2,399 research outputs found
Decomposition Based Search - A theoretical and experimental evaluation
In this paper we present and evaluate a search strategy called Decomposition
Based Search (DBS) which is based on two steps: subproblem generation and
subproblem solution. The generation of subproblems is done through value
ranking and domain splitting. Subdomains are explored so as to generate,
according to the heuristic chosen, promising subproblems first.
We show that two well known search strategies, Limited Discrepancy Search
(LDS) and Iterative Broadening (IB), can be seen as special cases of DBS. First
we present a tuning of DBS that visits the same search nodes as IB, but avoids
restarts. Then we compare both theoretically and computationally DBS and LDS
using the same heuristic. We prove that DBS has a higher probability of being
successful than LDS on a comparable number of nodes, under realistic
assumptions. Experiments on a constraint satisfaction problem and an
optimization problem show that DBS is indeed very effective if compared to LDS.Comment: 16 pages, 8 figures. LIA Technical Report LIA00203, University of
Bologna, 200
Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field
Numerical studies of the effect of a dc magnetic field on dynamo action
(development of magnetic fields with large spatial scales), due to
helically-driven magnetohydrodynamic turbulence, are reported. The apparent
effect of the dc magnetic field is to suppress the dynamo action, above a
relatively low threshold. However, the possibility that the suppression results
from an improper combination of rectangular triply spatially-periodic boundary
conditions and a uniform dc magnetic field is addressed: heretofore a common
and convenient computational convention in turbulence investigations. Physical
reasons for the observed suppression are suggested. Other geometries and
boundary conditions are offered for which the dynamo action is expected not to
be suppressed by the presence of a dc magnetic field component.Comment: To appear in Physics of Plasma
Federalism's Compromise: Inequity in Education from ESEA to ESSA
Thesis advisor: Michael HartneyThis thesis analyzes the sporadic and incoherent federal approaches to education policy over the last half century to reveal a theory of policymaking constraints that elucidates the tensions between political will for equity and implementation capacity that are intrinsic to federal education policy. I then apply this theory to the Every Student Succeeds Act (ESSA). I find that consistent with my theory of policymaking constraints, a majority of states responded to the increased flexibility in ESSA by retreating from equity concerns. I then explore possible predictive factors for cross-state variation, presenting evidence that wealthy citizen satisfaction with local schools was associated with a decrease in attention to equity in state ESSA plans and that states that evaded accountability measures under No Child Left Behind continued to do so under the Every Student Succeeds Act. These results contribute to an important debate about the retreat from equity commitments under ESSA and furthermore, the theoretical framework that explains the policy zigzag in education policy since 1965.Thesis (BA) — Boston College, 2018.Submitted to: Boston College. College of Arts and Sciences.Discipline: Departmental Honors.Discipline: Political Science
Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings
We report the discovery of several large "propeller" moons in the outer part
of Saturn's A ring, objects large enough to be followed over the 5-year
duration of the Cassini mission. These are the first objects ever discovered
that can be tracked as individual moons, but do not orbit in empty space. We
infer sizes up to 1--2 km for the unseen moonlets at the center of the
propeller-shaped structures, though many structural and photometric properties
of propeller structures remain unclear. Finally, we demonstrate that some
propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv
version of the paper contains supplementary tables that were left out of the
ApJL version due to lack of space).Comment: 9 pages, 4 figures; Published in ApJ
Rhodium catalyzed hydrogenation reactions in aqueous micellar systems as green solvents
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The hydrogenation of itaconic acid and dimethyl itaconate is transferred from methanol to aqueous micellar solutions of several surfactants, e.g., SDS and Triton X-100, in order to facilitate the recovery of the catalyst. The reaction rate and selectivity strongly depends on the chosen surfactant and in some cases also on the surfactant concentration. In the best case the selectivity is the same as in methanol but the reaction rate is still lower because of a lower hydrogen solubility in water. Repetitive semi-batch experiments are chosen to demonstrate that high turn-over-numbers (>1000) can be reached in aqueous micellar solutions. No notable catalyst deactivation is observed in these experiments. The performance of micellar reaction systems is controlled by the partition coefficient of the substrates between the micelles and the continuous aqueous phase which can be predicted using the Conductor-like Screening Model for Real Solvents (COSMO-RS).DFG, EXC 314, Unifying Concepts in Catalysi
Energy spectrum of turbulent fluctuations in boundary driven reduced magnetohydrodynamics
The nonlinear dynamics of a bundle of magnetic flux ropes driven by
stationary fluid motions at their endpoints is studied, by performing numerical
simulations of the magnetohydrodynamic (MHD) equations. The development of MHD
turbulence is shown, where the system reaches a state that is characterized by
the ratio between the Alfven time (the time for incompressible MHD waves to
travel along the field lines) and the convective time scale of the driving
motions. This ratio of time scales determines the energy spectra and the
relaxation toward different regimes ranging from weak to strong turbulence. A
connection is made with phenomenological theories for the energy spectra in MHD
turbulence.Comment: Published in Physics of Plasma
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