Energy in higher-derivative gravity via topological regularization

Indexación: Scopus.We give a novel definition of gravitational energy for an arbitrary theory of gravity including quadratic-curvature corrections to Einstein's equations. We focus on the theory in four dimensions, in the presence of a negative cosmological constant, and with asymptotically anti-de Sitter (AdS) boundary conditions. As a first example, we compute the gravitational energy and angular momentum of Schwarzschild-AdS black holes, for which we obtain results consistent with previous computations performed using different methods. However, our method is qualitatively different due to the fact that it is intrinsically nonlinear. It relies on the idea of adding to the gravity action topological invariant terms which suffice to regularize the Noether charges and render the variational problem well-posed. This is an idea that has been previously considered in the case of second-order theories, such as general relativity and which, as shown here, extends to higher-derivative theories. Besides black holes, we consider other solutions such as gravitational waves in AdS, for which we also find results that are in agreement. This enables us to investigate the consistency of this approach in the non-Einstein sector of the theory. © 2018 authors. Published by the American Physical Society.https://journals.aps.org/prd/abstract/10.1103/PhysRevD.98.04404

Correlation-Based Tuning of a Restricted-Complexity Controller for an Active Suspension System

A correlation-based controller tuning method is proposed for the ``Design and optimization of restricted-complexity controllers'' benchmark problem. The approach originally proposed for model following is extended to solve the disturbance rejection problem. The idea is to tune the controller parameters such that the closed-loop output be uncorrelated with the disturbance signal. Since perfect decorrelation between the closed-loop output and the disturbance signal is not attainable in the restricted-complexity controller design, the cross correlation between these two signals is minimized iteratively using the stochastic approximation method. Since control specifications can normally be expressed in terms of constraints on the sensitivity functions, a frequency-domain analysis of the criterion is performed. Straightforward implementation of the proposed approach on the active suspension system of the Automatic Control Laboratory of Grenoble (LAG) provides a 2nd-order controller that meets the control specifications very well

Iterative Correlation-Based Controller Tuning: Application to a Magnetic Suspension System

Iterative tuning of the parameters of a restricted-order controller using the data acquired in closed-loop operation seems to be a promising idea, especially for tuning PID controllers in industrial applications. In this paper, a new tuning approach based on decorrelation is proposed. The basic idea is to make the output error between the designed and achieved closed-loop systems uncorrelated with the reference signal. The controller parameters are calculated as the solution to correlation equations involving instrumental variables. Different choices of instrumental variables are proposed and compared via simulation. The stochastic properties of the correlation approach are compared with those of standard IFT using Monte-Carlo simulation. The proposed approach is also implemented on an experimental magnetic suspension system, and excellent performance using only a few real-time experiments is achieved

Torsion induces Gravity

In this work the Poincare-Chern Simons and Anti de Sitter Chern Simons gravities are studied. For both a solution that can be casted as a black hole with manifest torsion is found. Those solutions resemble Schwarzschild and Schwarzschild-AdS solutions respectively.Comment: 4 pages, RevTe

Randomized controlled trial investigating the effect of music on the virtual reality laparoscopic learning performance of novice surgeons

Background: Findings have shown that music affects cognitive performance, but little is known about its influence on surgical performance. The hypothesis of this randomized controlled trial was that arousing (activating) music has a beneficial effect on the surgical performance of novice surgeons in the setting of a laparoscopic virtual reality task. Methods: For this study, 45 junior surgeons with no previous laparoscopic experience were randomly assigned to three equal groups. Group 1 listened to activating music; group 2 listened to deactivating music; and group 3 had no music (control) while each participant solved a surgical task five times on a virtual laparoscopic simulator. The assessed global task score, the total task time, the instrument travel distances, and the surgeons' heart rate were assessed. Results: All surgical performance parameters improved significantly with experience (task repetition). The global score showed a trend for a between-groups difference, suggesting that the group listening to activating music had the worst performance. This observation was supported by a significant between-groups difference for the first trial but not subsequent trials (activating music, 35 points; deactivating music, 66 points; no music, 91 points; p=0.002). The global score (p=0.056) and total task time (p=0.065) showed a trend toward improvement when participants considered the music pleasant rather than unpleasant. Conclusions: Music in the operating theater may have a distracting effect on novice surgeons performing new tasks. Surgical trainers should consider categorically switching off music during teaching procedure

Effect of bisacodyl on postoperative bowel motility in elective colorectal surgery: a prospective, randomized trial

Background: Postoperative ileus is a common condition after abdominal surgery. Many prokinetic drugs have been evaluated including osmotic laxatives. The data on colon-stimulating laxatives are scarce. This prospective, randomized, double-blind trial investigates the effect of the colon-stimulating laxative bisacodyl on postoperative ileus in elective colorectal resections. Materials and methods: Between November 2004 and February 2007, 200 consecutive patients were randomly assigned to receive either bisacodyl or placebo. Primary endpoint was time to gastrointestinal recovery (mean time to first flatus passed, first defecation, and first solid food tolerated; GI-3). Secondary endpoints were incidence and duration of nasogastric tube reinsertion, incidence of vomiting, length of hospital stay, and visual analogue scores for pain, cramps, and nausea. Results: One hundred sixty-nine patients were analyzed, and 31 patients discontinued the study. Groups were comparable in baseline demographics. Time to GI-3 was significantly shorter in the bisacodyl group (3.0 versus 3.7days, P = 0.007). Of the single parameters defining GI-3, there was a 1-day difference in time to defecation in favor to the bisacodyl group (3.0 versus 4.0days, P = 0.001), whereas no significant difference in time to first flatus or tolerance of solid food was seen. No significant difference in the secondary endpoints was seen. Morbidity and mortality did not differ between groups. Conclusion: Bisacodyl accelerated gastrointestinal recovery and might be considered as part of multimodal recovery programs after colorectal surger

Gauge Identities and the Dirac Conjecture

The gauge symmetries of a general dynamical system can be systematically obtained following either a Hamiltonean or a Lagrangean approach. In the former case, these symmetries are generated, according to Dirac's conjecture, by the first class constraints. In the latter approach such local symmetries are reflected in the existence of so called gauge identities. The connection between the two becomes apparent, if one works with a first order Lagrangean formulation. Our analysis applies to purely first class systems. We show that Dirac's conjecture applies to first class constraints which are generated in a particular iterative way, regardless of the possible existence of bifurcations or multiple zeroes of these constraints. We illustrate these statements in terms of several examples.Comment: 21 page

Does monitor position influence visual-motor performance during minimally invasive surgery?

Background: In minimally invasive surgery (MIS), the natural relationship between hand and eye is disrupted, i.e. surgeons typically control tools inserted through the patient’s abdomen while viewing the workspace on a remote monitor, which can be located in a variety of positions. This separates the location of visual feedback from the area in which a motor action is executed. Previous studies suggest that the visual display should be placed directly ahead of the surgeon (i.e. to preserve visual-motor mapping). However, the extent of the impact of this rotation on surgical performance is unknown. Methods: Eighteen participants completed an aiming task on a tablet PC within a surgical box trainer using a laparoscopic tool in a controlled simulated environment. Visual feedback was presented on a remote monitor located at 0°, ±45° and ±90°, with order randomised using the Latin Square method. Results: Movements were significantly slower when the monitor was 90° relative to midline, but spatial accuracy was unaffected by monitor position. Interestingly, the effect of reduced speed in the 90° condition was transient, decreasing over time, suggesting rapid adaptation to the rotation. Conclusions: We conclude that the angle of the visual display in the context of MIS may require a surgeon to adapt to a changed mapping between visual inputs and motor outputs. While this adaptation occurs relatively quickly, it may interfere with skilled actions (e.g. intracorporeal suturing) in complex surgical procedures