Critical Phenomena Associated with Boson Stars

We present a brief synopsis of related work (gr-qc/0007039), describing a study of black hole threshold phenomena for a self-gravitating, massive complex scalar field in spherical symmetry. We construct Type I critical solutions dynamically by tuning a one-parameter family of initial data consisting of a boson star and a massless real scalar field, and numerically evolving this data. The resulting critical solutions appear to correspond to boson stars on the unstable branch, as we show via comparisons between our simulations and perturbation theory. For low-mass critical solutions, we find small ``halos'' of matter in the tails of the solutions, and these distort the profiles which otherwise agree with unstable boson stars. These halos seem to be artifacts of the collisions between the original boson stars and the massless fields, and do not appear to belong to the true critical solutions. From this study, it appears that unstable boson stars are unstable to dispersal (``explosion'') in addition to black hole formation. Given the similarities in macroscopic stability between boson stars and neutron stars, we suggest that similar phenomena could occur in models of neutron stars.Comment: 6 Pages, 5 Figures, LaTeX. To appear in Proceedings of the 20th Texas Symposium on Relativistic Astrophysics (Dec 9-15, 2000

A Nonparametric Bayesian Approach to Uncovering Rat Hippocampal Population Codes During Spatial Navigation

Rodent hippocampal population codes represent important spatial information about the environment during navigation. Several computational methods have been developed to uncover the neural representation of spatial topology embedded in rodent hippocampal ensemble spike activity. Here we extend our previous work and propose a nonparametric Bayesian approach to infer rat hippocampal population codes during spatial navigation. To tackle the model selection problem, we leverage a nonparametric Bayesian model. Specifically, to analyze rat hippocampal ensemble spiking activity, we apply a hierarchical Dirichlet process-hidden Markov model (HDP-HMM) using two Bayesian inference methods, one based on Markov chain Monte Carlo (MCMC) and the other based on variational Bayes (VB). We demonstrate the effectiveness of our Bayesian approaches on recordings from a freely-behaving rat navigating in an open field environment. We find that MCMC-based inference with Hamiltonian Monte Carlo (HMC) hyperparameter sampling is flexible and efficient, and outperforms VB and MCMC approaches with hyperparameters set by empirical Bayes

DID THE VOLUNTARY PRICE REPORTING SYSTEM FAIL TO PROVIDE PRICE TRANSPARENCY IN THE CASH MARKET FOR DRESSED STEERS: EVIDENCE FROM SOUTH DAKOTA

The information value of the former USDA voluntary price reporting system is investigated for dressed weight slaughter steers. The ability of the former system to promote market transparency and price discovery in the cash market is evaluated with state level mandatory price reporting data collected from September 1999 to April 2001. The empirical framework evaluates the informational value of public price reports according to the criteria established in the market integration literature. The empirical results indicate that in the cash market for dressed weight steers, in South Dakota and Nebraska the former voluntary price reporting system did foster market transparency and aided in the price discovery process.Livestock Production/Industries, Marketing,

The Voluntary Reporting System's Ability to Provide Price Transparency in the Cash Market for Dressed Steers: Evidence from South Dakota

The informational value of USDA's former voluntary price reporting system is investigated for dressed-weight slaughter steers sold by South Dakota producers. The ability of the former system to promote price transparency in the cash market is evaluated using state-level mandatory price reporting data collected from September 1999 to April 2001. The empirical framework examines the informational value of public price reports according to the criteria established in the market integration literature. The empirical results indicate that in the South Dakota cash market for dressed weight steers, the voluntary price reporting system fostered price transparency, and thus contributed to the price discovery process. Empirical evidence is also presented suggesting that strategic price reporting by market participants to influence the voluntary price reporting system was not detected during the period covered in this study.cointegration, competitive spatial equilibrium, error correction model, mandatory price reporting, marketing integration, price transparency, slaughter cattle spot market, voluntary price reporting, Demand and Price Analysis,

A sharp-front moving boundary model for malignant invasion

We analyse a novel mathematical model of malignant invasion which takes the form of a two-phase moving boundary problem describing the invasion of a population of malignant cells into a population of background tissue, such as skin. Cells in both populations undergo diffusive migration and logistic proliferation. The interface between the two populations moves according to a two-phase Stefan condition. Unlike many reaction-diffusion models of malignant invasion, the moving boundary model explicitly describes the motion of the sharp front between the cancer and surrounding tissues without needing to introduce degenerate nonlinear diffusion. Numerical simulations suggest the model gives rise to very interesting travelling wave solutions that move with speed $c$, and the model supports both malignant invasion and malignant retreat, where the travelling wave can move in either the positive or negative $x$-directions. Unlike the well-studied Fisher-Kolmogorov and Porous-Fisher models where travelling waves move with a minimum wave speed $c \ge c^* > 0$, the moving boundary model leads to travelling wave solutions with $|c| < c^{**}$. We interpret these travelling wave solutions in the phase plane and show that they are associated with several features of the classical Fisher-Kolmogorov phase plane that are often disregarded as being nonphysical. We show, numerically, that the phase plane analysis compares well with long time solutions from the full partial differential equation model as well as providing accurate perturbation approximations for the shape of the travelling waves.Comment: 48 pages, 21 figure

Behavioral biases when viewing multiplexed scenes:scene structure and frames of reference for inspection

Where people look when viewing a scene has been a much explored avenue of vision research (e.g., see Tatler, 2009). Current understanding of eye guidance suggests that a combination of high and low-level factors influence fixation selection (e.g., Torralba et al., 2006), but that there are also strong biases toward the center of an image (Tatler, 2007). However, situations where we view multiplexed scenes are becoming increasingly common, and it is unclear how visual inspection might be arranged when content lacks normal semantic or spatial structure. Here we use the central bias to examine how gaze behavior is organized in scenes that are presented in their normal format, or disrupted by scrambling the quadrants and separating them by space. In Experiment 1, scrambling scenes had the strongest influence on gaze allocation. Observers were highly biased by the quadrant center, although physical space did not enhance this bias. However, the center of the display still contributed to fixation selection above chance, and was most influential early in scene viewing. When the top left quadrant was held constant across all conditions in Experiment 2, fixation behavior was significantly influenced by the overall arrangement of the display, with fixations being biased toward the quadrant center when the other three quadrants were scrambled (despite the visual information in this quadrant being identical in all conditions). When scenes are scrambled into four quadrants and semantic contiguity is disrupted, observers no longer appear to view the content as a single scene (despite it consisting of the same visual information overall), but rather anchor visual inspection around the four separate “sub-scenes.” Moreover, the frame of reference that observers use when viewing the multiplex seems to change across viewing time: from an early bias toward the display center to a later bias toward quadrant centers

On the factors causing processing difficulty of multiple-scene displays

Multiplex viewing of static or dynamic scenes is an increasing feature of screen media. Most existing multiplex experiments have examined detection across increasing scene numbers, but currently no systematic evaluation of the factors that might produce difficulty in processing multiplexes exists. Across five experiments we provide such an evaluation. Experiment 1 characterises difficulty in change detection when the number of scenes is increased. Experiment 2 reveals that the increased difficulty across multiple-scene displays is caused by the total amount of visual information accounts for differences in change detection times, regardless of whether this information is presented across multiple scenes, or contained in one scene. Experiment 3 shows that whether quadrants of a display were drawn from the same, or different scenes did not affect change detection performance. Experiment 4 demonstrates that knowing which scene the change will occur in means participants can perform at monoplex level. Finally, Experiment 5 finds that changes of central interest in multiplexed scenes are detected far easier than marginal interest changes to such an extent that a centrally interesting object removal in nine screens is detected more rapidly than a marginally interesting object removal in four screens. Processing multiple-screen displays therefore seems dependent on the amount of information, and the importance of that information to the task, rather than simply the number of scenes in the display. We discuss the theoretical and applied implications of these findings

Public Price Reporting in the Cash Market for Live Cattle: A Spatial Market Approach

Legislative authorization for the Livestock Mandatory Reporting Act of 1999 was renewed in October of 2006. One of the cited justifications for implementing mandatory reporting was that the voluntary reporting system for the slaughter cattle cash market was unable to provide accurate and timely market information. We extend the spatial market analysis literature by developing a methodology for detecting distortions in spatial relationships across related price series. Using spatially linked regional markets, we compare state-level mandatory price-reporting data to the U.S. Department of Agriculture voluntarily reported state data to determine if the spatial relationship between price-reporting mechanisms was disrupted by market distortions prior to implementation of federal mandatory price reporting. We found no empirical evidence of system failure; therefore, we conclude that market thinning or noncompetitive behavior had not reached the level necessary to disrupt the ability of the voluntary price-reporting system to provide timely and accurate price information.public price reporting, spatial markets, market integration, price transparency, price discovery, livestock markets, Livestock Production/Industries, Marketing,