3,475 research outputs found
Conservation Laws in Cellular Automata
If X is a discrete abelian group and B a finite set, then a cellular
automaton (CA) is a continuous map F:B^X-->B^X that commutes with all X-shifts.
If g is a real-valued function on B, then, for any b in B^X, we define G(b) to
be the sum over all x in X of g(b_x) (if finite). We say g is `conserved' by F
if G is constant under the action of F. We characterize such `conservation
laws' in several ways, deriving both theoretical consequences and practical
tests, and provide a method for constructing all one-dimensional CA exhibiting
a given conservation law.Comment: 19 pages, LaTeX 2E with one (1) Encapsulated PostScript figure. To
appear in Nonlinearity. (v2) minor changes/corrections; new references added
to bibliograph
Class Attendance and Students’ Evaluations of Teaching: Do No-Shows Bias Course Ratings and Rankings?
Background: Many university departments use students’ evaluations of teaching (SET) to compare and rank courses. However, absenteeism from class is often nonrandom and, therefore, SET for different courses might not be comparable. Objective: The present study aims to answer two questions. Are SET positively biased due to absenteeism? Do procedures, which adjust for absenteeism, change course rankings? Research Design: The author discusses the problem from a missing data perspective and present empirical results from regression models to determine which factors are simultaneously associated with students’ class attendance and course ratings. In order to determine the extent of these biases, the author then corrects average ratings for students’ absenteeism and inspect changes in course rankings resulting from this adjustment. Subjects: The author analyzes SET data on the individual level. One or more course ratings are available for each student. Measures: Individual course ratings and absenteeism served as the key outcomes. Results: Absenteeism decreases with rising teaching quality. Furthermore, both factors are systematically related to student and course attributes. Weighting students’ ratings by actual absenteeism leads to mostly small changes in ranks, which follow a power law. Only a few, average courses are disproportionally influenced by the adjustment. Weighting by predicted absenteeism leads to very small changes in ranks. Again, average courses are more strongly affected than courses of very high or low in quality. Conclusions: No-shows bias course ratings and rankings. SET are more appropriate to identify high- and low-quality courses than to determine the exact ranks of average courses
A realistic two-lane traffic model for highway traffic
A two-lane extension of a recently proposed cellular automaton model for
traffic flow is discussed. The analysis focuses on the reproduction of the lane
usage inversion and the density dependence of the number of lane changes. It is
shown that the single-lane dynamics can be extended to the two-lane case
without changing the basic properties of the model which are known to be in
good agreement with empirical single-vehicle data. Therefore it is possible to
reproduce various empirically observed two-lane phenomena, like the
synchronization of the lanes, without fine-tuning of the model parameters
Surface Alfven Wave Damping in a 3D Simulation of the Solar Wind
Here we investigate the contribution of surface Alfven wave damping to the
heating of the solar wind in minima conditions. These waves are present in
regions of strong inhomogeneities in density or magnetic field (e. g., the
border between open and closed magnetic field lines). Using a 3-dimensional
Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping
contribution between 1-4 solar radii, the region of interest for both
acceleration and coronal heating. We consider waves with frequencies lower than
those that are damped in the chromosphere and on the order of those dominating
the heliosphere. In the region between open and closed field lines, within a
few solar radii of the surface, no other major source of damping has been
suggested for the low frequency waves we consider here. This work is the first
to study surface Alfven waves in a 3D environment without assuming a priori a
geometry of field lines or magnetic and density profiles. We determine that
waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In
quiet sun regions, surface Alfven waves are damped at further distances
compared to active regions, thus carrying additional wave energy into the
corona. We compare the surface Alfven wave contribution to the heating by a
variable polytropic index and find that it an order of magnitude larger than
needed for quiet sun regions. For active regions the contribution to the
heating is twenty percent. As it has been argued that a variable gamma acts as
turbulence, our results indicate that surface Alfven wave damping is comparable
to turbulence in the lower corona. This damping mechanism should be included
self consistently as an energy driver for the wind in global MHD models.Comment: Accepted to ApJ (scheduled September '09), 22 pages, 8 figure
Do high fetal catecholamine levels affect heart rate variability and tneconiutn passage during labour?
Objectives. To deternrine the relationship between Umbilical arterial catecholamine levels and fetal heart rate variability and meconium passage.Study design. A prospective descriptive study was perfonned. Umbilical artery catecholamine levels were measured in 55 newborns and correlated with fetal heart rate before delivery, Umbilical arterial pH, base excess and the presence of meconum-stained liquor.Results and conclusion. The range of catecholanrine levels was enonnous, with very high epinephrine or norepinephrine levels in several fetuses. We were unable to demonstrate an association between high catecholamine levels and the presence of nonnal fetal heart rate variability despite acidaemia. We postulate that high catecholamine levels may inhibit fetal meconiUITl passage
Anisotropic effect on two-dimensional cellular automaton traffic flow with periodic and open boundaries
By the use of computer simulations we investigate, in the cellular automaton
of two-dimensional traffic flow, the anisotropic effect of the probabilities of
the change of the move directions of cars, from up to right () and from
right to up (), on the dynamical jamming transition and velocities
under the periodic boundary conditions in one hand and the phase diagram under
the open boundary conditions in the other hand. However, in the former case,
the first order jamming transition disappears when the cars alter their
directions of move ( and/or ). In the open boundary
conditions, it is found that the first order line transition between jamming
and moving phases is curved. Hence, by increasing the anisotropy, the moving
phase region expand as well as the contraction of the jamming phase one.
Moreover, in the isotropic case, and when each car changes its direction of
move every time steps (), the transition from the jamming
phase (or moving phase) to the maximal current one is of first order.
Furthermore, the density profile decays, in the maximal current phase, with an
exponent .}Comment: 13 pages, 22 figure
Ca2+ signaling modulates cytolytic T lymphocyte effector functions
Cytolytic T cells use two mechanisms to kill virally infected cells, tumor cells, or other potentially autoreactive T cells in short-term in vitro assays. The perforin/granule exocytosis mechanism uses preformed cytolytic granules that are delivered to the target cell to induce apoptosis and eventual lysis. FasL/Fas (CD95 ligand/CD95)–mediated cytolysis requires de novo protein synthesis of FasL by the CTL and the presence of the death receptor Fas on the target cell to induce apoptosis. Using a CD8+ CTL clone that kills via both the perforin/granule exocytosis and FasL/Fas mechanisms, and a clone that kills via the FasL/Fas mechanism only, we have examined the requirement of intra- and extracellular Ca2+ in TCR-triggered cytolytic effector function. These two clones, a panel of Ca2+ antagonists, and agonists were used to determine that a large biphasic increase in intracellular calcium concentration, characterized by release of Ca2+ from intracellular stores followed by a sustained influx of extracellular Ca2+, is required for perforin/granule exocytosis. Only the sustained influx of extracellular Ca2+ is required for FasL induction and killing. Thapsigargin, at low concentrations, induces this small but sustained increase in [Ca2+]i and selectively induces FasL/Fas-mediated cytolysis but not granule exocytosis. These results further define the role of Ca2+ in perforin and FasL/Fas killing and demonstrate that differential Ca2+ signaling can modulate T cell effector functions
Measurement of polarization-transfer to bound protons in carbon and its virtuality dependence
We measured the ratio of the transverse to longitudinal
components of polarization transferred from electrons to bound protons in
by the process at the
Mainz Microtron (MAMI). We observed consistent deviations from unity of this
ratio normalized to the free-proton ratio,
, for both -
and -shell knocked out protons, even though they are embedded in averaged
local densities that differ by about a factor of two. The dependence of the
double ratio on proton virtuality is similar to the one for knocked out protons
from and , suggesting a universal behavior.
It further implies no dependence on average local nuclear density
- …