3,558 research outputs found
The NASA Space Station program plans
The design of a permanently manned space station is discussed. The role of the space shuttle, planning guidelines, international cooperation, and commercial possibilities are among the topics discussed
The modular variety of hyperelliptic curves of genus three
The modular variety of non singular and complete hyperelliptic curves with
level-two structure of genus 3 is a 5-dimensional quasi projective variety
which admits several standard compactifications. The first one, X, comes from
the realization of this variety as a sub-variety of the Siegel modular variety
of level two and genus three .We will be to describe the equations of X in a
suitable projective embedding and its Hilbert function. It will turn out that
X is normal. A further model comes from geometric invariant theory using
so-called semistable degenerated point configurations in (P^1)^8 . We denote
this GIT-compactification by Y. The equations of this variety in a suitable
projective embedding are known. This variety also can by identified with a
Baily-Borel compactified ball-quotient. We will describe these results in some
detail and obtain new proofs including some finer results for them. We have a
birational map between Y and X . In this paper we use the fact that there are
graded algebras (closely related to algebras of modular forms) A,B such that
X=proj(A) and Y=proj(B). This homomorphism rests on the theory of Thomae (19th
century), in which the thetanullwerte of hyperelliptic curves have been
computed. Using the explicit equations for we can compute the base locus
of the map from Y to X.
Blowing up the base locus and the singularity of Y, we get a dominant, smooth
model {\tilde Y}. We will see that {\tilde Y} is isomorphic to the
compactification of families of marked projective lines (P^1,x_1,...,x_8),
usually denoted by {\bar M_{0,8}}. There are several combinatorial similarities
between the models X and Y. These similarities can be described best, if one
uses the ball-model to describe Y.Comment: 39 page
Some Siegel threefolds with a Calabi-Yau model II
In the paper [FSM] we described some Siegel modular threefolds which admit a
Calabi-Yau model. Using a different method we give in this paper an enlarged
list of such varieties that admits a Calabi-Yau model in the following weak
sense: there exists a desingularization in the category of complex spaces of
the Satake compactification which admits a holomorphic three-form without zeros
and whose first Betti number vanishes Basic for our method is the paper [GN] of
van Geemen and Nygaard.Comment: 23 pages, no figure
A Siegel cusp form of degree 12 and weight 12
The theta series of the two unimodular even positive definite lattices of
rank 16 are known to be linearly dependent in degree at most 3 and linearly
independent in degree 4. In this paper we consider the next case of the 24
Niemeier lattices of rank 24. The associated theta series are linearly
dependent in degree at most 11 and linearly independent in degree 12. The
resulting Siegel cusp form of degree 12 and weight 12 is a Hecke eigenform
which seems to have interesting properties.Comment: 12 pages, plain te
Improved approximate inspirals of test-bodies into Kerr black holes
We present an improved version of the approximate scheme for generating
inspirals of test-bodies into a Kerr black hole recently developed by
Glampedakis, Hughes and Kennefick. Their original "hybrid" scheme was based on
combining exact relativistic expressions for the evolution of the orbital
elements (the semi-latus rectum p and eccentricity e) with approximate,
weak-field, formula for the energy and angular momentum fluxes, amended by the
assumption of constant inclination angle, iota, during the inspiral. Despite
the fact that the resulting inspirals were overall well-behaved, certain
pathologies remained for orbits in the strong field regime and for orbits which
are nearly circular and/or nearly polar. In this paper we eliminate these
problems by incorporating an array of improvements in the approximate fluxes.
Firstly, we add certain corrections which ensure the correct behaviour of the
fluxes in the limit of vanishing eccentricity and/or 90 degrees inclination.
Secondly, we use higher order post-Newtonian formulae, adapted for generic
orbits. Thirdly, we drop the assumption of constant inclination. Instead, we
first evolve the Carter constant by means of an approximate post-Newtonian
expression and subsequently extract the evolution of iota. Finally, we improve
the evolution of circular orbits by using fits to the angular momentum and
inclination evolution determined by Teukolsky based calculations. As an
application of the improved scheme we provide a sample of generic Kerr
inspirals and for the specific case of nearly circular orbits we locate the
critical radius where orbits begin to decircularise under radiation reaction.
These easy-to-generate inspirals should become a useful tool for exploring LISA
data analysis issues and may ultimately play a role in source detection.Comment: 25 pages, 14 figures, some typos corrected, short section on
conservative corrections added, minor changes for consistency with published
versio
Performance evaluation of wheels for lunar vehicles
Performance evaluation of wheels for lunar vehicle
CSM-365 - Using schema theory to explore interactions of multiple operators
In the last two years the schema theory for Genetic Programming (GP) has been applied to the problem of understanding the length biases of a variety of crossover and mutation operators on variable length linear structures. In these initial papers, operators were studied in isolation. In practice, however, they are typically used in various combinations, and in this paper we present the first schema theory analysis of the complex interactions of multiple operators. In particular we apply the schema theory to the use of standard subtree crossover, full mutation, and grow mutation (in varying proportions) to variable length linear structures in the one-then-zeros problem. We then show how the results can be used to guide choices about the relative proportion of these operators in order to achieve certain structural goals during a run
Harvesting Entities from the Web Using Unique Identifiers -- IBEX
In this paper we study the prevalence of unique entity identifiers on the
Web. These are, e.g., ISBNs (for books), GTINs (for commercial products), DOIs
(for documents), email addresses, and others. We show how these identifiers can
be harvested systematically from Web pages, and how they can be associated with
human-readable names for the entities at large scale.
Starting with a simple extraction of identifiers and names from Web pages, we
show how we can use the properties of unique identifiers to filter out noise
and clean up the extraction result on the entire corpus. The end result is a
database of millions of uniquely identified entities of different types, with
an accuracy of 73--96% and a very high coverage compared to existing knowledge
bases. We use this database to compute novel statistics on the presence of
products, people, and other entities on the Web.Comment: 30 pages, 5 figures, 9 tables. Complete technical report for A.
Talaika, J. A. Biega, A. Amarilli, and F. M. Suchanek. IBEX: Harvesting
Entities from the Web Using Unique Identifiers. WebDB workshop, 201
Carrier scattering, mobilities and electrostatic potential in mono-, bi- and tri-layer graphenes
The carrier density and temperature dependence of the Hall mobility in mono-,
bi- and tri-layer graphene has been systematically studied. We found that as
the carrier density increases, the mobility decreases for mono-layer graphene,
while it increases for bi-layer/tri-layer graphene. This can be explained by
the different density of states in mono-layer and bi-layer/tri-layer graphenes.
In mono-layer, the mobility also decreases with increasing temperature
primarily due to surface polar substrate phonon scattering. In
bi-layer/tri-layer graphene, on the other hand, the mobility increases with
temperature because the field of the substrate surface phonons is effectively
screened by the additional graphene layer(s) and the mobility is dominated by
Coulomb scattering.
We also find that the temperature dependence of the Hall coefficient in
mono-, bi- and tri-layer graphene can be explained by the formation of electron
and hole puddles in graphene. This model also explains the temperature
dependence of the minimum conductance of mono-, bi- and tri-layer graphene. The
electrostatic potential variations across the different graphene samples are
extracted.Comment: 18 pages, 7 figure
Nuclear Black Hole Formation in Clumpy Galaxies at High Redshift
Massive stellar clumps in high redshift galaxies interact and migrate to the
center to form a bulge and exponential disk in <1 Gyr. Here we consider the
fate of intermediate mass black holes (BHs) that might form by massive-star
coalescence in the dense young clusters of these disk clumps. We find that the
BHs move inward with the clumps and reach the inner few hundred parsecs in only
a few orbit times. There they could merge into a supermassive BH by dynamical
friction. The ratio of BH mass to stellar mass in the disk clumps is
approximately preserved in the final ratio of BH to bulge mass. Because this
ratio for individual clusters has been estimated to be ~10^{-3}, the observed
BH-to-bulge mass ratio results. We also obtain a relation between BH mass and
bulge velocity dispersion that is compatible with observations of present-day
galaxies.Comment: 10 pages, 3 figures, accepted by Ap
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