24,485 research outputs found
Recommendation Subgraphs for Web Discovery
Recommendations are central to the utility of many websites including
YouTube, Quora as well as popular e-commerce stores. Such sites typically
contain a set of recommendations on every product page that enables visitors to
easily navigate the website. Choosing an appropriate set of recommendations at
each page is one of the key features of backend engines that have been deployed
at several e-commerce sites.
Specifically at BloomReach, an engine consisting of several independent
components analyzes and optimizes its clients' websites. This paper focuses on
the structure optimizer component which improves the website navigation
experience that enables the discovery of novel content.
We begin by formalizing the concept of recommendations used for discovery. We
formulate this as a natural graph optimization problem which in its simplest
case, reduces to a bipartite matching problem. In practice, solving these
matching problems requires superlinear time and is not scalable. Also,
implementing simple algorithms is critical in practice because they are
significantly easier to maintain in production. This motivated us to analyze
three methods for solving the problem in increasing order of sophistication: a
sampling algorithm, a greedy algorithm and a more involved partitioning based
algorithm.
We first theoretically analyze the performance of these three methods on
random graph models characterizing when each method will yield a solution of
sufficient quality and the parameter ranges when more sophistication is needed.
We complement this by providing an empirical analysis of these algorithms on
simulated and real-world production data. Our results confirm that it is not
always necessary to implement complicated algorithms in the real-world and that
very good practical results can be obtained by using heuristics that are backed
by the confidence of concrete theoretical guarantees
Phonons in potassium doped graphene: the effects of electron-phonon interactions, dimensionality and ad-atom ordering
Graphene phonons are measured as a function of electron doping via the
addition of potassium adatoms. In the low doping regime, the in-plane carbon
G-peak hardens and narrows with increasing doping, analogous to the trend seen
in graphene doped via the field-effect. At high dopings, beyond those
accessible by the field-effect, the G-peak strongly softens and broadens. This
is interpreted as a dynamic, non-adiabatic renormalization of the phonon
self-energy. At dopings between the light and heavily doped regimes, we find a
robust inhomogeneous phase where the potassium coverage is segregated into
regions of high and low density. The phonon energies, linewidths and tunability
are remarkably similar for 1-4 layer graphene, but significantly different to
doped bulk graphite.Comment: Accepted in Phys. Rev. B as a Rapid Communication. 5 pages, 3
figures, revised text with additional dat
The Cretaceous-Tertiary boundary marine extinction and global primary productivity collapse
The extinction of marine phyto-and zoo-plankton across the K-T boundary has been well documented. Such an event may have resulted in decreased photosynthetic fixation of carbon in surface waters and a collapse of the food chain in the marine biosphere. Because the vertical and horizontal distribution of the carbon isotopic composition of total dissolved carton (TDC) in the modern ocean is controlled by the transfer of organic carbon from the surface to deep reservoirs, it follows that a major disruption of the marine biosphere would have had a major effect on the distribution of carbon isotopes in the ocean. Negative carbon isotope excursions have been identified at many marine K-T boundary sequences worldwide and are interpreted as a signal of decreased oceanic primary productivity. However, the magnitude, duration and consequences of this productivity crisis have been poorly constrained. On the basis of planktonic and benthic calcareous microfossil carbon isotope and other geochemical data from DSDP Site 577 located on the Shatsky Rise in the north-central Pacific, as well as other sites, researchers have been able to provide a reasonable estimate of the duration and magnitude of this event
Pinpointing the Position of the Post-AGB Star at the Core of RAFGL 2688 using Polarimetric Imaging with NICMOS
We have used infrared polarimetric imaging with NICMOS to determine precisely
the position of the star that illuminates (and presumably generated) the
bipolar, pre-planetary reflection nebula RAFGL 2688 (the Egg Nebula). The
polarimetric data pinpoint the illuminating star, which is not detected
directly at wavelengths less than or equal to 2 microns, at a position well
within the dark lane that bisects the nebula, 0.55" (about 550 AU) southwest of
the infrared peak which was previously detected at the southern tip of the
northern polar lobe. The inferred position of the central star corresponds to
the geometric center of the tips of the four principle lobes of near-infrared
H2 emission; identifying the central star at this position also reveals the
strong point symmetric structure of the nebula, as seen both in the intensity
and polarization structure of the polar lobes. The polarimetric and imaging
data indicate that the infrared peak directly detected in the NICMOS images is
a self-luminous source and, therefore, is most likely a distant binary
companion to the illuminating star. Although present theory predicts that
bipolar structure in pre-planetary and planetary nebulae is a consequence of
binary star evolution, the separation between the components of the RAFGL 2688
binary system, as deduced from these observations, is much too large for the
presence of the infrared companion to have influenced the structure of the
RAFGL 2688 nebula.Comment: 15 pages, 6 figures, to appear in The Astrophysical Journa
Economic Impact of the North Dakota University System in 2009
Financial Economics, Public Economics,
A position sensitive phoswich hard X-ray detector system
A prototype position sensitive phoswich hard X-ray detector, designed for eventual astronomical usage, was tested in the laboratory. The scintillation crystal geometry was designed on the basis of a Monte Carlo simulation of the internal optics and includes a 3mm thick NaI(T1) primary X-ray detector which is actively shielded by a 20 mm thick CsI(T1) scintillation crystal. This phoswich arrangement is viewed by a number two inch photomultipliers. Measured values of the positional and spectral resolution of incident X-ray photons are compared with calculation
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