658 research outputs found
Interactions between thresholds and spatial discretizations of snow: insights from estimates of wolverine denning habitat in the Colorado Rocky Mountains
Thresholds can be used to interpret environmental data in a way
that is easily communicated and useful for decision-making purposes.
However, thresholds are often developed for specific data products and time
periods, changing findings when the same threshold is applied to datasets or
periods with different characteristics. Here, we test the impact of
different spatial discretizations of snow on annual estimates of wolverine
denning opportunities in the Colorado Rocky Mountains, defined using a snow
water equivalent (SWE) threshold (0.20 m) and threshold date (15 May) from
previous habitat assessments. Annual potential wolverine denning area (PWDA)
was thresholded from a 36-year (1985–2020) snow reanalysis model with
three different spatial discretizations: (1) 480 m grid cells (D480), (2) 90 m
grid cells (D90), and (3) 480 m grid cells with implicit representations of
subgrid snow spatial heterogeneity (S480). Relative to the D480 and S480
discretizations, D90 resolved shallower snow deposits on slopes between 3050
and 3350 m elevation, decreasing PWDA by 10 %, on average. In years with
warmer and/or drier winters, S480 discretizations with subgrid
representations of snow heterogeneity increased PWDA, even within grid cells
where mean 15 May SWE was less than the SWE threshold. These simulations
increased PWDA by upwards of 30 % in low-snow years, as compared to the
D480 and D90 simulations without subgrid snow heterogeneity. Despite PWDA
sensitivity to different snow spatial discretizations, PWDA was controlled
more by annual variations in winter precipitation and temperature. However,
small changes to the SWE threshold (±0.07 m) and threshold date
(±2 weeks) also affected PWDA by as much as 82 %. Across these
threshold ranges, PWDA was approximately 18 % more sensitive to the SWE
threshold than the threshold date. However, the sensitivity to the threshold
date was larger in years with late spring snowfall, when PWDA depended on
whether modeled SWE was thresholded before, during, or after spring snow
accumulation. Our results demonstrate that snow thresholds are useful but
may not always provide a complete picture of the annual variability in
snow-adapted wildlife denning opportunities. Studies thresholding
spatiotemporal datasets could be improved by including (1) information about
the fidelity of thresholds across multiple spatial discretizations and (2) uncertainties related to ranges of realistic thresholds.</p
Entire curves avoiding given sets in C^n
Let be a proper closed subset of and
at most countable (). We give conditions
of and , under which there exists a holomorphic immersion (or a proper
holomorphic embedding) with .Comment: 10 page
Exhausting domains of the symmetrized bidisc
We show that the symmetrized bidisc may be exhausted by strongly linearly
convex domains. It shows in particular the existence of a strongly linearly
convex domain that cannot be exhausted by domains biholomorphic to convex ones.Comment: 6 page
Evolutionary multi-stage financial scenario tree generation
Multi-stage financial decision optimization under uncertainty depends on a
careful numerical approximation of the underlying stochastic process, which
describes the future returns of the selected assets or asset categories.
Various approaches towards an optimal generation of discrete-time,
discrete-state approximations (represented as scenario trees) have been
suggested in the literature. In this paper, a new evolutionary algorithm to
create scenario trees for multi-stage financial optimization models will be
presented. Numerical results and implementation details conclude the paper
Mean-risk models using two risk measures: A multi-objective approach
This paper proposes a model for portfolio optimisation, in which distributions are characterised and compared on the basis of three statistics: the expected value, the variance and the CVaR at a specified confidence level. The problem is multi-objective and transformed into a single objective problem in which variance is minimised while constraints are imposed on the expected value and CVaR. In the case of discrete random variables, the problem is a quadratic program. The mean-variance (mean-CVaR) efficient solutions that are not dominated with respect to CVaR (variance) are particular efficient solutions of the proposed model. In addition, the model has efficient solutions that are discarded by both mean-variance and mean-CVaR models, although they may improve the return distribution. The model is tested on real data drawn from the FTSE 100 index. An analysis of the return distribution of the chosen portfolios is presented
HMM based scenario generation for an investment optimisation problem
This is the post-print version of the article. The official published version can be accessed from the link below - Copyright @ 2012 Springer-Verlag.The Geometric Brownian motion (GBM) is a standard method for modelling financial time series. An important criticism of this method is that the parameters of the GBM are assumed to be constants; due to this fact, important features of the time series, like extreme behaviour or volatility clustering cannot be captured. We propose an approach by which the parameters of the GBM are able to switch between regimes, more precisely they are governed by a hidden Markov chain. Thus, we model the financial time series via a hidden Markov model (HMM) with a GBM in each state. Using this approach, we generate scenarios for a financial portfolio optimisation problem in which the portfolio CVaR is minimised. Numerical results are presented.This study was funded by NET ACE at OptiRisk Systems
Bergman kernel and complex singularity exponent
We give a precise estimate of the Bergman kernel for the model domain defined
by where
is a holomorphic map from to ,
in terms of the complex singularity exponent of .Comment: to appear in Science in China, a special issue dedicated to Professor
Zhong Tongde's 80th birthda
A remark on the dimension of the Bergman space of some Hartogs domains
Let D be a Hartogs domain of the form D={(z,w) \in CxC^N : |w| < e^{-u(z)}}
where u is a subharmonic function on C. We prove that the Bergman space of
holomorphic and square integrable functions on D is either trivial or infinite
dimensional.Comment: 12 page
Earliest land plants created modern levels of atmospheric oxygen
The progressive oxygenation of the Earth’s atmosphere was pivotal
to the evolution of life, but the puzzle of when and how
atmospheric oxygen (O2) first approached modern levels (~21%)
remains unresolved. Redox proxy data indicate the deep oceans
were oxygenated during 435-392 Ma, and the appearance of
fossil charcoal indicates O2>15-17% by 420-400 Ma. However,
existing models have failed to predict oxygenation at this time.
Here we show that the earliest plants, which colonized the land
surface from ~470 Ma onwards, were responsible for this mid-
Paleozoic oxygenation event, through greatly increasing global
organic carbon burial – the net long-term source of O2. We use
a trait-based ecophysiological model to predict that cryptogamic
vegetation cover could have achieved ~30% of today’s global
terrestrial net primary productivity by~445 Ma. Data from modern
bryophytes suggests this plentiful early plant material had a much
higher molar C:P ratio (~2000) than marine biomass (~100), such
that a given weathering flux of phosphorus could support more
organic carbon burial. Furthermore, recent experiments suggest
that early plants selectively increased the flux of phosphorus (relative
to alkalinity) weathered from rocks. Combining these effects
in a model of long-term biogeochemical cycling, we reproduce a
sustained +2‰ increase in the carbonate carbon isotope (δ13C)
record by ~445 Ma, and predict a corresponding rise in O2 to
present levels by 420-400 Ma, consistent with geochemical data.
This oxygen rise represents a permanent shift in regulatory regime
to one where fire-mediated negative feedbacks on organic carbon
burial stabilise high O2 levels
Identification of plasma lipid biomarkers for prostate cancer by lipidomics and bioinformatics
Background:
Lipids have critical functions in cellular energy storage, structure and signaling. Many individual lipid molecules have been associated with the evolution of prostate cancer; however, none of them has been approved to be used as a biomarker. The aim of this study is to identify lipid molecules from hundreds plasma apparent lipid species as biomarkers for diagnosis of prostate cancer.
Methodology/Principal Findings:
Using lipidomics, lipid profiling of 390 individual apparent lipid species was performed on 141 plasma samples from 105 patients with prostate cancer and 36 male controls. High throughput data generated from lipidomics were analyzed using bioinformatic and statistical methods. From 390 apparent lipid species, 35 species were demonstrated to have potential in differentiation of prostate cancer. Within the 35 species, 12 were identified as individual plasma lipid biomarkers for diagnosis of prostate cancer with a sensitivity above 80%, specificity above 50% and accuracy above 80%. Using top 15 of 35 potential biomarkers together increased predictive power dramatically in diagnosis of prostate cancer with a sensitivity of 93.6%, specificity of 90.1% and accuracy of 97.3%. Principal component analysis (PCA) and hierarchical clustering analysis (HCA) demonstrated that patient and control populations were visually separated by identified lipid biomarkers. RandomForest and 10-fold cross validation analyses demonstrated that the identified lipid biomarkers were able to predict unknown populations accurately, and this was not influenced by patient's age and race. Three out of 13 lipid classes, phosphatidylethanolamine (PE), ether-linked phosphatidylethanolamine (ePE) and ether-linked phosphatidylcholine (ePC) could be considered as biomarkers in diagnosis of prostate cancer.
Conclusions/Significance:
Using lipidomics and bioinformatic and statistical methods, we have identified a few out of hundreds plasma apparent lipid molecular species as biomarkers for diagnosis of prostate cancer with a high sensitivity, specificity and accuracy
- …