), in which
fibrations and weak equivalences are defined at the level of the underlying
collections. This allows us to treat R-module spectra (where R is a cofibrant
ring spectrum) as algebras over a cofibrant spectrum-valued operad with R as
its first term. Using this model structure, we give suficient conditions for
homotopical localizations in the category of symmetric spectra to preserve
module structures.Comment: 16 page
AI for the Public Sector: Opportunities and challenges of cross-sector collaboration
Public sector organisations are increasingly interested in using data science and artificial intelligence capabilities to deliver policy and generate efficiencies in high uncertainty environments. The long-term success of data science and AI in the public sector relies on effectively embedding it into delivery solutions for policy implementation. However, governments cannot do this integration of AI into public service delivery on their own. The UK Government Industrial Strategy is clear that delivering on the AI grand challenge requires collaboration between universities and public and private sectors. This cross-sectoral collaborative approach is the norm in applied AI centres of excellence around the world. Despite their popularity, cross-sector collaborations entail serious management challenges that hinder their success. In this article we discuss the opportunities and challenges from AI for public sector. Finally, we propose a series of strategies to successfully manage these cross-sectoral collaborations
Particle Creation If a Cosmic String Snaps
We calculate the Bogolubov coefficients for a metric which describes the
snapping of a cosmic string. If we insist on a matching condition for all times
{\it and} a particle interpretation, we find no particle creation.Comment: 10 pages, MRC.PH.17/9
Effects of Autism Spectrum Disorder Insurance Mandates on the Treated Prevalence of Autism Spectrum Disorder
Key Findings: State mandates requiring commercial health plans to cover services for children with autism spectrum disorder increased the number of children diagnosed with the disorder. However, diagnosis rates remain much lower than community estimates, suggesting that many commercially insured children with ASD remain undiagnosed or are insured through public plans
Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b
Oxygen (O) and carbon (C) have been inferred recently to be subsolar in
abundance from spectra of the atmosphere of the transiting hot Jupiter HD
189733b. Yet, the mass and radius of the planet coupled with structure models
indicate a strongly supersolar abundance of heavy elements in the interior of
this object. Here we explore the discrepancy between the large amount of heavy
elements suspected in the planet's interior and the paucity of volatiles
measured in its atmosphere. We describe the formation sequence of the icy
planetesimals formed beyond the snow line of the protoplanetary disk and
calculate the composition of ices ultimately accreted in the envelope of HD
189733b on its migration pathway. This allows us to reproduce the observed
volatile abundances by adjusting the mass of ices vaporized in the envelope.
The predicted elemental mixing ratios should be 0.15--0.3 times solar in the
envelope of HD 189733b if they are fitted to the recent O and C determinations.
However, our fit to the minimum mass of heavy elements predicted by internal
structure models gives elemental abundances that are 1.2--2.4 times oversolar
in the envelope of HD189733b. We propose that the most likely cause of this
discrepancy is irradiation from the central star leading to development of a
radiative zone in the planet's outer envelope which would induce gravitational
settling of elements. Hence, all strongly irradiated extrasolar planets should
present subsolar abundances of volatiles. We finally predict that the
abundances of nitrogen (N), sulfur (S) and phosphorus (P) are of , and relative to
H, respectively in the atmosphere of HD 189733b.Comment: Accepted for publication in Astronomy & Astrophysic
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