The Weil-Petersson gradient flow of renormalized volume and 3-dimensional convex cores

In this paper, we use the Weil-Petersson gradient flow for renormalized volume to study the space $CC(N;S,X)$ of convex cocompact hyperbolic structures on the relatively acylindrical 3-manifold $(N;S)$. Among the cases of interest are the deformation space of an acylindrical manifold and the Bers slice of quasi-Fuchsian space associated to a fixed surface. To treat the possibility of degeneration along flow-lines to peripherally cusped structures, we introduce a surgery procedure to yield a surgered gradient flow that limits to the unique structure $M_{\rm geod} \in CC(N;S,X)$ with totally geodesic convex core boundary facing $S$. Analyzing the geometry of structures along a flow line, we show that if $V_R(M)$ is the renormalized volume of $M$, then $V_R(M)-V_R(M_{\rm geod})$ is bounded below by a linear function of the Weil-Petersson distance $d_{\rm WP}(\partial_c M, \partial_c M_{\rm geod})$, with constants depending only on the topology of $S$. The surgered flow gives a unified approach to a number of problems in the study of hyperbolic 3-manifolds, providing new proofs and generalizations of well-known theorems such as Storm's result that $M_{\rm geod}$ has minimal volume for $N$ acylindrical and the second author's result comparing convex core volume and Weil-Petersson distance for quasifuchsian manifolds

Thermal and dynamic behaviour of supraglacial clasts and the origin of sorting in supraglacial debris covers

The transition zone from a discontinuous to a continuous debris cover is an extensive part of many glacier ablation zones. Although responsible for the highest specific melt rates of debris-covered glaciers, transition zones have received little research and are poorly understood. Here we consider the interactions between emergent clasts and melting ice surfaces at Glacier d'Estelette and Miage Glacier (Italian Alps). Debris-ice interactions are complex because dispersed heterogenous debris both enhances and retards melt rate in the same locality, depending on the distribution of clast sizes. Observations reveal that thermal and dynamic clast interactions with the glacier surface increase the transport rate of coarse clasts, and initiate vertical sorting at the point when a continuous debris layer forms. This happens because, in summer, clasts exceeding the critical thickness for melt slide over the glacier surface. In contrast finer thermally-embedded material is transported at ice surface velocity and become covered by coarser material from upslope. Once established, debris-cover texture allows sorting to develop as the cover thickens downglacier. A two-layer temperature profile results, in which a coarse, drier clast layer of low thermal conductivity overlies a finer-grained, moist layer of higher thermal conductivity. Transition-zone processes establish inverse grading at the initiation of a debris cover, allowing subsequent sorting to operate as the cover thickens downstream. The processes by which this occurs are unknown, but analogy with periglacial active layers suggests convection within a frost-susceptible lower fine layer and eluviation of fines supplied by aeolian deposition and in-situ clast distintegratio

Building a Culture of Evidence for Community College Student Success: Early Progress in the Achieving the Dream Initiative

Achieving the Dream is a multiyear, national initiative, launched by Lumina Foundation for Education, to help community college students stay in school and succeed. The 82 participating colleges commit to collecting and analyzing data to improve student outcomes, particularly for low-income students and students of color. This baseline report describes the early progress that the first 27 colleges have made after just one year of implementation

Improving Decision Speed, Accuracy and Group Cohesion through Early Information Gathering in House-Hunting Ants

BACKGROUND: Successful collective decision-making depends on groups of animals being able to make accurate choices while maintaining group cohesion. However, increasing accuracy and/or cohesion usually decreases decision speed and vice-versa. Such trade-offs are widespread in animal decision-making and result in various decision-making strategies that emphasize either speed or accuracy, depending on the context. Speed-accuracy trade-offs have been the object of many theoretical investigations, but these studies did not consider the possible effects of previous experience and/or knowledge of individuals on such trade-offs. In this study, we investigated how previous knowledge of their environment may affect emigration speed, nest choice and colony cohesion in emigrations of the house-hunting ant Temnothorax albipennis, a collective decision-making process subject to a classical speed-accuracy trade-off. METHODOLOGY/PRINCIPAL FINDINGS: Colonies allowed to explore a high quality nest site for one week before they were forced to emigrate found that nest and accepted it faster than emigrating naïve colonies. This resulted in increased speed in single choice emigrations and higher colony cohesion in binary choice emigrations. Additionally, colonies allowed to explore both high and low quality nest sites for one week prior to emigration remained more cohesive, made more accurate decisions and emigrated faster than emigrating naïve colonies. CONCLUSIONS/SIGNIFICANCE: These results show that colonies gather and store information about available nest sites while their nest is still intact, and later retrieve and use this information when they need to emigrate. This improves colony performance. Early gathering of information for later use is therefore an effective strategy allowing T. albipennis colonies to improve simultaneously all aspects of the decision-making process--i.e. speed, accuracy and cohesion--and partly circumvent the speed-accuracy trade-off classically observed during emigrations. These findings should be taken into account in future studies on speed-accuracy trade-offs

Do Gamma-Ray Burst Sources Repeat?

The demonstration of repeated gamma-ray bursts from an individual source would severely constrain burst source models. Recent reports (Quashnock and Lamb 1993; Wang and Lingenfelter 1993) of evidence for repetition in the first BATSE burst catalog have generated renewed interest in this issue. Here, we analyze the angular distribution of 585 bursts of the second BATSE catalog (Meegan et al. 1994). We search for evidence of burst recurrence using the nearest and farthest neighbor statistic and the two-point angular correlation function. We find the data to be consistent with the hypothesis that burst sources do not repeat; however, a repeater fraction of up to about 20% of the observed bursts cannot be excluded.Comment: ApJ Letters, in press, 13 pages, including three embedded figures. uuencoded Unix-compressed PostScrip

System data communication structures for active-control transport aircraft, volume 2

The application of communication structures to advanced transport aircraft are addressed. First, a set of avionic functional requirements is established, and a baseline set of avionics equipment is defined that will meet the requirements. Three alternative configurations for this equipment are then identified that represent the evolution toward more dispersed systems. Candidate communication structures are proposed for each system configuration, and these are compared using trade off analyses; these analyses emphasize reliability but also address complexity. Multiplex buses are recognized as the likely near term choice with mesh networks being desirable for advanced, highly dispersed systems

System data communication structures for active-control transport aircraft, volume 1

Candidate data communication techniques are identified, including dedicated links, local buses, broadcast buses, multiplex buses, and mesh networks. The design methodology for mesh networks is then discussed, including network topology and node architecture. Several concepts of power distribution are reviewed, including current limiting and mesh networks for power. The technology issues of packaging, transmission media, and lightning are addressed, and, finally, the analysis tools developed to aid in the communication design process are described. There are special tools to analyze the reliability and connectivity of networks and more general reliability analysis tools for all types of systems