1,672 research outputs found
Distributed Exact Shortest Paths in Sublinear Time
The distributed single-source shortest paths problem is one of the most
fundamental and central problems in the message-passing distributed computing.
Classical Bellman-Ford algorithm solves it in time, where is the
number of vertices in the input graph . Peleg and Rubinovich (FOCS'99)
showed a lower bound of for this problem, where
is the hop-diameter of .
Whether or not this problem can be solved in time when is
relatively small is a major notorious open question. Despite intensive research
\cite{LP13,N14,HKN15,EN16,BKKL16} that yielded near-optimal algorithms for the
approximate variant of this problem, no progress was reported for the original
problem.
In this paper we answer this question in the affirmative. We devise an
algorithm that requires time, for , and time, for larger . This
running time is sublinear in in almost the entire range of parameters,
specifically, for . For the all-pairs shortest paths
problem, our algorithm requires time, regardless of
the value of .
We also devise the first algorithm with non-trivial complexity guarantees for
computing exact shortest paths in the multipass semi-streaming model of
computation.
From the technical viewpoint, our algorithm computes a hopset of a
skeleton graph of without first computing itself. We then conduct
a Bellman-Ford exploration in , while computing the required edges
of on the fly. As a result, our algorithm computes exactly those edges of
that it really needs, rather than computing approximately the entire
Radiation-induced insulator discharge pulses in the CRRES internal discharge monitor satellite experiment
The Internal Discharge Monitor (IDM) was designed to observe electrical pulses from common electrical insulators in space service. The sixteen insulator samples included twelve planar printed circuit boards and four cables. The samples were fully enclosed, mutually isolated, and space radiation penetrated 0.02 cm of aluminum before striking the samples. Pulsing began on the seventh orbit, the maximum pulse rate occurred on the seventeenth orbit when 13 pulses occurred, and the pulses slowly diminished to about one per 3 orbits six months later. After 8 months, the radiation belts abruptly increased and the pulse rates attained a new high. These pulse rates were in agreement with laboratory experience on shorter time scales. Several of the samples never pulsed. If the pulses were not confined within IDM, the physical processes could spread to become a full spacecraft anomaly. The IDM results indicate the rate at which small insulator pulses occur. Small pulses are the seeds of larger satellite electrical anomalies. The pulse rates are compared with space radiation intensities, L shell location, and spectral distributions from the radiation spectrometers on the Combined Release and Radiation Effects Satellite
Bulletin No. 288 - Draingage and Irrigation, Soil, Economic, and Social Conditions, Delta Area, Utah Division 4: Social Conditions
This study is a part of a more comprehensive one which was organized in 1928 for the purpose of ascertaining what conditions existed in bonded irrigation and drainage districts which were unable to. liquidate obligations incurred.
The first such area to be studied was the Delta Area in Millard County. This area was selected for this study because of the pressing need for more detailed and wider information than was available to either the farmers or the bondholders and because available facts based on careful study might aid in achieving fairer settlements. These data might also assist other areas in eliminating wastes which multiply in hastily planned undertakings
Sorghum ergot - a sticky disease problem in southern Africa
Ergot disease of sorghum is not new to Africa - the causal
pathogen, Claviceps africana Frederickson, Mantle, and de Milliano was first recorded in Kenya as far back as 1923. However, ergot disease only began to gain recognition as a potential problem in sorghum production in the 1960s, when all the A-lines in Nigeria's national breeding program became infected, to the near complete exclusion of seed production (Futrell and Webster 1965)
Potential Use of Benomyl for Control of Ergot (Claviceps africana) in Sorghum A-lines in Zimbabwe
In the absence of a successful practice for the control of ergot (caused by C. africana) on sorghum A-lines (male-steriles) in Zimbabwe, 2 fungicides, benomyl as Benlate and thiram, were tested in greenhouse and field experiments conducted at the Matopos and Henderson Research Stations, Zimbabwe in 1993. Fungicides were either applied to sorghum panicles singly, at concentrations of 0.1 or 0.2% a.i., or combined in mixtures at 0.1% a.i. each. Fungicides were applied before inoculation at heading or stigma exsertion or after disease became visible. Treatment with either benomyl or thiram was ineffective if applied when disease first became visible. A significant reduction in initial disease severity, rate of disease increase and final disease severity was achieved with 1 application of benomyl at 0.2% a.i. at heading or stigma exsertion. At the concentrations tested benomyl and thiram did not reduce seed-set in R- (restorer) lines
Pearl Millet as an Alternate Host of the Sorghum Ergot Pathogen, Claviceps africana
The infectivity of C. africana on pearl millet (Pennisetum glaucum) lines in Africa was studied. C. africana consistently gave 100% disease incidence on male-sterile sorghum in experiments undertaken in Zimbabwe. C. fusiformis established disease with moderate incidence on most but not all lines tested. In contrast, C. africana established a parasitic association with all the pearl millet lines tested, with incidence as high as 23% in ICMSR 260, the genotype that also supported the highest incidence with C. fusiformis. However, the disease severities were always low, between 1 and 5%. All infections on pearl millet were verified as the sphacelial stage by virtue of their conidial characteristics. After 1 passage through a pearl millet host, C. africana did not apparently become more infectious on this host
Matroid and Knapsack Center Problems
In the classic -center problem, we are given a metric graph, and the
objective is to open nodes as centers such that the maximum distance from
any vertex to its closest center is minimized. In this paper, we consider two
important generalizations of -center, the matroid center problem and the
knapsack center problem. Both problems are motivated by recent content
distribution network applications. Our contributions can be summarized as
follows:
1. We consider the matroid center problem in which the centers are required
to form an independent set of a given matroid. We show this problem is NP-hard
even on a line. We present a 3-approximation algorithm for the problem on
general metrics. We also consider the outlier version of the problem where a
given number of vertices can be excluded as the outliers from the solution. We
present a 7-approximation for the outlier version.
2. We consider the (multi-)knapsack center problem in which the centers are
required to satisfy one (or more) knapsack constraint(s). It is known that the
knapsack center problem with a single knapsack constraint admits a
3-approximation. However, when there are at least two knapsack constraints, we
show this problem is not approximable at all. To complement the hardness
result, we present a polynomial time algorithm that gives a 3-approximate
solution such that one knapsack constraint is satisfied and the others may be
violated by at most a factor of . We also obtain a 3-approximation
for the outlier version that may violate the knapsack constraint by
.Comment: A preliminary version of this paper is accepted to IPCO 201
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