421 research outputs found
Building Medical Homes: Lessons From Eight States With Emerging Programs
Profiles states' progress in developing and implementing medical home programs, strategies to encourage primary care providers' adoption, and states' ability to convene stakeholders, help improve and evaluate performance, and address antitrust concerns
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Slow carbon and nutrient accumulation in trees established following fire exclusion in the southwestern United States.
Increasing tree density that followed fire exclusion after the 1880s in the southwestern United States may have also altered nutrient cycles and led to a carbon (C) sink that constitutes a significant component of the U.S. C budget. Yet, empirical data quantifying century-scale changes in C or nutrients due to fire exclusion are rare. We used tree-ring reconstructions of stand structure from five ponderosa pine-dominated sites from across northern Arizona to compare live tree C, nitrogen (N), and phosphorus (P) storage between the 1880s and 1990s. Live tree biomass in the 1990s contained up to three times more C, N, and P than in 1880s. However, the increase in C storage was smaller than values used in recent U.S. C budgets. Furthermore, trees that had established prior to the 1880s accounted for a large fraction (28-66%) of the C, N, and P stored in contemporary stands. Overall, our century-scale analysis revealed that forests of the 1880s were on a trajectory to accumulate C and nutrients in trees even in the absence of fire exclusion, either because growing conditions became more favorable after the 1880s or because forests in the 1880s included age or size cohorts poised for accelerated growth. These results may lead to a reduction in the C sink attributed to fire exclusion, and they refine our understanding of reference conditions for restoration management of fire-prone forests
Low rank compression in the numerical solution of the nonequilibrium Dyson equation
We propose a method to improve the computational and memory efficiency of
numerical solvers for the nonequilibrium Dyson equation in the Keldysh
formalism. It is based on the empirical observation that the nonequilibrium
Green's functions and self energies arising in many problems of physical
interest, discretized as matrices, have low rank off-diagonal blocks, and can
therefore be compressed using a hierarchical low rank data structure. We
describe an efficient algorithm to build this compressed representation on the
fly during the course of time stepping, and use the representation to reduce
the cost of computing history integrals, which is the main computational
bottleneck. For systems with the hierarchical low rank property, our method
reduces the computational complexity of solving the nonequilibrium Dyson
equation from cubic to near quadratic, and the memory complexity from quadratic
to near linear. We demonstrate the full solver for the Falicov-Kimball model
exposed to a rapid ramp and Floquet driving of system parameters, and are able
to increase feasible propagation times substantially. We present examples with
262144 time steps, which would require approximately five months of computing
time and 2.2 TB of memory using the direct time stepping method, but can be
completed in just over a day on a laptop with less than 4 GB of memory using
our method. We also confirm the hierarchical low rank property for the driven
Hubbard model in the weak coupling regime within the GW approximation, and in
the strong coupling regime within dynamical mean-field theory.Comment: 14 page
Mapping as assemblage for cultural research
This article introduces a novel approach to the practice of mapping for the Cultural Researcher. Mapping is typically defined as a spatial concept where definitions of territory are represented. Recent interest in mapping and the affordances of Global Positioning Software (GPS) technologies offer new directions for exploring connections and flows across economic, social and cultural spheres. These new developments offer exciting ways to re-engage with spatial definitions and representations; however they can also be seen to maintain existing power relations that are inherent within previous modes. This article explores how the practice of mapping offers some methodological and conceptual revisions to what may constitute Cultural Research; that is, to situate the Researcher in a space of subject/object relations, rendered as fields or domains of data. Central to this, is to understand that the Research perspective is embedded within its ‘object’; not sitting on the outside, looking down or around, but an integral agent within the data mapping process and whose role might be to record, emphasise, direct and facilitate selected connections and flows between networks
Ecological restoration alters nitrogen transformations in a ponderosa pine-bunchgrass ecosystem
Ponderosa pinebunchgrass ecosystems of the western United States were altered following Euro-American settlement as grazing and fire suppression facilitated pine invasion of grassy openings. Pine invasion changed stand structure and fire regimes, motivating restoration through forest thinning and prescribed burning. To determine effects of restoration on soil nitrogen (N) transformations, we replicated (0.25-ha plots) the following experimental restoration treatments within a ponderosa pinebunchgrass community near Flagstaff, Arizona: (1) partial restorationthinning to presettlement conditions, (2) complete restorationremoval of trees and forest floor to presettlement conditions, native grass litter addition, and a prescribed burn, and (3) control. Within treatments, we stratified sampling to assess effects of canopy cover on N transformations. Forest floor net N mineralization and nitrification were similar among treatments on an areal basis, but higher in restoration treatments on a mass basis. In the mineral soil (015 cm), restoration treatments had 23 times greater annual net N mineralization and 35 times greater annual net nitrification than the control. Gross N transformation measurements indicate that elevated net N mineralization may be due to increased gross N mineralization, while elevated net nitrification may be due to decreased microbial immobilization of nitrate. Net N transformation rates beneath relict grassy openings were twice those beneath postsettlement pines. These short-term (1 yr) results suggest that ecological restoration increases N transformation rates and that prescribed burning may not be necessary to restore N cycling processes
Fast prediction and evaluation of gravitational waveforms using surrogate models
[Abridged] We propose a solution to the problem of quickly and accurately
predicting gravitational waveforms within any given physical model. The method
is relevant for both real-time applications and in more traditional scenarios
where the generation of waveforms using standard methods can be prohibitively
expensive. Our approach is based on three offline steps resulting in an
accurate reduced-order model that can be used as a surrogate for the
true/fiducial waveform family. First, a set of m parameter values is determined
using a greedy algorithm from which a reduced basis representation is
constructed. Second, these m parameters induce the selection of m time values
for interpolating a waveform time series using an empirical interpolant. Third,
a fit in the parameter dimension is performed for the waveform's value at each
of these m times. The cost of predicting L waveform time samples for a generic
parameter choice is of order m L + m c_f online operations where c_f denotes
the fitting function operation count and, typically, m << L. We generate
accurate surrogate models for Effective One Body (EOB) waveforms of
non-spinning binary black hole coalescences with durations as long as 10^5 M,
mass ratios from 1 to 10, and for multiple harmonic modes. We find that these
surrogates are three orders of magnitude faster to evaluate as compared to the
cost of generating EOB waveforms in standard ways. Surrogate model building for
other waveform models follow the same steps and have the same low online
scaling cost. For expensive numerical simulations of binary black hole
coalescences we thus anticipate large speedups in generating new waveforms with
a surrogate. As waveform generation is one of the dominant costs in parameter
estimation algorithms and parameter space exploration, surrogate models offer a
new and practical way to dramatically accelerate such studies without impacting
accuracy.Comment: 20 pages, 17 figures, uses revtex 4.1. Version 2 includes new
numerical experiments for longer waveform durations, larger regions of
parameter space and multi-mode model
A fast time domain solver for the equilibrium Dyson equation
We consider the numerical solution of the real time equilibrium Dyson
equation, which is used in calculations of the dynamical properties of quantum
many-body systems. We show that this equation can be written as a system of
coupled, nonlinear, convolutional Volterra integro-differential equations, for
which the kernel depends self-consistently on the solution. As is typical in
the numerical solution of Volterra-type equations, the computational bottleneck
is the quadratic-scaling cost of history integration. However, the structure of
the nonlinear Volterra integral operator precludes the use of standard fast
algorithms. We propose a quasilinear-scaling FFT-based algorithm which respects
the structure of the nonlinear integral operator. The resulting method can
reach large propagation times, and is thus well-suited to explore quantum
many-body phenomena at low energy scales. We demonstrate the solver with two
standard model systems: the Bethe graph, and the Sachdev-Ye-Kitaev model
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