2,697 research outputs found
Combinatorial Assortment Optimization
Assortment optimization refers to the problem of designing a slate of
products to offer potential customers, such as stocking the shelves in a
convenience store. The price of each product is fixed in advance, and a
probabilistic choice function describes which product a customer will choose
from any given subset. We introduce the combinatorial assortment problem, where
each customer may select a bundle of products. We consider a model of consumer
choice where the relative value of different bundles is described by a
valuation function, while individual customers may differ in their absolute
willingness to pay, and study the complexity of the resulting optimization
problem. We show that any sub-polynomial approximation to the problem requires
exponentially many demand queries when the valuation function is XOS, and that
no FPTAS exists even for succinctly-representable submodular valuations. On the
positive side, we show how to obtain constant approximations under a
"well-priced" condition, where each product's price is sufficiently high. We
also provide an exact algorithm for -additive valuations, and show how to
extend our results to a learning setting where the seller must infer the
customers' preferences from their purchasing behavior
On the galactic rotation curves problem within an axisymmetric approach
In U. Nucamendi et al. Phys. Rev. D63 (2001) 125016 and K. Lake, Phys. Rev.
Lett. 92 (2004) 051101 it has been shown that galactic potentials can be
kinematically linked to the observed red/blue shifts of the corresponding
galactic rotation curves under a minimal set of assumptions: the emitted
photons come from stable timelike circular geodesic orbits of stars in a static
spherically symmetric gravitational field, and propagate to us along null
geodesics. It is remarkable that this relation can be established without
appealing at all to a concrete theory of gravitational interaction. Here we
generalize this kinematical spherically symmetric approach to the galactic
rotation curves problem to the stationary axisymmetric realm since this is
precisely the symmetry that spiral galaxies possess. Thus, by making use of the
most general stationary axisymmetric metric, we also consider stable circular
orbits of stars that emit signals which travel to a distant observer along null
geodesics and express the galactic red/blue shifts in terms of three arbitrary
metric functions, clarifying the contribution of the rotation as well as the
dragging of the gravitational field. This stationary axisymmetric approach
distinguishes between red and blue shifts emitted by circularly orbiting
receding and approaching stars, respectively, even when they are considered
with respect to the center of a spiral galaxy, indicating the need of precise
measurements in order to confront predictions with observations. We also point
out the difficulties one encounters in the attempt of determining the metric
functions from observations and list some possible strategies to overcome them.Comment: 7 pages in latex (MNRAS format), no figures, discussion and
references adde
Two-years of home based functional electrical stimulation recovers epidermis from atrophy and flattening after years of complete Conus-Cauda Syndrome
Meson-Baryon Unitarized Coupled Channel Chiral Perturbation Theory and the (1405) and (1670) Resonances
The wave meson-baryon scattering is analyzed for the strangeness
and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism
incorporating Chiral Symmetry. Four channels have been considered: , , and . The required input to solve
the Bethe-Salpeter equation is taken from lowest order Chiral Perturbation
Theory in a relativistic formalism. There appear undetermined low energy
constants, as a consequence of the renormalization of the amplitudes, which are
obtained from fits to the mass-spectrum, to the elastic
and --matrices and to the
cross section data. The position and residues of the
complex poles in the second Riemann Sheet of the scattering amplitude determine
masses, widths and branching ratios of the (1405) and
(1670) resonances, in reasonable agreement with experiment. A good
overall description of data, from threshold up to 1.75 GeV, is
achieved despite the fact that three-body channels have not been explicitly
included.Comment: 23 pages, Latex, 10 Figures. In this revised version a new subsection
3.6 on Heavy Baryon Expansion and new references have been adde
CP Violation in Hyperon Nonleptonic Decays within the Standard Model
We calculate the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in
nonleptonic hyperon decay within the Standard Model using the framework of
heavy-baryon chiral perturbation theory (chiPT). We identify those terms that
correspond to previous calculations and discover several errors in the existing
literature. We present a new result for the lowest-order (in chiPT)
contribution of the penguin operator to these asymmetries, as well as an
estimate for the uncertainty of our result that is based on the calculation of
the leading nonanalytic corrections.Comment: 21 pages, 2 figures; discussion clarified, results & conclusions
unchanged, to appear in Phys. Rev.
New Physics and CP Violation in Hyperon Nonleptonic Decays
The sum of the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in
hyperon nonleptonic decays is presently being measured by the E871 experiment.
We evaluate contributions to the asymmetries induced by chromomagnetic-penguin
operators, whose coefficients can be enhanced in certain models of new physics.
Incorporating recent information on the strong phases in Xi->Lambda pi decay,
we show that new-physics contributions to the two asymmetries can be
comparable. We explore how the upcoming results of E871 may constrain the
coefficients of the operators. We find that its preliminary measurement is
already better than the epsilon parameter of K-Kbar mixing in bounding the
parity-conserving contributions.Comment: 12 pages, 2 figure
The abundance of C18O and HDO in the envelope and hot core of the intermediate mass protostar NGC 7129 FIRS 2
NGC 7129 FIRS 2 is a young intermediate-mass (IM) protostar, which is
associated with two energetic bipolar outflows and displays clear signs of the
presence of a hot core. It has been extensively observed with ground based
telescopes and within the WISH Guaranteed Time Herschel Key Program. We present
new observations of the C18O 3-2 and the HDO 3_{12}-2_{21} lines towards NGC
7129 FIRS 2. Combining these observations with Herschel data and modeling their
emissions, we constrain the C18O and HDO abundance profiles across the
protostellar envelope. In particular, we derive the abundance of C18O and HDO
in the hot core. The intensities of the C18O lines are well reproduced assuming
that the C18O abundance decreases through the protostellar envelope from the
outer edge towards the centre until the point where the gas and dust reach the
CO evaporation temperature (~20-25 K) where the C18O is released back to the
gas phase. Once the C18O is released to the gas phase, the modelled C18O
abundance is found to be ~1.6x10^{-8}, which is a factor of 10 lower than the
reference abundance. This result is supported by the non-detection of C18O 9-8,
which proves that even in the hot core (T_k>100 K) the CO abundance must be 10
times lower than the reference value. Several scenarios are discussed to
explain this C18O deficiency. One possible explanation is that during the
pre-stellar and protostellar phase, the CO is removed from the grain mantles by
reactions to form more complex molecules. Our HDO modeling shows that the
emission of HDO 3_{12}-2_{21} line is maser and comes from the hot core
(T_k>100 K). Assuming the physical structure derived by Crimier et al. (2010),
we determine a HDO abundance of ~0.4 - 1x10^{-7} in the hot core of this IM
protostar, similar to that found in the hot corinos NGC 1333 IRAS 2A and IRAS
16293-2422.Comment: 10 pages, 7 figure
Haemodynamics and flow modiïŹcation stents for peripheral arterial disease:a review
Endovascular stents are widely used for the treatment of peripheral arterial disease (PAD). However, the development of in-stent restenosis and downstream PAD progression remain a challenge. Stent revascularisation of PAD causes arterial trauma and introduces abnormal haemodynamics, which initiate complicated biological processes detrimental to the arterial wall. The interaction between stent struts and arterial cells in contact, and the blood flow field created in a stented region, are highly affected by stent design. Spiral flow is known as a normal physiologic characteristic of arterial circulation and is believed to prevent the development of flow disturbances. This secondary flow motion is lost in atheromatous disease and its re-introduction after endovascular treatment of PAD has been suggested as a method to induce stabilised and coherent haemodynamics. Stent designs able to generate spiral flow may support endothelial function and therefore increase patency rates. This review is focused on secondary flow phenomena in arteries and the development of flow modification stent technologies for the treatment of PAD
Initial experience with a synthetic sealant PleuraSealâą after pulmonary resections: a prospective study with retrospective case matched controls
The objective of this study was to evaluate postoperative outcome and efficacy of a hydrogel tissue sealant for prevention of alveolar leakage after open lung resections
The ionizing photon production efficiency of bright z2-5 galaxies
We investigate the production efficiency of ionizing photons ()
of 1174 galaxies with secure redshift at z=2-5 from the VANDELS survey to
determine the relation between ionizing emission and physical properties of
bright and massive sources. We constrain and galaxy physical
parameters by means of spectro-photometric fits performed with the BEAGLE code.
The analysis exploits the multi-band photometry in the VANDELS fields, and the
measurement of UV rest-frame emission lines (CIII], HeII, OIII]) from deep VIMOS spectra. We find no clear evolution
of with redshift within the probed range. The ionizing efficiency
slightly increases at fainter , and bluer UV slopes, but these trends
are less evident when restricting the analysis to a complete subsample at
log(M/M)9.5. We find a significant trend of increasing
with increasing EW(Ly), with an average
log(/Hz erg)25 at EW50\AA, and a higher ionizing
efficiency for high-EW CIII] and OIII] emitters.
The most significant correlations are found with respect to stellar mass,
specific star-formation rate (sSFR) and SFR surface density ().
The relation between and sSFR shows a monotonic increase from
log(/Hz erg) 24.5 at log(sSFR)-9.5 to
25.5 at log(sSFR)-7.5, a low scatter and little dependence
on mass. The objects above the main-sequence of star-formation consistently
have higher-than-average . A clear increase of with
is also found, with log(/Hz erg)25 for
objects at 10 M.(Abridged)Comment: 11 pages, 9 figures, accepted for publication in A&
- âŠ