2,033 research outputs found
Random trees with superexponential branching weights
We study rooted planar random trees with a probability distribution which is
proportional to a product of weight factors associated to the vertices of
the tree and depending only on their individual degrees . We focus on the
case when grows faster than exponentially with . In this case the
measures on trees of finite size converge weakly as tends to infinity
to a measure which is concentrated on a single tree with one vertex of infinite
degree. For explicit weight factors of the form with
we obtain more refined results about the approach to the infinite
volume limit.Comment: 19 page
Mean-field methods in evolutionary duplication-innovation-loss models for the genome-level repertoire of protein domains
We present a combined mean-field and simulation approach to different models
describing the dynamics of classes formed by elements that can appear,
disappear or copy themselves. These models, related to a paradigm
duplication-innovation model known as Chinese Restaurant Process, are devised
to reproduce the scaling behavior observed in the genome-wide repertoire of
protein domains of all known species. In view of these data, we discuss the
qualitative and quantitative differences of the alternative model formulations,
focusing in particular on the roles of element loss and of the specificity of
empirical domain classes.Comment: 10 Figures, 2 Table
Impact of land surface initialization approach on subseasonal forecast skill: A regional analysis in the southern hemisphere
The authors use a sophisticated coupled landâatmosphere modeling system for a Southern Hemisphere subdomain centered over southeastern Australia to evaluate differences in simulation skill from two different land surface initialization approaches. The first approach uses equilibrated land surface states obtained from offline simulations of the land surface model, and the second uses land surface states obtained from reanalyses. The authors find that land surface initialization using prior offline simulations contribute to relative gains in subseasonal forecast skill. In particular, relative gains in forecast skill for temperature of 10%â20% within the first 30 days of the forecast can be attributed to the land surface initialization method using offline states. For precipitation there is no distinct preference for the land surface initialization method, with limited gains in forecast skill irrespective of the lead time. The authors evaluated the asymmetry between maximum and minimum temperatures and found that maximum temperatures had the largest gains in relative forecast skill, exceeding 20% in some regions. These results were statistically significant at the 98% confidence level at up to 60 days into the forecast period. For minimum temperature, using reanalyses to initialize the land surface contributed to relative gains in forecast skill, reaching 40% in parts of the domain that were statistically significant at the 98% confidence level. The contrasting impact of the land surface initialization method between maximum and minimum temperature was associated with different soil moisture coupling mechanisms. Therefore, land surface initialization from prior offline simulations does improve predictability for temperature, particularly maximum temperature, but with less obvious improvements for precipitation and minimum temperature over southeastern Australia
Extreme value statistics from the Real Space Renormalization Group: Brownian Motion, Bessel Processes and Continuous Time Random Walks
We use the Real Space Renormalization Group (RSRG) method to study extreme
value statistics for a variety of Brownian motions, free or constrained such as
the Brownian bridge, excursion, meander and reflected bridge, recovering some
standard results, and extending others. We apply the same method to compute the
distribution of extrema of Bessel processes. We briefly show how the continuous
time random walk (CTRW) corresponds to a non standard fixed point of the RSRG
transformation.Comment: 24 pages, 5 figure
Levy-Student Distributions for Halos in Accelerator Beams
We describe the transverse beam distribution in particle accelerators within
the controlled, stochastic dynamical scheme of the Stochastic Mechanics (SM)
which produces time reversal invariant diffusion processes. This leads to a
linearized theory summarized in a Shchr\"odinger--like (\Sl) equation. The
space charge effects have been introduced in a recent paper~\cite{prstab} by
coupling this \Sl equation with the Maxwell equations. We analyze the space
charge effects to understand how the dynamics produces the actual beam
distributions, and in particular we show how the stationary, self--consistent
solutions are related to the (external, and space--charge) potentials both when
we suppose that the external field is harmonic (\emph{constant focusing}), and
when we \emph{a priori} prescribe the shape of the stationary solution. We then
proceed to discuss a few new ideas~\cite{epac04} by introducing the generalized
Student distributions, namely non--Gaussian, L\'evy \emph{infinitely divisible}
(but not \emph{stable}) distributions. We will discuss this idea from two
different standpoints: (a) first by supposing that the stationary distribution
of our (Wiener powered) SM model is a Student distribution; (b) by supposing
that our model is based on a (non--Gaussian) L\'evy process whose increments
are Student distributed. We show that in the case (a) the longer tails of the
power decay of the Student laws, and in the case (b) the discontinuities of the
L\'evy--Student process can well account for the rare escape of particles from
the beam core, and hence for the formation of a halo in intense beams.Comment: revtex4, 18 pages, 12 figure
Velocity profile of granular flows inside silos and hoppers
We measure the flow of granular materials inside a quasi-two dimensional silo
as it drains and compare the data with some existing models. The particles
inside the silo are imaged and tracked with unprecedented resolution in both
space and time to obtain their velocity and diffusion properties. The data
obtained by varying the orifice width and the hopper angle allows us to
thoroughly test models of gravity driven flows inside these geometries. All of
our measured velocity profiles are smooth and free of the shock-like
discontinuities ("rupture zones") predicted by critical state soil mechanics.
On the other hand, we find that the simple Kinematic Model accurately captures
the mean velocity profile near the orifice, although it fails to describe the
rapid transition to plug flow far away from the orifice. The measured diffusion
length , the only free parameter in the model, is not constant as usually
assumed, but increases with both the height above the orifice and the angle of
the hopper. We discuss improvements to the model to account for the
differences. From our data, we also directly measure the diffusion of the
particles and find it to be significantly less than predicted by the Void
Model, which provides the classical microscopic derivation of the Kinematic
Model in terms of diffusing voids in the packing. However, the experimental
data is consistent with the recently proposed Spot Model, based on a simple
mechanism for cooperative diffusion. Finally, we discuss the flow rate as a
function of the orifice width and hopper angles. We find that the flow rate
scales with the orifice size to the power of 1.5, consistent with dimensional
analysis. Interestingly, the flow rate increases when the funnel angle is
increased.Comment: 17 pages, 8 figure
Challenges to Domesticating Native Forage Legumes
If ruminant production from cultivated and natural grasslands is to depend less on petroleum-based products, forage legumes must serve as protein sources. Commercially available legumes for warm-dry climate grasslands are, however, very limited and resources available for developing such legumes are inadequate. Indeterminate flowering and dehiscent seed pods combined with the need for specialized seed harvesting equipment are major impediments (Butler and Muir 2012). Warm climates often present environmental challenges such as poor rainfall distribution, extended dry seasons, temperature extremes and aggressive grass species (Muir et al. 2011). Erosion of indigenous knowledge and replacement with inappropriate land management approaches from moist-temperate regions compound the challenges
Lattice permutations and Poisson-Dirichlet distribution of cycle lengths
We study random spatial permutations on Z^3 where each jump x -> \pi(x) is
penalized by a factor exp(-T ||x-\pi(x)||^2). The system is known to exhibit a
phase transition for low enough T where macroscopic cycles appear. We observe
that the lengths of such cycles are distributed according to Poisson-Dirichlet.
This can be explained heuristically using a stochastic
coagulation-fragmentation process for long cycles, which is supported by
numerical data.Comment: 18 pages, 14 figure
Using the Conservation Planning Tool to Effectively Recover Northern Bobwhites: An Example for States to Effectively Step-Down the NBCI Plan
The National Bobwhite Conservation Initiative (NBCI) 2.0 provides a sound foundation for recovering northern bobwhites (Colinus virginianus) range-wide, regionally and, to some extent, even locally. However, the NBCI does not provide detailed guidance to states on how to step-down the plan for efficacious delivery of on-the-ground management actions prescribed via biologists within the plan itself. States often must incorporate multiple planning efforts (e.g., state wildlife action plans) and geospatial layers not directly included in the NBCI plan (see NBCI Appendix in these Proceedings) to make tenable decisions which best guide allocation of resources and benefit multiple species of greatest conservation concern. The Conservation Planning Tool (CPT), developed as part of NBCI 2.0, provides the infrastructure for states and conservation organizations to capture biologist information coalesced in the plan while incorporating other data (e.g., species emphasis areas, current CRP implementation, etc.) germane to conservation planning. We use 3 states (Kansas, Florida, and Virginia) to demonstrate the utility of the CPT and to develop a step-down implementation plan, via creation of a habitat prioritization model, for recovery of bobwhites in each state. We explore the implications associated with creation of focal areas with respect to high versus medium ranked areas and underscore the importance of inclusion of major land-use opportunities and constraints prescribed within the plan to garner successful bobwhite recovery. We propose a framework for the integration of monitoring efforts into the step-down model to assess bird response and evaluate NBCI success through estimating bobwhite population density
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