224,648 research outputs found
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Elicitation and representation of expert knowledge for computer aided diagnosis in mammography
To study how professional radiologists describe, interpret and make decisions about micro-calcifications in mammograms. The purpose was to develop a model of the radiologists' decision making for use in CADMIUM II, a computerized aid for mammogram interpretation that combines symbolic reasoning with image processing
Renormalon Subtraction from the Average Plaquette and the Gluon Condensate
A Borel resummation scheme of subtracting the perturbative contribution from
the average plaquette is proposed using the bilocal expansion of Borel
transform. It is shown that the remnant of the average plaquette, after
subtraction of the perturbative contribution, scales as a dim-4 condensate. A
critical review of the existing procedure of renormalon subtraction is
presented.Comment: 7pages,one figure; Journal version (added references and introductory
remarks
Hardness of Graph Pricing through Generalized Max-Dicut
The Graph Pricing problem is among the fundamental problems whose
approximability is not well-understood. While there is a simple combinatorial
1/4-approximation algorithm, the best hardness result remains at 1/2 assuming
the Unique Games Conjecture (UGC). We show that it is NP-hard to approximate
within a factor better than 1/4 under the UGC, so that the simple combinatorial
algorithm might be the best possible. We also prove that for any , there exists such that the integrality gap of
-rounds of the Sherali-Adams hierarchy of linear programming for
Graph Pricing is at most 1/2 + .
This work is based on the effort to view the Graph Pricing problem as a
Constraint Satisfaction Problem (CSP) simpler than the standard and complicated
formulation. We propose the problem called Generalized Max-Dicut(), which
has a domain size for every . Generalized Max-Dicut(1) is
well-known Max-Dicut. There is an approximation-preserving reduction from
Generalized Max-Dicut on directed acyclic graphs (DAGs) to Graph Pricing, and
both our results are achieved through this reduction. Besides its connection to
Graph Pricing, the hardness of Generalized Max-Dicut is interesting in its own
right since in most arity two CSPs studied in the literature, SDP-based
algorithms perform better than LP-based or combinatorial algorithms --- for
this arity two CSP, a simple combinatorial algorithm does the best.Comment: 28 page
Charged stripes from alternating static magnetic field
We motivate and perform a calculation of the energy of a cold fluid of
charged fermions in the presence of a striped magnetic background. We find that
a non-trivial value for the doping density on the walls is preferredComment: RevTeX, 3 pages, 3 encapsulated PostScript figure
A paradigmatic flow for small-scale magnetohydrodynamics: properties of the ideal case and the collision of current sheets
We propose two sets of initial conditions for magnetohydrodynamics (MHD) in
which both the velocity and the magnetic fields have spatial symmetries that
are preserved by the dynamical equations as the system evolves. When
implemented numerically they allow for substantial savings in CPU time and
memory storage requirements for a given resolved scale separation. Basic
properties of these Taylor-Green flows generalized to MHD are given, and the
ideal non-dissipative case is studied up to the equivalent of 2048^3 grid
points for one of these flows. The temporal evolution of the logarithmic
decrements, delta, of the energy spectrum remains exponential at the highest
spatial resolution considered, for which an acceleration is observed briefly
before the grid resolution is reached. Up to the end of the exponential decay
of delta, the behavior is consistent with a regular flow with no appearance of
a singularity. The subsequent short acceleration in the formation of small
magnetic scales can be associated with a near collision of two current sheets
driven together by magnetic pressure. It leads to strong gradients with a fast
rotation of the direction of the magnetic field, a feature also observed in the
solar wind.Comment: 8 pages, 4 figure
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Manufacturing Mechatronics Using Thermal Spray Shape Deposition
A new technology for manufacturing mechatronics is described. The technique is based on recursive
masking and deposition of thermally sprayed materials. Using these methods, mechanical structures
can be created that embed and interconnect electronic components. This results in highly integrated
mechatronic devices. A simple, electromechanical artifact was designed and produced to assess the
feasibility of these techniques. The details and limitations of this project will be discussed. Areas of
future research are identified which are aimed at realizing the full potential of this emerging manufacturing
process.Mechanical Engineerin
Particle-in-cell and weak turbulence simulations of plasma emission
The plasma emission process, which is the mechanism for solar type II and
type III radio bursts phenomena, is studied by means of particle-in-cell and
weak turbulence simulation methods. By plasma emission, it is meant as a loose
description of a series of processes, starting from the solar flare associated
electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent
partial conversion of beam energy into the radiation energy by nonlinear
processes. Particle-in-cell (PIC) simulation is rigorous but the method is
computationally intense, and it is difficult to diagnose the results. Numerical
solution of equations of weak turbulence (WT) theory, termed WT simulation, on
the other hand, is efficient and naturally lends itself to diagnostics since
various terms in the equation can be turned on or off. Nevertheless, WT theory
is based upon a number of assumptions. It is, therefore, desirable to compare
the two methods, which is carried out for the first time in the present paper
with numerical solutions of the complete set of equations of the WT theory and
with two-dimensional electromagnetic PIC simulation. Upon making quantitative
comparisons it is found that WT theory is largely valid, although some
discrepancies are also found. The present study also indicates that it requires
large computational resources in order to accurately simulate the radiation
emission processes, especially for low electron beam speeds. Findings from the
present paper thus imply that both methods may be useful for the study of solar
radio emissions as they are complementary.Comment: 21 pages, 9 figure
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