13,515 research outputs found
Resource Letter: Dark Energy and the Accelerating Universe
This Resource Letter provides a guide to the literature on dark energy and
the accelerating universe. It is intended to be of use to researchers,
teachers, and students at several levels. Journal articles, books, and websites
are cited for the following topics: Einstein's cosmological constant,
quintessence or dynamical scalar fields, modified cosmic gravity, relations to
high energy physics, cosmological probes and observations, terrestrial probes,
calculational tools and parameter estimation, teaching strategies and
educational resources, and the fate of the universe.Comment: Resource Letter for AAPT/AJP, 11 pages, 99 reference
Cohort aggregation modelling for complex forest stands: Spruce-aspen mixtures in British Columbia
Mixed-species growth models are needed as a synthesis of ecological knowledge
and for guiding forest management. Individual-tree models have been commonly
used, but the difficulties of reliably scaling from the individual to the stand
level are often underestimated. Emergent properties and statistical issues
limit their effectiveness. A more holistic modelling of aggregates at the whole
stand level is a potentially attractive alternative. This work explores
methodology for developing biologically consistent dynamic mixture models where
the state is described by aggregate stand-level variables for species or
age/size cohorts. The methods are demonstrated and tested with a two-cohort
model for spruce-aspen mixtures named SAM. The models combine single-species
submodels and submodels for resource partitioning among the cohorts. The
partitioning allows for differences in competitive strength among species and
size classes, and for complementarity effects. Height growth reduction in
suppressed cohorts is also modelled. SAM fits well the available data, and
exhibits behaviors consistent with current ecological knowledge. The general
framework can be applied to any number of cohorts, and should be useful as a
basis for modelling other mixed-species or uneven-aged stands.Comment: Accepted manuscript, to appear in Ecological Modellin
Metabolic network percolation quantifies biosynthetic capabilities across the human oral microbiome
The biosynthetic capabilities of microbes underlie their growth and interactions, playing a prominent role in microbial community structure. For large, diverse microbial communities, prediction of these capabilities is limited by uncertainty about metabolic functions and environmental conditions. To address this challenge, we propose a probabilistic method, inspired by percolation theory, to computationally quantify how robustly a genome-derived metabolic network produces a given set of metabolites under an ensemble of variable environments. We used this method to compile an atlas of predicted biosynthetic capabilities for 97 metabolites across 456 human oral microbes. This atlas captures taxonomically-related trends in biomass composition, and makes it possible to estimate inter-microbial metabolic distances that correlate with microbial co-occurrences. We also found a distinct cluster of fastidious/uncultivated taxa, including several Saccharibacteria (TM7) species, characterized by their abundant metabolic deficiencies. By embracing uncertainty, our approach can be broadly applied to understanding metabolic interactions in complex microbial ecosystems.T32GM008764 - NIGMS NIH HHS; T32 GM008764 - NIGMS NIH HHS; R01 DE024468 - NIDCR NIH HHS; R01 GM121950 - NIGMS NIH HHS; DE-SC0012627 - Biological and Environmental Research; RGP0020/2016 - Human Frontier Science Program; NSFOCE-BSF 1635070 - National Science Foundation; HR0011-15-C-0091 - Defense Advanced Research Projects Agency; R37DE016937 - NIDCR NIH HHS; R37 DE016937 - NIDCR NIH HHS; R01GM121950 - NIGMS NIH HHS; R01DE024468 - NIDCR NIH HHS; 1457695 - National Science FoundationPublished versio
An exact solution method for binary equilibrium problems with compensation and the power market uplift problem
We propose a novel method to find Nash equilibria in games with binary
decision variables by including compensation payments and
incentive-compatibility constraints from non-cooperative game theory directly
into an optimization framework in lieu of using first order conditions of a
linearization, or relaxation of integrality conditions. The reformulation
offers a new approach to obtain and interpret dual variables to binary
constraints using the benefit or loss from deviation rather than marginal
relaxations. The method endogenizes the trade-off between overall (societal)
efficiency and compensation payments necessary to align incentives of
individual players. We provide existence results and conditions under which
this problem can be solved as a mixed-binary linear program.
We apply the solution approach to a stylized nodal power-market equilibrium
problem with binary on-off decisions. This illustrative example shows that our
approach yields an exact solution to the binary Nash game with compensation. We
compare different implementations of actual market rules within our model, in
particular constraints ensuring non-negative profits (no-loss rule) and
restrictions on the compensation payments to non-dispatched generators. We
discuss the resulting equilibria in terms of overall welfare, efficiency, and
allocational equity
Integrated pest management portfolios in UK arable farming: results of a farmer survey.
Farmers are faced with a wide range of pest management (PM) options that can be adopted in isolation or alongside complementary or substitute strategies. This paper presents the results of a survey of UK cereal producers, focusing on the character and diversity of PM strategies currently used by, or available to, farmers. In addition, the survey asked various questions pertaining to agricultural policy participation, attitude towards environmental issues, sources of PM advice and information and the important characteristics of PM technologies. The results indicate that many farmers do make use of a suite of PM techniques, and that their choice of integrated PM (IPM) portfolio appears to be jointly dictated by farm characteristics and government policy. Results also indicate that portfolio choice does affect the number of subsequent insecticide applications per crop. These results help to identify the type of IPM portfolios considered to be adoptable by farmers and highlight the importance of substitution in IPM portfolios. As such, these results will help to direct R&D effort towards the realisation of more sustainable PM approaches and aid the identification of potential portfolio adopters. These findings highlight the opportunity that a revised agri-environmental policy design could generate in terms of enhancing coherent IPM portfolio adoption
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