7,685 research outputs found
Arithmetic of singular Enriques Surfaces
We study the arithmetic of Enriques surfaces whose universal covers are
singular K3 surfaces. If a singular K3 surface X has discriminant d, then it
has a model over the ring class field d. Our main theorem is that the same
holds true for any Enriques quotient of X. It is based on a study on
Neron-Severi groups of singular K3 surfaces. We also comment on Galois actions
on divisors of Enriques surfaces.Comment: 32 pages; v2: Section 2 expanded, minor additions and edit
QSO-galaxy correlations due to weak lensing in arbitrary Friedmann-Lemaitre cosmologies
We calculate the angular cross-correlation function between background QSOs
and foreground galaxies induced by the weak lensing effect of large-scale
structures. Results are given for arbitrary Friedmann-Lemaitre cosmologies. The
non-linear growth of density perturbations is included. Compared to the linear
growth, the non-linear growth increases the correlation amplitude by about an
order of magnitude in an Einstein-de Sitter universe, and by even more for
lower Omega_0. The dependence of the correlation amplitude on the cosmological
parameters strongly depends on the normalization of the power spectrum. The
QSO-galaxy cross-correlation function is most sensitive to density structures
on scales in the range (1-10) Mpc/h, where the normalization of the power
spectrum to the observed cluster abundance appears most appropriate. In that
case, the correlation strength changes by less than a factor of <~ 2 when
Omega_0 varies between 0.3 and 1, quite independent of the value of
Omega_Lambda. For Omega_0 <~ 0.3, the correlation strength increases with
decreasing Omega_0, and it scales approximately linearly with the Hubble
constant h.Comment: revised version, accepted by MNRA
Charge correlations in the weakly doped t-J model calculated by projection technique
We study frequency- and wave-vector dependent charge correlations in weakly
doped antiferromagnets using Mori-Zwanzig projection technique. The system is
described by the two-dimensional t-J model. The ground state is expressed
within a cumulant formalism which has been successfully applied to study
magnetic properties of the weakly doped system. Within this approach the ground
state contains independent spin-bag quasiparticles (magnetic polarons). We
present results for the charge-density response function and for the optical
conductivity at zero temperature for different values of t/J. They agree well
with numerical results calculated by exact diagonalization techniques. The
density response function for intermediate and large momenta shows a broad
continuum on energy scales of order of several t whereas the optical
conductivity for \omega > 0 is dominated by low energy excitations (at 1.5 - 2
J). We show that these weak-doping properties can be well understood by
transitions between excited states of spin-bag quasiparticles.Comment: 10 pages, 5 figs., to appear in Europ. Phys. J.
Modularity of Calabi-Yau varieties
In this paper we discuss recent progress on the modularity of Calabi-Yau
varieties. We focus mostly on the case of surfaces and threefolds. We will also
discuss some progress on the structure of the L-function in connection with
mirror symmetry. Finally, we address some questions and open problems.Comment: Further references adde
Sodium chloride transport of normal and dietary enlarged rat cecum in vitro
Sodium chloride transport across isolated cecum mucosa was investigated in normal rats and rats with adaptive cecum growth induced by dietary polyethylene glycol (PEG). The normal cecum absorbed CI in excess of Na with a small short-circuit current (Isc). Dietary adaptation led to large equivalent increments of Na and Cl net absorption without adequate Ise change. Inhibitor studies (mucosal amiloride 10(-3) and 10(-4) M; mucosal 4,4-diisolhiocyanatostilbene-2,2-disulfonic acid 5 x 10(-5) M;serosal furosemide 10(-3) M;serosal ouabain 10(-3) M) suggested that normal cecal NaCl absorption involves electroneutral Na/N and Cl/HCO3 exchange at the apical and Na-K-ATPase-mediated exit across the basolateral cell membrane. Dietary adaptation stimulates the loosely coupled antiports and possibly activates a small serosally located NaCl cotransport. Comparative histology showed flattening of all tissue layers and widening of crypts in PEG animals. Crypt widening may facilitate ion access to underutilized transport sites and, at least in part, explain the increased absorption of the enlarged cecum
The temperature-mass relation in magnetized galaxy clusters
We use cosmological, magneto-hydrodynamic simulations of galaxy clusters to
quantify the dynamical importance of magnetic fields in these clusters. The
set-up of initial magnetic field strengths at high redshifts is chosen such
that observed Faraday-rotation measurements in low-redshift clusters are well
reproduced in the simulations. We compute the radial profiles of the
intracluster gas temperature and of the thermal and magnetic pressure in a set
of clusters simulated in the framework of an Einstein-de Sitter and a
low-density, spatially-flat CDM cosmological model. We find that, for a
realistic range of initial magnetic field strengths, the temperature of the
intracluster gas changes by less than .Comment: Accepted for publication in A&
This Far, No Further: Introducing Virtual Borders to Mobile Robots Using a Laser Pointer
We address the problem of controlling the workspace of a 3-DoF mobile robot.
In a human-robot shared space, robots should navigate in a human-acceptable way
according to the users' demands. For this purpose, we employ virtual borders,
that are non-physical borders, to allow a user the restriction of the robot's
workspace. To this end, we propose an interaction method based on a laser
pointer to intuitively define virtual borders. This interaction method uses a
previously developed framework based on robot guidance to change the robot's
navigational behavior. Furthermore, we extend this framework to increase the
flexibility by considering different types of virtual borders, i.e. polygons
and curves separating an area. We evaluated our method with 15 non-expert users
concerning correctness, accuracy and teaching time. The experimental results
revealed a high accuracy and linear teaching time with respect to the border
length while correctly incorporating the borders into the robot's navigational
map. Finally, our user study showed that non-expert users can employ our
interaction method.Comment: Accepted at 2019 Third IEEE International Conference on Robotic
Computing (IRC), supplementary video: https://youtu.be/lKsGp8xtyI
Virtual Borders: Accurate Definition of a Mobile Robot's Workspace Using Augmented Reality
We address the problem of interactively controlling the workspace of a mobile
robot to ensure a human-aware navigation. This is especially of relevance for
non-expert users living in human-robot shared spaces, e.g. home environments,
since they want to keep the control of their mobile robots, such as vacuum
cleaning or companion robots. Therefore, we introduce virtual borders that are
respected by a robot while performing its tasks. For this purpose, we employ a
RGB-D Google Tango tablet as human-robot interface in combination with an
augmented reality application to flexibly define virtual borders. We evaluated
our system with 15 non-expert users concerning accuracy, teaching time and
correctness and compared the results with other baseline methods based on
visual markers and a laser pointer. The experimental results show that our
method features an equally high accuracy while reducing the teaching time
significantly compared to the baseline methods. This holds for different border
lengths, shapes and variations in the teaching process. Finally, we
demonstrated the correctness of the approach, i.e. the mobile robot changes its
navigational behavior according to the user-defined virtual borders.Comment: Accepted on 2018 IEEE/RSJ International Conference on Intelligent
Robots and Systems (IROS), supplementary video: https://youtu.be/oQO8sQ0JBR
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