18,570 research outputs found
Apollo spacecraft systems analysis program. A comparison of the computed LR received signal spectrum and the 1967 PEARL data
Similarities between experimentally measured power spectral density curves of landing radar signal returned from rough surface and computer simulated curves for sinusoidal surface
Short-Term Dynamical Interactions Among Extrasolar Planets
We show that short-term perturbations among massive planets in multiple
planet systems can result in radial velocity variations of the central star
which differ substantially from velocity variations derived assuming the
planets are executing independent Keplerian motions. We discuss two alternate
fitting methods which can lead to an improved dynamical description of multiple
planet systems. In the first method, the osculating orbital elements are
determined via a Levenberg-Marquardt minimization scheme driving an N-body
integrator. The second method is an improved analytic model in which orbital
elements are allowed to vary according to a simple model for resonant
interactions between the planets. Both of these methods can determine the true
masses for the planets by eliminating the sin(i) degeneracy inherent in fits
that assume independent Keplerian motions. We apply our fitting methods to the
GJ876 radial velocity data (Marcy et al. 2001), and argue that the mass factors
for the two planets are likely in the 1.25-2.0 rangeComment: 13 pages, including 4 figures and 3 tables Accepted by Astrophyiscal
Journal Letter
Economic Growth and Threatened and Endangered Species Listings: A VAR Analysis
We conduct several analyses to examine the link between threatened and endangered species listings and macroeconomic activity. Preliminary tests using ordinary least squares are run on both time series data on the national level and cross sectional data at the state level. The analysis is then extended using vector autoregressive (VAR) techniques. VAR results, impulse response functions and variance decompositions are reported to shed more light on the causal relationships between threatened and endangered species, GDP and population. Our results indicate that there is little or no empirical evidence that GDP growth rates lead to changes in the number of threatened and endangered species listings.
Economic Growth and Threatened and Endangered Species Listings: A VAR Analysis
We conduct several analyses to examine the link between threatened and endangered species listings and macroeconomic activity. Preliminary tests using ordinary least squares are run on both time series data on the national level and cross sectional data at the state level. The analysis is then extended using vector autoregressive (VAR) techniques. VAR results, impulse response functions and variance decompositions are reported to shed more light on the causal relationships between threatened and endangered species, GDP and population. Our results indicate that there is little or no empirical evidence that GDP growth rates lead to changes in the number of threatened and endangered species listings. Key Words: Economic growth, endangered and threatened species, vector autoregression
Pseudo-High-Order Symplectic Integrators
Symplectic N-body integrators are widely used to study problems in celestial
mechanics. The most popular algorithms are of 2nd and 4th order, requiring 2
and 6 substeps per timestep, respectively. The number of substeps increases
rapidly with order in timestep, rendering higher-order methods impractical.
However, symplectic integrators are often applied to systems in which
perturbations between bodies are a small factor of the force due to a dominant
central mass. In this case, it is possible to create optimized symplectic
algorithms that require fewer substeps per timestep. This is achieved by only
considering error terms of order epsilon, and neglecting those of order
epsilon^2, epsilon^3 etc. Here we devise symplectic algorithms with 4 and 6
substeps per step which effectively behave as 4th and 6th-order integrators
when epsilon is small. These algorithms are more efficient than the usual 2nd
and 4th-order methods when applied to planetary systems.Comment: 14 pages, 5 figures. Accepted for publication in the Astronomical
Journa
A follow-up study of the social adjustment of referred children after group termination
Thesis (M.S.)--Boston Universit
A follow-up study of the social adjustment of referred children after group termination
Thesis (M.S.)--Boston Universit
Stochastic Dominance, Entropy and Biodiversity Management
In this paper we develop a model of population dynamics using the Shannon entropy index, a measure of diversity that allows for global and specific population shocks. We model the effects of increasing the number of parcels on biodiversity, varying the number of spatially diverse parcels to capture risk diversification. We discuss the concepts of stochastic dominance as a means of project selection, in order to model biodiversity returns and risks. Using a Monte Carlo simulation we find that stochastic dominance may be a useful theoretical construct for project selections but it is unable to rank every case.
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