9,021 research outputs found
Universal persistence exponents in an extremally driven system
The local persistence R(t), defined as the proportion of the system still in
its initial state at time t, is measured for the Bak--Sneppen model. For 1 and
2 dimensions, it is found that the decay of R(t) depends on one of two classes
of initial configuration. For a subcritical initial state, R(t)\sim
t^{-\theta}, where the persistence exponent \theta can be expressed in terms of
a known universal exponent. Hence \theta is universal. Conversely, starting
from a supercritical state, R(t) decays by the anomalous form 1-R(t)\sim
t^{\tau_{\rm ALL}} until a finite time t_{0}, where \tau_{\rm ALL} is also a
known exponent. Finally, for the high dimensional model R(t) decays
exponentially with a non--universal decay constant.Comment: 4 pages, 6 figures. To appear in Phys. Rev.
The Anisotropic Bak-Sneppen model
The Bak-Sneppen model is shown to fall into a different universality class with the introduction of a preferred direction, mirroring the situation in spin systems. This is first demonstrated by numerical simulations and subsequently confirmed by analysis of the multitrait version of the model, which admits exact solutions in the extremes of zero and maximal anisotropy. For intermediate anisotropies, we show that the spatiotemporal evolution of the avalanche has a power law `tail' which passes through the system for any non-zero anisotropy but remains fixed for the isotropic case, thus explaining the crossover in behaviour. Finally, we identify the maximally anisotropic model which is more tractable and yet more generally applicable than the isotropic system
Extremal driving as a mechanism for generating long-term memory
It is argued that systems whose elements are renewed according to an extremal
criterion can generally be expected to exhibit long-term memory. This is
verified for the minimal extremally driven model, which is first defined and
then solved for all system sizes N\geq2 and times t\geq0, yielding exact
expressions for the persistence R(t)=[1+t/(N-1)]^{-1} and the two-time
correlation function C(t_{\rm w}+t,t_{\rm w})=(1-1/N)(N+t_{\rm w})/(N+t_{\rm
w}+t-1). The existence of long-term memory is inferred from the scaling of
C(t_{\rm w}+t,t_{\rm w})\sim f(t/t_{\rm w}), denoting {\em aging}. Finally, we
suggest ways of investigating the robustness of this mechanism when competing
processes are present.Comment: 5 pages, no figures; requires IOP style files. To appear as a J.
Phys. A. lette
Choral reading of poetry
Thesis (Ed.M.)--Boston UniversityThe purpose of this study is twofold:
(1) to review professional literature concerning methods and materials
for the teaching of poetry, particularly those used in choral reading;
and (2) to compile a workbook with appropriate materials for use in the
junior high school
Chemical differentiation on one-plate planets: Predictions and geologic observations for Venus
Recent studies have examined the partial melting of planetary interiors on one-plate planets and the implications for the formation and evolution of basaltic crust and the complementary residual mantle layer. In contrast to the Earth, where the crust and residual layer move laterally and are returned to the interior following subduction, one-plate planets such as Venus are characterized by vertical accretion of the crust and residual layer. The residual mantle layer is depleted and compositionally buoyant, being less dense than undepleted mantle due to its reduced Fe/Mg and dense Al-bearing minerals; its melting temperature is also increased. As the crust and depleted mantle layer grow vertically during the thermal evolution of the planet, several stages develop. As a step in the investigation and testing of these theoretical treatments of crustal development on Venus, we investigate the predictions deriving from two of these stages (a stable thick crust and depleted layer, and a thick unstable depleted layer) and compare these to geologic and geophysical observations, speculating on how these might be interpreted in the context of the vertical crustal accretion models. In each case, we conclude with an outline of further tests and observations of these models
Volume-controlled buckling of thin elastic shells: Application to crusts formed on evaporating partially-wetted droplets
Motivated by the buckling of glassy crusts formed on evaporating droplets of
polymer and colloid solutions, we numerically model the deformation and
buckling of spherical elastic caps controlled by varying the volume between the
shell and the substrate. This volume constraint mimics the incompressibility of
the unevaporated solvent. Discontinuous buckling is found to occur for
sufficiently thin and/or large contact angle shells, and robustly takes the
form of a single circular region near the boundary that `snaps' to an inverted
shape, in contrast to externally pressurised shells. Scaling theory for shallow
shells is shown to well approximate the critical buckling volume, the
subsequent enlargement of the inverted region and the contact line force.Comment: 7 pages in J. Phys. Cond. Mat. spec; 4 figs (2 low-quality to reach
LANL's over-restrictive size limits; ask for high-detailed versions if
required
Energy Distribution in disordered elastic Networks
Disordered networks are found in many natural and artificial materials, from gels or cytoskeletal structures to metallic foams or bones. Here, the energy distribution in this type of networks is modeled, taking into account the orientation of the struts. A correlation between the orientation and the energy per unit volume is found and described as a function of the connectivity in the network and the relative bending stiffness of the struts. If one or both parameters have relatively large values, the struts aligned in the loading direction present the highest values of energy. On the contrary, if these have relatively small values, the highest values of energy can be reached in the struts oriented transversally. This result allows explaining in a simple way remodeling processes in biological materials, for example, the remodeling of trabecular bone and the reorganization in the cytoskeleton. Additionally, the correlation between the orientation, the affinity, and the bending-stretching ratio in the network is discussed
Evaluation of Mobility Modes on Lunar Exploration Traverses - Marius Hills, Copernicus Peaks, and Hadley Apennines Missions
Energy and time costs of lunar walking or riding traverses, and scientific tasks on J-type missions, and capabilities of A7L suits and life support system
Parallel algorithms for solving linear equations using Givens transformations
AbstractThe use of the Givens method to solve linear equations on a parallel computer is reviewed, and a new algorithm which requires fewer time steps in the infinite processor case is presented
Mary Kenneth Keller: First US PhD in Computer Science
The first two doctoral-level degrees in Computer Science in the US were
awarded in June 1965. This paper discusses one of the degree recipients, Sister
Mary Kenneth Keller, BVM
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