400 research outputs found
Reduction of Soil-Borne Plant Pathogens Using Lime and Ammonia Evolved from Broiler Litter
In laboratory and micro-plots simulations and in a commercial greenhouse, soil ammonia (NH3) and pH were manipulated as means to control soil-borne fungal pathogens and nematodes. Soil ammonification capacity was increased by applying low C/N ratio broiler litter at 1â8% (w/w). Soil pH was increased using lime at 0.5â1% (w/w). This reduced fungi (Fusarium oxysporum f. sp. dianthi and Sclerotium rolfsii) and root-knot nematode (Meloidogyne javanica) in lab tests below detection. In a commercial greenhouse, broiler litter (25 Mg haâ1) and lime (12.5 Mg haâ1) addition to soil in combination with solarization significantly reduced M. javanica induced root galling of tomato test plants from 47% in the control plots (solarization only) to 7% in treated plots. Root galling index of pepper plants, measured 178 days after planting in the treated and control plots, were 0.8 and 1.5, respectively, which was statistically significantly different. However, the numbers of nematode juveniles in the root zone soil counted 83 and 127 days after pepper planting were not significantly different between treatments. Pepper fruit yield was not different between treatments. Soil disinfection and curing was completed within one month, and by the time of bell-pepper planting the pH and ammonia values were normal
Lithium-6 from Solar Flares
By introducing a hitherto ignored Li-6 producing process, due to accelerated
He-3 reactions with He-4, we show that accelerated particle interactions in
solar flares produce much more Li-6 than Li-7. By normalizing our calculations
to gamma-ray data we demonstrate that the Li-6 produced in solar flares,
combined with photospheric Li-7, can account for the recently determined solar
wind lithium isotopic ratio, obtained from measurements in lunar soil, provided
that the bulk of the flare produced lithium is evacuated by the solar wind.
Further research in this area could provide unique information on a variety of
problems, including solar atmospheric transport and mixing, solar convection
and the lithium depletion issue, and solar wind and solar particle
acceleration.Comment: latex 9 pages, 2 figures, ApJ Letters in pres
Adaptive Importance Sampling Simulation of Queueing Networks
In this paper, a method is presented for the efficient estimation of rare-event (overflow) probabilities in Jackson queueing networks using importance sampling. The method differs in two ways from methods discussed in most earlier literature: the change of measure is state-dependent, i.e., it is a function of the content of the buffers, and the change of measure is determined using a cross-entropy-based adaptive procedure. This method yields asymptotically efficient estimation of overflow probabilities of queueing models for which it has been shown that methods using a stateindependent change of measure are not asymptotically efficient. Numerical results demonstrating the effectiveness of the method are presented as well
Width of percolation transition in complex networks
It is known that the critical probability for the percolation transition is
not a sharp threshold, actually it is a region of non-zero width
for systems of finite size. Here we present evidence that for complex networks
, where is the average
length of the percolation cluster, and is the number of nodes in the
network. For Erd\H{o}s-R\'enyi (ER) graphs , while for
scale-free (SF) networks with a degree distribution
and , . We show analytically
and numerically that the \textit{survivability} , which is the
probability of a cluster to survive chemical shells at probability ,
behaves near criticality as . Thus
for probabilities inside the region the behavior of the
system is indistinguishable from that of the critical point
Optimal Paths in Complex Networks with Correlated Weights: The World-wide Airport Network
We study complex networks with weights, , associated with each link
connecting node and . The weights are chosen to be correlated with the
network topology in the form found in two real world examples, (a) the
world-wide airport network, and (b) the {\it E. Coli} metabolic network. Here
, where and are the degrees of
nodes and , is a random number and represents the
strength of the correlations. The case represents correlation
between weights and degree, while represents anti-correlation and
the case reduces to the case of no correlations. We study the
scaling of the lengths of the optimal paths, , with the system
size in strong disorder for scale-free networks for different . We
calculate the robustness of correlated scale-free networks with different
, and find the networks with to be the most robust
networks when compared to the other values of . We propose an
analytical method to study percolation phenomena on networks with this kind of
correlation. We compare our simulation results with the real world-wide airport
network, and we find good agreement
Numerical evaluation of the upper critical dimension of percolation in scale-free networks
We propose a numerical method to evaluate the upper critical dimension
of random percolation clusters in Erd\H{o}s-R\'{e}nyi networks and in
scale-free networks with degree distribution ,
where is the degree of a node and is the broadness of the degree
distribution. Our results report the theoretical prediction, for scale-free networks with and
for Erd\H{o}s-R\'{e}nyi networks and scale-free networks with .
When the removal of nodes is not random but targeted on removing the highest
degree nodes we obtain for all . Our method also yields
a better numerical evaluation of the critical percolation threshold, , for
scale-free networks. Our results suggest that the finite size effects increases
when approaches 3 from above.Comment: 10 pages, 5 figure
Limited path percolation in complex networks
We study the stability of network communication after removal of
links under the assumption that communication is effective only if the shortest
path between nodes and after removal is shorter than where is the shortest path before removal. For a large
class of networks, we find a new percolation transition at
, where and
is the node degree. Below , only a fraction of
the network nodes can communicate, where , while above , order nodes can
communicate within the limited path length . Our analytical results
are supported by simulations on Erd\H{o}s-R\'{e}nyi and scale-free network
models. We expect our results to influence the design of networks, routing
algorithms, and immunization strategies, where short paths are most relevant.Comment: 11 pages, 3 figures, 1 tabl
Percolation theory applied to measures of fragmentation in social networks
We apply percolation theory to a recently proposed measure of fragmentation
for social networks. The measure is defined as the ratio between the
number of pairs of nodes that are not connected in the fragmented network after
removing a fraction of nodes and the total number of pairs in the original
fully connected network. We compare with the traditional measure used in
percolation theory, , the fraction of nodes in the largest cluster
relative to the total number of nodes. Using both analytical and numerical
methods from percolation, we study Erd\H{o}s-R\'{e}nyi (ER) and scale-free (SF)
networks under various types of node removal strategies. The removal strategies
are: random removal, high degree removal and high betweenness centrality
removal. We find that for a network obtained after removal (all strategies) of
a fraction of nodes above percolation threshold, . For fixed and close to percolation threshold
(), we show that better reflects the actual fragmentation. Close
to , for a given , has a broad distribution and it is
thus possible to improve the fragmentation of the network. We also study and
compare the fragmentation measure and the percolation measure
for a real social network of workplaces linked by the households of the
employees and find similar results.Comment: submitted to PR
The Energy of a Plasma in the Classical Limit
When \lambda_{T} << d_{T}, where \lambda_{T} is the de Broglie wavelength and
d_{T}, the distance of closest approach of thermal electrons, a classical
analysis of the energy of a plasma can be made. In all the classical analysis
made until now, it was assumed that the frequency of the fluctuations \omega <<
T (k_{B}=\hbar=1). Using the fluctuation-dissipation theorem, we evaluate the
energy of a plasma, allowing the frequency of the fluctuations to be arbitrary.
We find that the energy density is appreciably larger than previously thought
for many interesting plasmas, such as the plasma of the Universe before the
recombination era.Comment: 10 pages, 2 figures, accepted for publication in Phys.Rev.Let
Counterintuitive transitions in multistate curve crossing involving linear potentials
Two problems incorporating a set of horizontal linear potentials crossed by a
sloped linear potential are analytically solved and compared with numerical
results: (a) the case where boundary conditions are specified at the ends of a
finite interval, and (b) the case where the sloped linear potential is replaced
by a piecewise-linear sloped potential and the boundary conditions are
specified at infinity. In the approximation of small gaps between the
horizontal potentials, an approach similar to the one used for the degenerate
problem (Yurovsky V A and Ben-Reuven A 1998 J. Phys. B 31,1) is applicable for
both problems. The resulting scattering matrix has a form different from the
semiclassical result obtained by taking the product of Landau-Zener amplitudes.
Counterintuitive transitions involving a pair of successive crossings, in which
the second crossing precedes the first one along the direction of motion, are
allowed in both models considered here.Comment: LaTeX 2.09 using ioplppt.sty and psfig.sty, 16 pages with 5 figures.
Submitted to J. Phys.
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