3,289 research outputs found
Combining Traditional Marketing and Viral Marketing with Amphibious Influence Maximization
In this paper, we propose the amphibious influence maximization (AIM) model
that combines traditional marketing via content providers and viral marketing
to consumers in social networks in a single framework. In AIM, a set of content
providers and consumers form a bipartite network while consumers also form
their social network, and influence propagates from the content providers to
consumers and among consumers in the social network following the independent
cascade model. An advertiser needs to select a subset of seed content providers
and a subset of seed consumers, such that the influence from the seed providers
passing through the seed consumers could reach a large number of consumers in
the social network in expectation.
We prove that the AIM problem is NP-hard to approximate to within any
constant factor via a reduction from Feige's k-prover proof system for 3-SAT5.
We also give evidence that even when the social network graph is trivial (i.e.
has no edges), a polynomial time constant factor approximation for AIM is
unlikely. However, when we assume that the weighted bi-adjacency matrix that
describes the influence of content providers on consumers is of constant rank,
a common assumption often used in recommender systems, we provide a
polynomial-time algorithm that achieves approximation ratio of
for any (polynomially small) . Our
algorithmic results still hold for a more general model where cascades in
social network follow a general monotone and submodular function.Comment: An extended abstract appeared in the Proceedings of the 16th ACM
Conference on Economics and Computation (EC), 201
Strongly nonlinear waves in capillary electrophoresis
In capillary electrophoresis, sample ions migrate along a micro-capillary
filled with a background electrolyte under the influence of an applied electric
field. If the sample concentration is sufficiently high, the electrical
conductivity in the sample zone could differ significantly from the
background.Under such conditions, the local migration velocity of sample ions
becomes concentration dependent resulting in a nonlinear wave that exhibits
shock like features. If the nonlinearity is weak, the sample concentration
profile, under certain simplifying assumptions, can be shown to obey Burgers'
equation (S. Ghosal and Z. Chen Bull. Math. Biol. 2010, 72(8), pg. 2047) which
has an exact analytical solution for arbitrary initial condition.In this paper,
we use a numerical method to study the problem in the more general case where
the sample concentration is not small in comparison to the concentration of
background ions. In the case of low concentrations, the numerical results agree
with the weakly nonlinear theory presented earlier, but at high concentrations,
the wave evolves in a way that is qualitatively different.Comment: 7 pages, 5 figures, 1 Appendix, 2 videos (supplementary material
Direct Numerical Simulations of the Navier-Stokes Alpha Model
We explore the utility of the recently proposed alpha equations in providing
a subgrid model for fluid turbulence. Our principal results are comparisons of
direct numerical simulations of fluid turbulence using several values of the
parameter alpha, including the limiting case where the Navier-Stokes equations
are recovered. Our studies show that the large scale features, including
statistics and structures, are preserved by the alpha models, even at coarser
resolutions where the fine scales are not fully resolved. We also describe the
differences that appear in simulations. We provide a summary of the principal
features of the alpha equations, and offer some explanation of the
effectiveness of these equations used as a subgrid model for three-dimensional
fluid turbulence
Profile of bubbling solutions to a Liouville system
In several fields of Physics, Chemistry and Ecology, some models are
described by Liouville systems. In this article we first prove a uniqueness
result for a Liouville system in . Then we establish an uniform
estimate for bubbling solutions of a locally defined Liouville system near an
isolated blowup point. The uniqueness result, as well as the local uniform
estimates are crucial ingredients for obtaining a priori estimate, degree
counting formulas and existence results for Liouville systems defined on
Riemann surfaces.Comment: 32 page
The Navier-Stokes-alpha model of fluid turbulence
We review the properties of the nonlinearly dispersive Navier-Stokes-alpha
(NS-alpha) model of incompressible fluid turbulence -- also called the viscous
Camassa-Holm equations and the LANS equations in the literature. We first
re-derive the NS-alpha model by filtering the velocity of the fluid loop in
Kelvin's circulation theorem for the Navier-Stokes equations. Then we show that
this filtering causes the wavenumber spectrum of the translational kinetic
energy for the NS-alpha model to roll off as k^{-3} for k \alpha > 1 in three
dimensions, instead of continuing along the slower Kolmogorov scaling law,
k^{-5/3}, that it follows for k \alpha < 1. This rolloff at higher wavenumbers
shortens the inertial range for the NS-alpha model and thereby makes it more
computable. We also explain how the NS-alpha model is related to large eddy
simulation (LES) turbulence modeling and to the stress tensor for second-grade
fluids. We close by surveying recent results in the literature for the NS-alpha
model and its inviscid limit (the Euler-alpha model).Comment: 22 pages, 1 figure. Dedicated to V. E. Zakharov on the occasion of
his 60th birthday. To appear in Physica
Thermopower peak in phase transition region of (1-x)LaCaMnO/xYSZ
The thermoelectric power (TEP) and the electrical resistivity of the
intergranular magnetoresistance (IGMR) composite,
(1-x)LaCaMnO/xYSZ (LCMO/YSZ) with x = 0, 0.75%, 1.25%,
4.5%, 13% 15% and 80% of the yttria-stabalized zirconia (YSZ), have been
measured from 300 K down to 77 K. Pronounced TEP peak appears during the phase
transition for the samples of x 0, while not observed for x = 0. We suggest
that this is due to the magnetic structure variation induced by the lattice
strain which is resulting from the LCMO/YSZ boundary layers. The transition
width in temperature derived from , with being the AC magnetic
susceptibility, supports this interpretation.Comment: 4 pages, 4 eps figures, Latex, J. Appl. Phys 94, 7206 (2003
Polymer translocation out of confined environments
We consider the dynamics of polymer translocation out of confined
environments. Analytic scaling arguments lead to the prediction that the
translocation time scales like for translocation out of a planar
confinement between two walls with separation into a 3D environment, and
for translocation out of two strips with separation
into a 2D environment. Here, is the chain length, and
are the Flory exponents in 3D and 2D, and is the scaling exponent of
translocation velocity with , whose value for the present choice of
parameters is based on Langevin dynamics simulations. These
scaling exponents improve on earlier predictions.Comment: 5 pages, 5 figures. To appear in Phys. Rev.
Electroweak Matrix Elements in the Two-Nucleon Sector from Lattice QCD
We demonstrate how to make rigorous predictions for electroweak matrix
elements in nuclear systems directly from QCD. More precisely, we show how to
determine the short-distance contributions to low-momentum transfer electroweak
matrix elements in the two-nucleon sector from lattice QCD. In potential model
descriptions of multi-nucleon systems, this is equivalent to uniquely
determining the meson-exchange currents, while in the context of nuclear
effective field theory, this translates into determining the coefficients of
local, gauge-invariant, multi-nucleon-electroweak current operators. The
energies of the lowest-lying states of two nucleons on a finite volume lattice
with periodic boundary conditions in the presence of a background magnetic
field are sufficient to determine the local four-nucleon operators that
contribute to the deuteron magnetic moment and to the threshold cross-section
of n + p -> d + gamma. Similarly, the energy-levels of two nucleons immersed in
a background isovector axial weak field can be used to determine the
coefficient of the leading local four-nucleon operator contributing to the
neutral- and charged-current break-up of the deuteron. This is required for the
extraction of solar neutrino fluxes at SNO and future neutrino experiments.Comment: 22 pages, 4 figure
Fine-grained Complexity Meets IP = PSPACE
In this paper we study the fine-grained complexity of finding exact and
approximate solutions to problems in P. Our main contribution is showing
reductions from exact to approximate solution for a host of such problems.
As one (notable) example, we show that the Closest-LCS-Pair problem (Given
two sets of strings and , compute exactly the maximum with ) is equivalent to its approximation version
(under near-linear time reductions, and with a constant approximation factor).
More generally, we identify a class of problems, which we call BP-Pair-Class,
comprising both exact and approximate solutions, and show that they are all
equivalent under near-linear time reductions.
Exploring this class and its properties, we also show:
Under the NC-SETH assumption (a significantly more relaxed
assumption than SETH), solving any of the problems in this class requires
essentially quadratic time.
Modest improvements on the running time of known algorithms
(shaving log factors) would imply that NEXP is not in non-uniform
.
Finally, we leverage our techniques to show new barriers for
deterministic approximation algorithms for LCS.
At the heart of these new results is a deep connection between interactive
proof systems for bounded-space computations and the fine-grained complexity of
exact and approximate solutions to problems in P. In particular, our results
build on the proof techniques from the classical IP = PSPACE result
Emerging applications for OCT in the head and neck
Objectives: To describe the current and promising new applications of Optical Coherence Tomography (OCT) as a helpful tool when imaging the different sites in the head and neck. We used the OCT Niris system, which is the first commercially available OCT device for applications outside the field of ophthalmology. Methods: OCT images were obtained of normal, benign, premalignant and malignant lesions in different areas of the head and neck. The OCT imaging system has a tissue penetration depth of approximately 1-2mm, a scanning range of 2mm and a spatial depth resolution of approximately 10-20μm. Imaging was performed using a flexible probe in two different settings, the outpatient clinic and the operating room. Results: High-resolution cross-sectional images from the larynx were obtained with the patient awake, without the need for general anesthesia, under direct visualization with a flexible fiberoptic endoscope. The OCT probe was inserted through the nasal cavity and placed in slight contact with the laryngeal tissue. In the ears, cholesteatoma was differentiated from inflamed middle ear mucosa by the different hyperintensity. In the neck, normal as well as different pathologies of the thyroid were identified. Conclusions: This system is non invasive and easy to incorporate into the operating room setting as well as the outpatient clinic. It requires minimal set-up and only one person is required to operate the system. OCT has the distinctive capability to obtain highresolution images, and the microanatomy of different sites can be observed. OCT technology has the potential to offer a quick, efficient and reliable imaging method to help the surgeon not only in the operating room but also in the clinical setting to guide surgical biopsies and aid in clinical decision making of different head and neck pathologies, especially those arising form the larynx. © 2010 Copyright SPIE - The International Society for Optical Engineering
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