18,354 research outputs found
Pricing for Online Resource Allocation: Intervals and Paths
We present pricing mechanisms for several online resource allocation problems
which obtain tight or nearly tight approximations to social welfare. In our
settings, buyers arrive online and purchase bundles of items; buyers' values
for the bundles are drawn from known distributions. This problem is closely
related to the so-called prophet-inequality of Krengel and Sucheston and its
extensions in recent literature. Motivated by applications to cloud economics,
we consider two kinds of buyer preferences. In the first, items correspond to
different units of time at which a resource is available; the items are
arranged in a total order and buyers desire intervals of items. The second
corresponds to bandwidth allocation over a tree network; the items are edges in
the network and buyers desire paths.
Because buyers' preferences have complementarities in the settings we
consider, recent constant-factor approximations via item prices do not apply,
and indeed strong negative results are known. We develop static, anonymous
bundle pricing mechanisms.
For the interval preferences setting, we show that static, anonymous bundle
pricings achieve a sublogarithmic competitive ratio, which is optimal (within
constant factors) over the class of all online allocation algorithms, truthful
or not. For the path preferences setting, we obtain a nearly-tight logarithmic
competitive ratio. Both of these results exhibit an exponential improvement
over item pricings for these settings. Our results extend to settings where the
seller has multiple copies of each item, with the competitive ratio decreasing
linearly with supply. Such a gradual tradeoff between supply and the
competitive ratio for welfare was previously known only for the single item
prophet inequality
Mobility: a double-edged sword for HSPA networks
This paper presents an empirical study on the performance of mobile High Speed Packet Access (HSPA, a 3.5G cellular standard) networks in Hong Kong via extensive field tests. Our study, from the viewpoint of end users, covers virtually all possible mobile scenarios in urban areas, including subways, trains, off-shore ferries and city buses. We have confirmed that mobility has largely negative impacts on the performance of HSPA networks, as fast-changing wireless environment causes serious service deterioration or even interruption. Meanwhile our field experiment results have shown unexpected new findings and thereby exposed new features of the mobile HSPA networks, which contradict commonly held views. We surprisingly find out that mobility can improve fairness of bandwidth sharing among users and traffic flows. Also the triggering and final results of handoffs in mobile HSPA networks are unpredictable and often inappropriate, thus calling for fast reacting fallover mechanisms. We have conducted in-depth research to furnish detailed analysis and explanations to what we have observed. We conclude that mobility is a double-edged sword for HSPA networks. To the best of our knowledge, this is the first public report on a large scale empirical study on the performance of commercial mobile HSPA networks
Cultural Competency in Capacity Building
Discusses different capacity building approaches to improving cultural competency that are informed by community participation and multicultural organizational development
A Supernova Factory in the Merger System Arp 299
We have imaged the nearby galaxy merger Arp 299 at arcsecond and
milliarcsecond resolution, using both the Very Large Array and the Very Long
Baseline Array. The large-scale radio emission from the merger contains 5
bright, compact radio sources embedded in diffuse emission, with diameters less
than 200 pc. Supernova rates of 0.1 to 1 per year are required to produce the
VLA-detected radio emission in these sources. Two of the compact VLA radio
sources, designated Source A and Source D, also have been detected and imaged
at milliarcsecond scales. Source A, which is associated with the nucleus of one
of the merging galaxies, contains five milliarcsecond-scale sources, each with
a radio power between 100 and 1000 times that of the Galactic supernova remnant
Cassiopeia A. Four of these have flat or inverted spectra and appear to be
young supernovae. Three of the VLBI-scale sources are located within 10 pc
(projected) of one another, and two are separated by less than 3 pc, indicating
that they all may be within the same super starcluster or complex of such
clusters. The brightest VLBI-scale source, A0, has an extremely inverted
pectrum, with alpha larger than +2 at gigahertz frequencies. It seems to be the
youngest supernova, which has not yet broken out of its circumstellar shell.
The milliarcsecond radio sources within Source A appear to constitute a
upernova factory, confirming the presence of an extreme starburst that peaked
at least a few million years ago.Comment: Accepted for the Astrophysical Journal, 22 pages, 10 figure
Stationary Cycling Induced by Switched Functional Electrical Stimulation Control
Functional electrical stimulation (FES) is used to activate the dysfunctional
lower limb muscles of individuals with neuromuscular disorders to produce
cycling as a means of exercise and rehabilitation. However, FES-cycling is
still metabolically inefficient and yields low power output at the cycle crank
compared to able-bodied cycling. Previous literature suggests that these
problems are symptomatic of poor muscle control and non-physiological muscle
fiber recruitment. The latter is a known problem with FES in general, and the
former motivates investigation of better control methods for FES-cycling.In
this paper, a stimulation pattern for quadriceps femoris-only FES-cycling is
derived based on the effectiveness of knee joint torque in producing forward
pedaling. In addition, a switched sliding-mode controller is designed for the
uncertain, nonlinear cycle-rider system with autonomous state-dependent
switching. The switched controller yields ultimately bounded tracking of a
desired trajectory in the presence of an unknown, time-varying, bounded
disturbance, provided a reverse dwell-time condition is satisfied by
appropriate choice of the control gains and a sufficient desired cadence.
Stability is derived through Lyapunov methods for switched systems, and
experimental results demonstrate the performance of the switched control system
under typical cycling conditions.Comment: 8 pages, 3 figures, submitted to ACC 201
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