19,024 research outputs found
Unimodular Trees versus Einstein Trees
The maximally helicity violating (MHV) tree level scattering amplitudes
involving three, four or five gravitons are worked out in Unimodular Gravity.
They are found to coincide with the corresponding amplitudes in General
Relativity. This a remarkable result, insofar as both the propagators and the
vertices are quite different in both theories.Comment: 20 pages, 5 figure
Main Memory Adaptive Indexing for Multi-core Systems
Adaptive indexing is a concept that considers index creation in databases as
a by-product of query processing; as opposed to traditional full index creation
where the indexing effort is performed up front before answering any queries.
Adaptive indexing has received a considerable amount of attention, and several
algorithms have been proposed over the past few years; including a recent
experimental study comparing a large number of existing methods. Until now,
however, most adaptive indexing algorithms have been designed single-threaded,
yet with multi-core systems already well established, the idea of designing
parallel algorithms for adaptive indexing is very natural. In this regard only
one parallel algorithm for adaptive indexing has recently appeared in the
literature: The parallel version of standard cracking. In this paper we
describe three alternative parallel algorithms for adaptive indexing, including
a second variant of a parallel standard cracking algorithm. Additionally, we
describe a hybrid parallel sorting algorithm, and a NUMA-aware method based on
sorting. We then thoroughly compare all these algorithms experimentally; along
a variant of a recently published parallel version of radix sort. Parallel
sorting algorithms serve as a realistic baseline for multi-threaded adaptive
indexing techniques. In total we experimentally compare seven parallel
algorithms. Additionally, we extensively profile all considered algorithms. The
initial set of experiments considered in this paper indicates that our parallel
algorithms significantly improve over previously known ones. Our results
suggest that, although adaptive indexing algorithms are a good design choice in
single-threaded environments, the rules change considerably in the parallel
case. That is, in future highly-parallel environments, sorting algorithms could
be serious alternatives to adaptive indexing.Comment: 26 pages, 7 figure
A stochastic individual based model for the growth of a stand of Japanese knotweed including mowing as a management technique
Invasive alien species are a growing threat for environment and health. They
also have a major economic impact, as they can damage many infrastructures. The
Japanese knotweed (Fallopia japonica), present in North America, Northern and
Central Europe as well as in Australia and New Zealand, is listed by the World
Conservation Union as one of the world's worst invasive species. So far, most
models have dealt with how the invasion spreads without management. This paper
aims at providing a model able to study and predict the dynamics of a stand of
Japanese knotweed taking into account mowing as a management technique. The
model we propose is stochastic and individual-based, which allows us taking
into account the behaviour of individuals depending on their size and location,
as well as individual stochasticity. We set plant dynamics parameters thanks to
a calibration with field data, and study the influence of the initial
population size, the mean number of mowing events a year and the management
project duration on mean area and mean number of crowns of stands. In
particular, our results provide the sets of parameters for which it is possible
to obtain the stand eradication, and the minimal duration of the management
project necessary to achieve this latter
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