Rank-based estimation for all-pass time series models

An autoregressive-moving average model in which all roots of the autoregressive polynomial are reciprocals of roots of the moving average polynomial and vice versa is called an all-pass time series model. All-pass models are useful for identifying and modeling noncausal and noninvertible autoregressive-moving average processes. We establish asymptotic normality and consistency for rank-based estimators of all-pass model parameters. The estimators are obtained by minimizing the rank-based residual dispersion function given by Jaeckel [Ann. Math. Statist. 43 (1972) 1449--1458]. These estimators can have the same asymptotic efficiency as maximum likelihood estimators and are robust. The behavior of the estimators for finite samples is studied via simulation and rank estimation is used in the deconvolution of a simulated water gun seismogram.Comment: Published at http://dx.doi.org/10.1214/009053606000001316 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

The Enrichment History of Hot Gas in Poor Galaxy Groups

We have analyzed the ASCA SIS and GIS data for seventeen groups and determined the average temperature and abundance of the hot x-ray emitting gas. For groups with gas temperatures less than 1.5 keV we find that the abundance is correlated with the gas temperature and luminosity. We have also determined the abundance of the alpha-elements and iron independently for those groups with sufficient counts. We find that for the cool groups (i.e. kT <1.5 keV) the ratio of alpha-elements to iron is ~1, about half that seen in clusters. Spectral fits with the S, Si and Fe abundances allowed to vary separately suggest the S/Fe ratio is similar to that seen in clusters while the Si/Fe ratio in groups is half the value determined for richer systems. The mass of metals per unit blue luminosity drops rapidly in groups as the temperature drops. There are two possible explanations for this decrease. One is that the star formation in groups is very different from that in rich clusters. The other explanation is that groups lose much of their enriched material via winds during the early evolution of ellipticals. If the latter is true, we find that poor groups will have contributed significantly (roughly 1/3 of the metals) to the enrichment of the intergalactic medium.Comment: 19 Pages with 2 figures, Accepted for publication in the Astrophysical Journa

A Microcantilever Device to Assess the Effect of Force on the Lifetime of Selectin-Carbohydrate Bonds

A microcantilever technique was used to apply force to receptor-ligand molecules involved in leukocyte rolling on blood vessel walls. E-selectin was adsorbed onto 3-μm-diameter, 4-mm-long glass fibers, and the selectin ligand, sialyl Lewisx, was coupled to latex microspheres. After binding, the microsphere and bound fiber were retracted using a computerized loading protocol that combines hydrodynamic and Hookean forces on the fiber to produce a range of force loading rates (force/time), rf. From the distribution of forces at failure, the average force was determined and plotted as a function of ln rf. The slope and intercept of the plot yield the unstressed reverse reaction rate, kro , and a parameter that describes the force dependence of reverse reaction rates, ro. The ligand was titrated so adhesion occurred in ~30% of tests, implying that \u3e80% of adhesive events involve single bonds. Monte Carlo simulations show that this level of multiple bonding has little effect on parameter estimation. The estimates are ro = 0.048 and 0.016 nm and kro = 0.72 and 2.2 s-1 for loading rates in the ranges 200–1000 and 1000–5000 pN s-1, respectively. Levenberg-Marquardt fitting across all values of rf gives ro = 0.034 nm and kro = 0.82 s-1. The values of these parameters are in the range required for rolling, as suggested by adhesive dynamics simulations

Genetic and Neuroanatomical Support for Functional Brain Network Dynamics in Epilepsy

Focal epilepsy is a devastating neurological disorder that affects an overwhelming number of patients worldwide, many of whom prove resistant to medication. The efficacy of current innovative technologies for the treatment of these patients has been stalled by the lack of accurate and effective methods to fuse multimodal neuroimaging data to map anatomical targets driving seizure dynamics. Here we propose a parsimonious model that explains how large-scale anatomical networks and shared genetic constraints shape inter-regional communication in focal epilepsy. In extensive ECoG recordings acquired from a group of patients with medically refractory focal-onset epilepsy, we find that ictal and preictal functional brain network dynamics can be accurately predicted from features of brain anatomy and geometry, patterns of white matter connectivity, and constraints complicit in patterns of gene coexpression, all of which are conserved across healthy adult populations. Moreover, we uncover evidence that markers of non-conserved architecture, potentially driven by idiosyncratic pathology of single subjects, are most prevalent in high frequency ictal dynamics and low frequency preictal dynamics. Finally, we find that ictal dynamics are better predicted by white matter features and more poorly predicted by geometry and genetic constraints than preictal dynamics, suggesting that the functional brain network dynamics manifest in seizures rely on - and may directly propagate along - underlying white matter structure that is largely conserved across humans. Broadly, our work offers insights into the generic architectural principles of the human brain that impact seizure dynamics, and could be extended to further our understanding, models, and predictions of subject-level pathology and response to intervention

THE FUTURE OF INFORMATION SYSTEMS AS AN ACADEMIC FIELD: YOUR FATE IN 1998

The academic study of information systems is dynamic and exciting. It tends to have very fluid boundaries. Researchers in information systems venture into problem areas associated with such diverse fields as computer science, communications, cognitive psychology, and sociology. Information systems are studied in the context of innovation, organizational change, and competitive advantage. The changing technology provides new and revisited opportunities for investigation and problem solving. Until quite recently, the information systems faculty were the custodians in schools of management of most of the technical knowledge of organizational computing. That technical knowledge is being rapidly diffused to the entire faculty. Faculty in accounting at one time fled from computers; they now embrace them. The same is true of other functional areas in schools of management. What will happen to the academic field of information systems when the computer expertise is shared by most faculty members

Students’ Perceptions of Factors that Affect College Funding Decisions

This exploratory study examines the factors that college students perceive are important in helping them make good financial decisions about paying for a college education. The study categorizes and summarizes students\u27 self-reported responses to an openended survey question about recommendations for changes in financial aid counseling practices. The 335 student responses had a recurring theme of better information provided through individual counseling sessions

Some families of density matrices for which separability is easily tested

We reconsider density matrices of graphs as defined in [quant-ph/0406165]. The density matrix of a graph is the combinatorial laplacian of the graph normalized to have unit trace. We describe a simple combinatorial condition (the "degree condition") to test separability of density matrices of graphs. The condition is directly related to the PPT-criterion. We prove that the degree condition is necessary for separability and we conjecture that it is also sufficient. We prove special cases of the conjecture involving nearest point graphs and perfect matchings. We observe that the degree condition appears to have value beyond density matrices of graphs. In fact, we point out that circulant density matrices and other matrices constructed from groups always satisfy the condition and indeed are separable with respect to any split. The paper isolates a number of problems and delineates further generalizations.Comment: 14 pages, 4 figure

Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation

To reach Earth’s surface, magma must ascend from the hot, ductile asthenosphere through cold and brittle rock in the lithosphere. It does so via fluid-filled fractures called dykes. While the continuum mechanics of ductile asthenosphere is well established, there has been little theoretical work on the cold and brittle regime where dyking and faulting occurs. Geodynamic models use plasticity to model fault-like behaviour; plasticity also shows promise for modelling dykes. Here we build on an existing model to develop a poro-viscoelastic–viscoplastic theory for two-phase flow across the lithosphere. Our theory addresses the deficiencies of previous work by incorporating (i) a hyperbolic yield surface, (ii) a plastic potential with control of dilatancy and (iii) a viscous regularization of plastic failure. We use analytical and numerical solutions to investigate the behaviour of this theory. Through idealized models and a comparison to linear elastic fracture mechanics, we demonstrate that this behaviour includes a continuum representation of dyking. Finally, we consider a model scenario reminiscent of continental rifting and demonstrate the consequences of dyke injection into the cold, upper lithosphere: a sharp reduction in the force required to rift

Early career job choice in logistics: Comparing shifts in attribute importance between internship and full-time roles

Maximum difference scaling was used to analyze the importance logistics students attach to 17 job attributes for internships as well as for full-time, entry-level positions. Significant differences in importance were found on nine of the 17 attributes tested. Room for advancement was the most important criterion for full-time positions while atmosphere/work environment was most important for internships. Implications for practitioners, academics and students are discussed. It is believed the results of the current study will provide useful insight to logistics employers to assist them in developing more attractive, entry-level job and internship opportunities and help them communicate more effective recruiting messages