26,094 research outputs found
Modelling Citation Networks
The distribution of the number of academic publications as a function of
citation count for a given year is remarkably similar from year to year. We
measure this similarity as a width of the distribution and find it to be
approximately constant from year to year. We show that simple citation models
fail to capture this behaviour. We then provide a simple three parameter
citation network model using a mixture of local and global search processes
which can reproduce the correct distribution over time. We use the citation
network of papers from the hep-th section of arXiv to test our model. For this
data, around 20% of citations use global information to reference recently
published papers, while the remaining 80% are found using local searches. We
note that this is consistent with other studies though our motivation is very
different from previous work. Finally, we also find that the fluctuations in
the size of an academic publication's bibliography is important for the model.
This is not addressed in most models and needs further work.Comment: 29 pages, 22 figure
Microlensing and the Search for Extraterrestrial Life
Are microlensing searches likely to discover planets that harbor life? Given
our present state of knowledge, this is a difficult question to answer. We
therefore begin by asking a more narrowly focused question: are conditions on
planets discovered via microlensing likely to be similar to those we experience
on Earth? In this paper I link the microlensing observations to the well-known
"Goldilocks Problem" (conditions on the Earth-like planets need to be "just
right"), to find that Earth-like planets discovered via microlensing are likely
to be orbiting stars more luminous than the sun. This means that light from the
planetary system's central star may contribute a significant fraction of the
baseline flux relative to the star that is lensed. Such blending of light from
the lens with light from the lensed source can, in principle, limit our ability
to detect these events. This turns out not to be a significant problem,
however. A second consequence of blending is the opportunity to determine the
spectral type of the lensed spectral type of the lensed star. This
circumstance, plus the possibility that finite-source-size effects are
important, implies that some meaningful follow-up observations are likely to be
possible for a subset Earth-like planets discovered via microlensing. In
addition, calculations indicate that reasonable requirements on the planet's
density and surface gravity imply that the mass of Earth-like planets is likely
to be within a factor of of an Earth mass.Comment: 15 pages, 2 figures. To be published in the Astrophysical Journa
The Spin Holonomy Group In General Relativity
It has recently been shown by Goldberg et al that the holonomy group of the
chiral spin-connection is preserved under time evolution in vacuum general
relativity. Here, the underlying reason for the time-independence of the
holonomy group is traced to the self-duality of the curvature 2-form for an
Einstein space. This observation reveals that the holonomy group is
time-independent not only in vacuum, but also in the presence of a cosmological
constant. It also shows that once matter is coupled to gravity, the
"conservation of holonomy" is lost. When the fundamental group of space is
non-trivial, the holonomy group need not be connected. For each homotopy class
of loops, the holonomies comprise a coset of the full holonomy group modulo its
connected component. These cosets are also time-independent. All possible
holonomy groups that can arise are classified, and examples are given of
connections with these holonomy groups. The classification of local and global
solutions with given holonomy groups is discussed.Comment: 21 page
Multi-agent system for dynamic manufacturing system optimization
This paper deals with the application of multi-agent system concept for optimization of dynamic uncertain process. These problems are known to have a computationally demanding objective function, which could turn to be infeasible when large problems are considered. Therefore, fast approximations to the objective function are required. This paper employs bundle of intelligent systems algorithms tied together in a multi-agent system. In order to demonstrate the system, a metal reheat furnace scheduling problem is adopted for highly demanded optimization problem. The proposed multi-agent approach has been evaluated for different settings of the reheat furnace scheduling problem. Particle Swarm Optimization, Genetic Algorithm with different classic and advanced versions: GA with chromosome differentiation, Age GA, and Sexual GA, and finally a Mimetic GA, which is based on combining the GA as a global optimizer and the PSO as a local optimizer. Experimentation has been performed to validate the multi-agent system on the reheat furnace scheduling problem
Hydrogen and fluorine in the surfaces of lunar samples
The resonant nuclear reaction F-19 (p, alpha gamma)0-16 has been used to perform depth sensitive analyses for both fluorine and hydrogen in lunar samples. The resonance at 0.83 MeV (center-of-mass) in this reaction has been applied to the measurement of the distribution of trapped solar protons in lunar samples to depths of about 1/2 micrometer. These results are interpreted in terms of terrestrial H2O surface contamination and a redistribution of the implanted solar H which has been influenced by heavy radiation damage in the surface region. Results are also presented for an experiment to test the penetration of H2O into laboratory glass samples which have been irradiated with 0-16 to simulate the radiation damaged surfaces of lunar glasses. Fluorine determinations have been performed in a 1 pm surface layer on lunar samples using the same F-19 alpha gamma)0-16 resonance. The data are discussed from the standpoint of lunar fluorine and Teflon contamination
L^p boundedness of the wave operator for the one dimensional Schroedinger operator
Given a one dimensional perturbed Schroedinger operator H=-(d/dx)^2+V(x) we
consider the associated wave operators W_+, W_- defined as the strong L^2
limits as s-> \pm\infty of the operators e^{isH} e^{-isH_0} We prove that the
wave operators are bounded operators on L^p for all 1<p<\infty, provided
(1+|x|)^2 V(x) is integrable, or else (1+|x|)V(x) is integrable and 0 is not a
resonance. For p=\infty we obtain an estimate in terms of the Hilbert
transform. Some applications to dispersive estimates for equations with
variable rough coefficients are given.Comment: 26 page
An Empirical Study of Finding Approximate Equilibria in Bimatrix Games
While there have been a number of studies about the efficacy of methods to
find exact Nash equilibria in bimatrix games, there has been little empirical
work on finding approximate Nash equilibria. Here we provide such a study that
compares a number of approximation methods and exact methods. In particular, we
explore the trade-off between the quality of approximate equilibrium and the
required running time to find one. We found that the existing library GAMUT,
which has been the de facto standard that has been used to test exact methods,
is insufficient as a test bed for approximation methods since many of its games
have pure equilibria or other easy-to-find good approximate equilibria. We
extend the breadth and depth of our study by including new interesting families
of bimatrix games, and studying bimatrix games upto size .
Finally, we provide new close-to-worst-case examples for the best-performing
algorithms for finding approximate Nash equilibria
Evidence of Skyrmion excitations about in n-Modulation Doped Single Quantum Wells by Inter-band Optical Transmission
We observe a dramatic reduction in the degree of spin-polarization of a
two-dimensional electron gas in a magnetic field when the Fermi energy moves
off the mid-point of the spin-gap of the lowest Landau level, . This
rapid decay of spin alignment to an unpolarized state occurs over small changes
to both higher and lower magnetic field. The degree of electron spin
polarization as a function of is measured through the magneto-absorption
spectra which distinguish the occupancy of the two electron spin states. The
data provide experimental evidence for the presence of Skyrmion excitations
where exchange energy dominates Zeeman energy in the integer quantum Hall
regime at
A Histological Examination of the Ovaries of Pacific Sanddab, Citharichthys sordidus, Captured at Two Oil Platforms and Two Natural Sites in the Southern California Bight
A number of the 26 offshore oil and gas platforms off California may be nearing the end of their economic lives. Decisions as to the disposition of these platforms will be based on a number of parameters, including the biological role of the structures. One issue that has arisen is the possible contamination of fishes living around platforms resulting from contaminants released during drilling and production. If significant contamination is occurring, it would be expected to impair the reproductive abilities of impacted fishes. One form of reproductive impairment is atresia, the abnormal reabsorption of oocytes that are destined to be spawned. Atresia has been widely used as an indicator of pollutant-related reproductive impairment in fishes. We examined the occurrence of atretic oocytes in Pacific sanddab, Citharichthys sordidus, collected near two offshore platforms in the Santa Barbara Channel (B and Gilda) and from two natural reference sites (off the east end of Santa Cruz Island and in mid-channel off Rincon). While pronounced atresia was observed in a few fish at one natural site and one platform, there was no evidence of widespread pronounced atresia at any of the four sites
A New Approach to Black Hole Microstates
If one encodes the gravitational degrees of freedom in an orthonormal frame
field there is a very natural first order action one can write down (which in
four dimensions is known as the Goldberg action). In this essay we will show
that this action contains a boundary action for certain microscopic degrees of
freedom living at the horizon of a black hole, and argue that these degrees of
freedom hold great promise for explaining the microstates responsible for black
hole entropy, in any number of spacetime dimensions. This approach faces many
interesting challenges, both technical and conceptual.Comment: 6 pages, 0 figures, LaTeX; submitted to Mod. Phys. Lett. A.; this
essay received "honorable mention" from the Gravity Research Foundation, 199
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