32,699 research outputs found
Chaotic Properties of Subshifts Generated by a Non-Periodic Recurrent Orbit
The chaotic properties of some subshift maps are investigated. These
subshifts are the orbit closures of certain non-periodic recurrent points of a
shift map. We first provide a review of basic concepts for dynamics of
continuous maps in metric spaces. These concepts include nonwandering point,
recurrent point, eventually periodic point, scrambled set, sensitive dependence
on initial conditions, Robinson chaos, and topological entropy. Next we review
the notion of shift maps and subshifts. Then we show that the one-sided
subshifts generated by a non-periodic recurrent point are chaotic in the sense
of Robinson. Moreover, we show that such a subshift has an infinite scrambled
set if it has a periodic point. Finally, we give some examples and discuss the
topological entropy of these subshifts, and present two open problems on the
dynamics of subshifts
Neural-Network Vector Controller for Permanent-Magnet Synchronous Motor Drives: Simulated and Hardware-Validated Results
This paper focuses on current control in a permanentmagnet synchronous motor (PMSM). The paper has two main objectives: The first objective is to develop a neural-network (NN) vector controller to overcome the decoupling inaccuracy problem associated with conventional PI-based vector-control methods. The NN is developed using the full dynamic equation of a PMSM, and trained to implement optimal control based on approximate dynamic programming. The second objective is to evaluate the robust and adaptive performance of the NN controller against that of the conventional standard vector controller under motor parameter variation and dynamic control conditions by (a) simulating the behavior of a PMSM typically used in realistic electric vehicle applications and (b) building an experimental system for hardware validation as well as combined hardware and simulation evaluation. The results demonstrate that the NN controller outperforms conventional vector controllers in both simulation and hardware implementation
Dedifferentiation: A new approach in stem cell research
Dedifferentiation is an important biological phenomenon whereby cells regress from a specialized function to a simpler state reminiscent of stem cells. Stem cells are self-renewing cells capable of giving rise to differentiated cells when supplied with the appropriate factors. Stem cells that are derived by dedifferentiation of one's own cells could be a new resource for regenerative medicine, one that poses no risk of genetic incompatibility or immune rejection and provokes fewer ethical debates than the use of stem cells derived from embryonic tissue. Until now, it has not been quite clear why some differentiated cell types can dedifferentiate and proliferate, whereas others cannot. A better understanding of the mechanisms involved in dedifferentiation may enable scientists to control and possibly alter the plasticity of the differentiated state, which may lead to benefits not only in stem cell research but also in regenerative medicine and even tumor biology. If so, dedifferentiation will offer an ethically acceptable alternative route to obtain an abundant source of stem cells. Dedifferentiation is likely to become a new focus of stem cell research. Here we compile recent advances in this emerging but significant research, highlighting its central concepts, research findings, possible signaling pathways, and potential applications.published_or_final_versio
Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlGaN/AlN/GaN heterostructures
We studied the persistent photoconductivity (PPC) effect in AlGaN/AlN/GaN
heterostructures with two different Al-compositions (x=0.15 and x=0.25). The
two-dimensional electron gas formed at the AlN/GaN heterointerface was
characterized by Shubnikov-de Haas and Hall measurements. Using optical
illumination, we were able to increase the carrier density of the
Al0.15Ga0.85N/AlN/GaN sample from 1.6x10^{12} cm^{-2} to 5.9x1012 cm^{-2},
while the electron mobility was enhanced from 9540 cm2/Vs to 21400 cm2/Vs at T
= 1.6 K. The persistent photocurrent in both samples exhibited a strong
dependence on illumination wavelength, being highest close to the bandgap and
decreasing at longer wavelengths. The PPC effect became fairly weak for
illumination wavelengths longer than 530 nm and showed a more complex response
with an initial negative photoconductivity in the infrared region of the
spectrum (>700 nm). The maximum PPC-efficiency for 390 nm illumination was
0.011% and 0.005% for Al0.25Ga0.75N/AlN/GaN and Al0.15Ga0.85N/AlN/GaN samples,
respectively. After illumination, the carrier density could be reduced by
annealing the sample. Annealing characteristics of the PPC effect were studied
in the 20-280 K temperature range. We found that annealing at 280 K was not
sufficient for full recovery of the carrier density. In fact, the PPC effect
occurs in these samples even at room temperature. Comparing the measurement
results of two samples, the Al0.25Ga0.75N/AlN/GaN sample had a larger response
to illumination and displayed a smaller recovery with thermal annealing. This
result suggests that the energy scales of the defect configuration-coordinate
diagrams for these samples are different, depending on their Al-composition.Comment: 27 pages, 8 figure
Effects of tai chi on postural control during dual-task stair negotiation in knee osteoarthritis : a randomised controlled trial protocol
Stair ascent and descent require complex integration between sensory and motor systems; individuals with knee osteoarthritis (KOA) have an elevated risk for falls and fall injuries, which may be in part due to poor dynamic postural control during locomotion. Tai chi exercise has been shown to reduce fall risks in the ageing population and is recommended as one of the non-pharmocological therapies for people with KOA. However, neuromuscular mechanisms underlying the benefits of tai chi for persons with KOA are not clearly understood. Postural control deficits in performing a primary motor task may be more pronounced when required to simultaneously attend to a cognitive task. This single-blind, parallel design randomised controlled trial (RCT) aims to evaluate the effects of a 12-week tai chi programme versus balance and postural control training on neuromechanical characteristics during dual-task stair negotiation. Sixty-six participants with KOA will be randomised into either tai chi or balance and postural control training, each at 60 min per session, twice weekly for 12 weeks. Assessed at baseline and 12 weeks (ie, postintervention), the primary outcomes are attention cost and dynamic postural stability during dual-task stair negotiation. Secondary outcomes include balance and proprioception, foot clearances, self-reported symptoms and function. A telephone follow-up to assess symptoms and function will be conducted at 20 weeks. The findings will help determine whether tai chi is beneficial on dynamic stability and in reducing fall risks in older adults with KOA patients in community. Ethics approval was obtained from the Ethics Committee of the Affiliated Rehabilitation Hospital of Fujian University of Traditional Chinese Medicine (#2018KY-006-1). Study findings will be disseminated through presentations at scientific conferences or publications in peer-reviewed journals. ChiCTR1800018028. [Abstract copyright: © Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.
Temporal and Spatial Changing Trends on the Fluvial Sedimentation in the Three Gorges Reservoir, Yangtze River in China
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchive
Two years of measurements of atmospheric total gaseous mercury (TGM) at a remote site in Mt. Changbai area, Northeastern China
Total gaseous mercury (TGM) was continuously monitored at a remote site (CBS) in Mt. Changbai area, Northeastern China from 24 October 2008 to 31 October 2010. The overall mean TGM concentration was 1.60&plusmn;0.51 ng m<sup>−3</sup>, which is lower than those reported from remote sites in Eastern, Southwestern, and Western China, indicating a relatively lower regional anthropogenic mercury (Hg) emission intensity in Northeastern China. Measurements at a site in the vicinity (~1.2 km) of CBS station from August 2005 to July 2006 showed a significantly higher mean TGM concentration of 3.58&plusmn;1.78 ng m<sup>−3</sup>. The divergent result was partially attributed to fluctuations in the relatively frequencies of surface winds during the two study periods and moreover an effect of local emission sources. The temporal variation of TGM at CBS was influenced by regional sources as well as long-range transported Hg. Regional sources frequently contributing to episodical high TGM concentrations were pin-pointed as a large iron mining district in Northern North Korea and two large power plants and urban areas to the southwest of the sampling site. Source areas in Beijing, Tianjin, southern Liaoning, Hebei, northwestern Shanxi, and northwestern Shandong were found to contribute to elevated TGM observations at CBS via long-range transport. Diurnal pattern of TGM at CBS was mainly controlled by regional sources, likely as well as intrusion of air masses from the free troposphere during summer season. There are no consistent seasonal pattern of TGM at CBS, and the monthly TGM variations showed links with the patterns of regional air movements and long-range transport
Spin and orbital angular momentum in gauge theories (II): QCD and nucleon spin structure
Parallel to the construction of gauge invariant spin and orbital angular
momentum for QED in paper (I) of this series, we present here an analogous but
non-trivial solution for QCD. Explicitly gauge invariant spin and orbital
angular momentum operators of quarks and gluons are obtained. This was
previously thought to be an impossible task, and opens a more promising avenue
towards the understanding of the nucleon spin structure.Comment: 3 pages, no figure; presented by F. Wang at NSTAR200
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