Semiclassical String Solutions on 1/2 BPS Geometries

We study semiclassical string solutions on the 1/2 BPS geometry of type IIB string theory characterized by concentric rings on the boundary plane. We consider both folded rotating strings carrying nonzero R-charge and circular pulsating strings. We find that unlike rotating strings, as far as circular pulsating strings are concerned, the dynamics remains qualitatively unchanged when the concentric rings replace AdS_5\times S^5. Using the gravity dual we have also studied the Wilson loop of the corresponding gauge theory. The result is qualitatively the same as that in AdS_5\times S^5 in the global coordinates where the corresponding gauge theory is defined on S^3\times R. We show that there is a correction to 1/L leading order behavior of the potential between external objects.Comment: 17 pages, latex file, v2: refs. adde

Who Goes to College? Differential Enrollment by Race and Family Background

While trends in college enrollment for blacks and whites have been the subject of study for a number of years, little attention has been paid to the variation in college enrollment by socioeconomic status (SES). It is well documented that, controlling for family background, blacks are more likely to enroll in college than whites. This relationship is somewhat deceptive, however. Upon closer examination, we find that blacks are more likely to enroll in college than their white counterparts only among low-SES individuals. Among high SES individuals, this pattern is reversed. We also find that this relationship is strongest in the 1970s and appears to disappear over time; by the 1990s, blacks are no more likely to attend college than whites at any end of the SES distribution. This paper first documents this phenomenon and then attempts to understand what is driving these differences across the distribution of family background characteristics and why the relationship is changing over time. Although they have a significant impact on college enrollment behavior, tuition costs and local labor markets explain very little of racial differences in college entry. We do uncover different responses to tuition and labor markets by individuals from different ends of the SES distribution, an important consideration for policies targeted at improving college enrollment for low-SES individuals.

Safe Robotic Grasping: Minimum Impact-Force Grasp Selection

This paper addresses the problem of selecting from a choice of possible grasps, so that impact forces will be minimised if a collision occurs while the robot is moving the grasped object along a post-grasp trajectory. Such considerations are important for safety in human-robot interaction, where even a certified "human-safe" (e.g. compliant) arm may become hazardous once it grasps and begins moving an object, which may have significant mass, sharp edges or other dangers. Additionally, minimising collision forces is critical to preserving the longevity of robots which operate in uncertain and hazardous environments, e.g. robots deployed for nuclear decommissioning, where removing a damaged robot from a contaminated zone for repairs may be extremely difficult and costly. Also, unwanted collisions between a robot and critical infrastructure (e.g. pipework) in such high-consequence environments can be disastrous. In this paper, we investigate how the safety of the post-grasp motion can be considered during the pre-grasp approach phase, so that the selected grasp is optimal in terms applying minimum impact forces if a collision occurs during a desired post-grasp manipulation. We build on the methods of augmented robot-object dynamics models and "effective mass" and propose a method for combining these concepts with modern grasp and trajectory planners, to enable the robot to achieve a grasp which maximises the safety of the post-grasp trajectory, by minimising potential collision forces. We demonstrate the effectiveness of our approach through several experiments with both simulated and real robots.Comment: To be appeared in IEEE/RAS IROS 201

Quasi-Normal Modes of Extremal BTZ Black Holes in TMG

We study the spectrum of tensor perturbations on extremal BTZ black holes in topologically massive gravity for arbitrary values of the coefficient of the Chern-Simons term, $\mu$. Imposing proper boundary conditions at the boundary of the space and at the horizon, we find that the spectrum contains quasi-normal modes.Comment: 18 pages, Latex, Ref's added, Typos correcte