1,147 research outputs found
Old and New Fields on Super Riemann Surfaces
The ``new fields" or ``superconformal functions" on super Riemann
surfaces introduced recently by Rogers and Langer are shown to coincide with
the Abelian differentials (plus constants), viewed as a subset of the functions
on the associated super Riemann surface. We confirm that, as originally
defined, they do not form a super vector space.Comment: 9 pages, LaTex. Published version: minor changes for clarity, two new
reference
Vortices and the entrainment transition in the 2D Kuramoto model
We study synchronization in the two-dimensional lattice of coupled phase
oscillators with random intrinsic frequencies. When the coupling is larger
than a threshold , there is a macroscopic cluster of
frequency-synchronized oscillators. We explain why the macroscopic cluster
disappears at . We view the system in terms of vortices, since cluster
boundaries are delineated by the motion of these topological defects. In the
entrained phase (), vortices move in fixed paths around clusters, while
in the unentrained phase (), vortices sometimes wander off. These
deviant vortices are responsible for the disappearance of the macroscopic
cluster. The regularity of vortex motion is determined by whether clusters
behave as single effective oscillators. The unentrained phase is also
characterized by time-dependent cluster structure and the presence of chaos.
Thus, the entrainment transition is actually an order-chaos transition. We
present an analytical argument for the scaling for small
lattices, where is the threshold for phase-locking. By also deriving the
scaling , we thus show that for small , in
agreement with numerics. In addition, we show how to use the linearized model
to predict where vortices are generated.Comment: 11 pages, 8 figure
Universality in the one-dimensional chain of phase-coupled oscillators
We apply a recently developed renormalization group (RG) method to study
synchronization in a one-dimensional chain of phase-coupled oscillators in the
regime of weak randomness. The RG predicts how oscillators with randomly
distributed frequencies and couplings form frequency-synchronized clusters.
Although the RG was originally intended for strong randomness, i.e. for
distributions with long tails, we find good agreement with numerical
simulations even in the regime of weak randomness. We use the RG flow to derive
how the correlation length scales with the width of the coupling distribution
in the limit of large coupling. This leads to the identification of a
universality class of distributions with the same critical exponent . We
also find universal scaling for small coupling. Finally, we show that the RG
flow is characterized by a universal approach to the unsynchronized fixed
point, which provides physical insight into low-frequency clusters.Comment: 14 pages, 10 figure
Atmospheric circulation and cyclone frequency variations linked to the primary modes of Greenland snow accumulation
Data from 34 Greenland firn cores, extending from 1982 to 1996, are used to identify spatial accumulation variability patterns and their associated atmospheric circulation and cyclone frequencies. The first principal component, representing west-central Greenland accumulation, is correlated to NAO variability, having increased southwesterly (northeasterly) flow over that area during high (low) accumulation winters. The flow is linked to a relative increase in cyclone activity on the west central region of the ice sheet during high accumulation periods. The second principal component represents accumulation over southeastern Greenland where strong westerly flow leads to high accumulation and an increase in lee cyclones on the east and southeast coast. The study provides evidence that increased cyclone activity occurs over, or immediately adjacent to, areas experiencing anomalously high accumulation and it is important to distinguish lee cyclones from ‘‘Icelandic’’ cyclones, as they produce opposite precipitation effects over the ice sheet
Characterization of wild and captive baboon gut microbiota and their antibiotic resistomes
Antibiotic exposure results in acute and persistent shifts in the composition and function of microbial communities associated with vertebrate hosts. However, little is known about the state of these communities in the era before the widespread introduction of antibiotics into clinical and agricultural practice. We characterized the fecal microbiota and antibiotic resistomes of wild and captive baboon populations to understand the effect of human exposure and to understand how the primate microbiota may have been altered during the antibiotic era. We used culture-independent and bioinformatics methods to identify functional resistance genes in the guts of wild and captive baboons and show that exposure to humans is associated with changes in microbiota composition and resistome expansion compared to wild baboon groups. Our results suggest that captivity and lifestyle changes associated with human contact can lead to marked changes in the ecology of primate gut communities.Environmental microbes have harbored the capacity for antibiotic production for millions of years, spanning the evolution of humans and other vertebrates. However, the industrial-scale use of antibiotics in clinical and agricultural practice over the past century has led to a substantial increase in exposure of these agents to human and environmental microbiota. This perturbation is predicted to alter the ecology of microbial communities and to promote the evolution and transfer of antibiotic resistance (AR) genes. We studied wild and captive baboon populations to understand the effects of exposure to humans and human activities (e.g., antibiotic therapy) on the composition of the primate fecal microbiota and the antibiotic-resistant genes that it collectively harbors (the “resistome”). Using a culture-independent metagenomic approach, we identified functional antibiotic resistance genes in the gut microbiota of wild and captive baboon groups and saw marked variation in microbiota architecture and resistomes across habitats and lifeways. Our results support the view that antibiotic resistance is an ancient feature of gut microbial communities and that sharing habitats with humans may have important effects on the structure and function of the primate microbiota
List Mode EM Reconstruction of Compton Scatter Camera Images in 3-D
A method has been developed for List Mode EM reconstruction of Compton scattering camera images in 3D, using a previously reported 2-D technique and refining and adapting it to three dimensions. Spatial variation in the system sensitivity is determined by an approximate numerical integration which accounts for solid angle effects, absorption and escape probabilities, and variation in the differential angular scattering cross section. The method for computing the system transition probabilities uses a similar method to determine values in pixels along exact back-projected cones for each detected event, and uses pre-computed values of the inherent system resolution (which includes the effects of spatial and energy measurement resolution and Doppler broadening) to model the response in pixels neighboring the back-projected cone. The algorithm has been parallelized, permitting reconstruction of images using larger number of detected events in relatively constant time by adding additional processors. Results are presented using 3-D simulated data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85814/1/Fessler162.pd
Preliminary Studies on the Feasibility of Addition of Vertex View to Conventional Brain SPECT
We have investigated the improvement in resolution and sensitivity for brain imaging which would result by the addition of a single stationary vertex view to the tomographic data. This method has the practical advantage of being relatively inexpensive and easy to implement. The uniform Cramer Rao bound is a plot of the minimum achievable standard deviation for estimating the pixel intensity as a function of the bias gradient length. Uniform CR bound analysis indicated an improvement in performance when the vertex detector is added, especially for centrally located pixels for which improvement is seen over the useful depth for brain imaging. Simulation experiments were done with a simple six slice phantom and with the Hoffman brain phantom. Visual inspection of the reconstructed images showed improved resolution and noise characteristics over reconstructed images without the vertex data. Quantitatively, substantial reduction in mean square error was observed for a plane close to the vertex detector. Improvement reduced as distance from the vertex detector is increased. Background activities inside the field of view of the vertex detector but not the tomograph were represented by several blobs of activity on a plane lying outside the reconstruction volume. This activity was estimated by 3D spline fitting jointly with the image reconstruction process. Adding the vertex view to conventional brain SPECT should lead to improved cortical imaging, and to moderate improvement for deep structures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85857/1/Fessler142.pd
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