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"Hugh Thomas"
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Unwinding the amplituhedron in binary
A
bstract
We present new, fundamentally combinatorial and topological characterizations of the amplituhedron. Upon projecting external data through the amplituhedron, the resulting configuration of points has a specified (and maximal) generalized “winding number”. Equivalently, the amplituhedron can be fully described in binary: canonical projections of the geometry down to one dimension have a specified (and maximal) number of “sign flips” of the projected data. The locality and unitarity of scattering amplitudes are easily derived as elementary consequences of this binary code. Minimal winding defines a natural “dual” of the amplituhedron. This picture gives us an avatar of the amplituhedron purely in the configuration space of points in vector space (momentum-twistor space in the physics), a new interpretation of the canonical amplituhedron form, and a direct bosonic understanding of the scattering super-amplitude in planar
N
= 4 SYM as a differential form on the space of physical kinematical data.
Journal Article
Acoustic Estimation of the Direction of Arrival of an Unmanned Aerial Vehicle Based on Frequency Tracking in the Time-Frequency Plane
The development of unmanned aerial vehicles (UAVs) opens up a lot of opportunities but also brings some threats. Dealing with these threats is not easy and requires some good techniques. Knowing the location of the threat is essential to deal with an UAV that is displaying disturbing behavior. Many methods exist but can be very limited due to the size of UAVs or due to technological improvements over the years. However, the noise produced by the UAVs is still predominant, so it gives a good opening for the development of acoustic methods. The method presented here takes advantage of a microphone array with a processing based on time domain Delay and Sum Beamforming. In order to obtain a better signal to noise ratio, the UAV’s acoustic signature is taken into account in the processing by using a time-frequency representation of the beamformer’s output. Then, only the content related to this signature is considered to calculate the energy in one direction. This method enables to have a good robustness to noise and to localize an UAV with a poor spectral content or to separate two UAVs with different spectral contents. Simulation results and those of a real flight experiment are reported.
Journal Article
The artery first technique: re-examining the critical view of safety during laparoscopic cholecystectomy
IntroductionSignificant discrepancies exist between surgeon-documented and actual rates of critical view of safety (CVS) achievement on retrospective review following laparoscopic cholecystectomy. This discrepancy may be due to surgeon utilisation of the artery first technique (AFT), an exception to the CVS first described by Strasberg et al. The present study aims to characterise the use of the AFT, hypothesising it is used as an adjunct in difficult dissections to maximise exposure of the hepato-cystic triangle ensuring safe cholecystectomy.MethodsProspective digital recording of the operative procedure of patients’ undergoing laparoscopic cholecystectomy were undertaken at Christchurch Public Hospital, New Zealand and North Shore Private Hospital, Sydney, Australia. Videos were uploaded to Touch Surgery™ Enterprise. Difficulty was graded, annotated and indications for the AFT quantified using a standardised protocol.ResultsA total of 275 annotated procedures were included in this study. The AFT was employed in 54 (20%) patients; in 13 (24%) patients for bleeding, in 35 (65%) patients where windows one and two were visible, and in 6 (11%) patients no windows were visible within the hepato-cystic triangle. There were significant differences in utilisation across operative grade and by seniority of operator (p < 0.005).ConclusionsThe data presented here demonstrate the AFT is frequently used, particularly with Grade 3 cholecystectomy. However, more data are needed to confirm the utility and safety of this approach. Analysis of the AFT shows that to understand and improve safety in laparoscopic cholecystectomy appreciating how the operation was undertaken and not just that the CVS was achieved is crucial.
Journal Article
The golden empire : Spain, Charles V, and the creation of America
A narrative chronicle of Spain's dominant years traces Latin America's exploration, conquest, and economic development between 1522 and 1556, offering insight into how period accomplishments remain influential in today's world.
Genetic Algorithm-Based Acoustic Array Optimization for Estimating UAV DOA Using Beamforming
The localization of unmanned aerial vehicles is an important topic due to several threats near sensitive sites. Localization based on their sounds has been a particular point of interest in past studies for many years. It requires the use of a microphone array. The positioning of the various microphones making up an antenna defines the intrinsic directivity of the array. In this study, a genetic algorithm is used to determine the microphone positions that optimize directivity in a focus direction and for a frequency, by favoring the narrowness of the main lobe and the reduction of the secondary lobes. The optimization leads to several antennas with a 3D structure similar to that designed in a previous study. A method estimating the direction of arrival of a drone was also presented in that study making use of its acoustic signature to enhance the signal-to-noise ratio and thus improving the estimations. In this paper, an improvement to the method is proposed for tracking the drone’s trajectory. Measurements were conducted to compare the drone locations given by the first designed antenna and the one optimized by the genetic algorithm. Performance on the direction of arrival found is characterized in terms of mean error, standard deviation and root mean square error relative to the GPS reference onboard the UAV. An experiment with the optimized antenna has also been conducted with the drone at a great distance to the antenna to characterize the maximal distance for possible estimations of the direction of arrival. Results show that the method used for the direction of arrival estimation can give a mean error below 10° in azimuth and 5° in elevation. The maximum distance between the antenna and the drone for which the method is able to give estimations is between 240 and 340 m.
Journal Article
Noncrossing partitions and representations of quivers
We situate the noncrossing partitions associated with a finite Coxeter group within the context of the representation theory of quivers. We describe Reading’s bijection between noncrossing partitions and clusters in this context, and show that it extends to the extended Dynkin case. Our setup also yields a new proof that the noncrossing partitions associated with a finite Coxeter group form a lattice. We also prove some new results within the theory of quiver representations. We show that the finitely generated, exact abelian, and extension-closed subcategories of the representations of a quiver Q without oriented cycles are in natural bijection with the cluster tilting objects in the associated cluster category. We also show that these subcategories are exactly the finitely generated categories that can be obtained as the semistable objects with respect to some stability condition.
Journal Article