排序方式: 共有90条查询结果,搜索用时 15 毫秒
51.
Muhammad USMAN Bingyu ZHANG 《系统科学与复杂性》2007,20(2):284-292
It has been observed in laboratory experiments that when nonlinear dispersive waves are forced periodically from one end of undisturbed stretch of the medium of propagation, the signal eventually becomes temporally periodic at each spatial point. The observation has been confirmed mathematically in the context of the damped Korteweg-de Vries (KdV) equation and the damped Benjamin-Bona-Mahony (BBM) equation. In this paper we intend to show the same results hold for the pure KdV equation (without the damping terms) posed on a finite domain. Consideration is given to the initial-boundary-value problem {ut+ux+uux+uxxx=0, u(x,0)=φ(x), 0〈x〈1, t〉0,u(0,t)=h(t), u(1,t) = 0, ux(1,t) = 0, t〉0.It is shown that if the boundary forcing h is periodic with small ampitude, then the small amplitude solution u of (*) becomes eventually time-periodic. Viewing (*) (without the initial condition) as an infinite-dimensional dynamical system in the Hilbert space L^2(0, 1), we also demonstrate that for a given periodic boundary forcing with small amplitude, the system (*) admits a (locally) unique limit cycle, or forced oscillation, which is locally exponentially stable. A list of open problems are included for the interested readers to conduct further investigations. 相似文献
52.
Torgerson DG Ampleford EJ Chiu GY Gauderman WJ Gignoux CR Graves PE Himes BE Levin AM Mathias RA Hancock DB Baurley JW Eng C Stern DA Celedón JC Rafaels N Capurso D Conti DV Roth LA Soto-Quiros M Togias A Li X Myers RA Romieu I Van Den Berg DJ Hu D Hansel NN Hernandez RD Israel E Salam MT Galanter J Avila PC Avila L Rodriquez-Santana JR Chapela R Rodriguez-Cintron W Diette GB Adkinson NF Abel RA Ross KD Shi M Faruque MU Dunston GM Watson HR Mantese VJ Ezurum SC Liang L Ruczinski I Ford JG 《Nature genetics》2011,43(9):887-892
Asthma is a common disease with a complex risk architecture including both genetic and environmental factors. We performed a meta-analysis of North American genome-wide association studies of asthma in 5,416 individuals with asthma (cases) including individuals of European American, African American or African Caribbean, and Latino ancestry, with replication in an additional 12,649 individuals from the same ethnic groups. We identified five susceptibility loci. Four were at previously reported loci on 17q21, near IL1RL1, TSLP and IL33, but we report for the first time, to our knowledge, that these loci are associated with asthma risk in three ethnic groups. In addition, we identified a new asthma susceptibility locus at PYHIN1, with the association being specific to individuals of African descent (P = 3.9 × 10(-9)). These results suggest that some asthma susceptibility loci are robust to differences in ancestry when sufficiently large samples sizes are investigated, and that ancestry-specific associations also contribute to the complex genetic architecture of asthma. 相似文献
53.
Nicholas AK Khurshid M Désir J Carvalho OP Cox JJ Thornton G Kausar R Ansar M Ahmad W Verloes A Passemard S Misson JP Lindsay S Gergely F Dobyns WB Roberts E Abramowicz M Woods CG 《Nature genetics》2010,42(11):1010-1014
Autosomal recessive primary microcephaly (MCPH) is a disorder of neurodevelopment resulting in a small brain. We identified WDR62 as the second most common cause of MCPH after finding homozygous missense and frame-shifting mutations in seven MCPH families. In human cell lines, we found that WDR62 is a spindle pole protein, as are ASPM and STIL, the MCPH7 and MCHP7 proteins. Mutant WDR62 proteins failed to localize to the mitotic spindle pole. In human and mouse embryonic brain, we found that WDR62 expression was restricted to neural precursors undergoing mitosis. These data lend support to the hypothesis that the exquisite control of the cleavage furrow orientation in mammalian neural precursor cell mitosis, controlled in great part by the centrosomes and spindle poles, is critical both in causing MCPH when perturbed and, when modulated, generating the evolutionarily enlarged human brain. 相似文献
54.
The present paper reports on a systematic study of the influence of Zn alloying on the
structural and optical properties of Cd1 xZnxS thin films. X-ray diffraction study for structural
analysis reveals that the two binary compounds have been completely transformed into ternary
compound with hexagonal (wurtzite) structure with preferred orientation along c-direction with
(002) planes. The optical properties such as optical constants and band gap energy of the films were
examined by using spectroscopic ellipsometer and Photospectrometery. It was found that the
optical constants (n and k) decrease with the addition of Zn content in the alloy. It was also
confirmed that the band gap increases with increasing Zn amount in the alloy and is attributed to
quantum size effect in the grain size. Raman spectroscopy analysis shows one dominant phonon
band at 326 cm 1, the so-called longitudinal optical (LO) mode for all the alloy composition (x).
The appearance of a single phonon band in the Raman spectra established the formation of single
phase hexagonal structured Cd1 xZnxS thin film. The LO band is asymmetrically broaden and high
frequency shifted due to potential fluctuation caused by the dopant material. The AFM results
showed that the surface roughness was decreased with increasing Zn content. 相似文献
55.
A microRNA regulon that mediates endothelial recruitment and metastasis by cancer cells 总被引:1,自引:0,他引:1
Metastatic progression of cancer is a complex and clinically daunting process. We previously identified a set of human microRNAs (miRNAs) that robustly suppress breast cancer metastasis to lung and bone and which display expression levels that predict human metastasis. Although these findings revealed miRNAs as suppressors of cell-autonomous metastatic phenotypes, the roles of non-coding RNAs in non-cell-autonomous cancer progression processes remain unknown. Here we reveal that endogenous miR-126, an miRNA silenced in a variety of common human cancers, non-cell-autonomously regulates endothelial cell recruitment to metastatic breast cancer cells, in vitro and in vivo. It suppresses metastatic endothelial recruitment, metastatic angiogenesis and metastatic colonization through coordinate targeting of IGFBP2, PITPNC1 and MERTK--novel pro-angiogenic genes and biomarkers of human metastasis. Insulin-like growth factor binding protein 2 (IGFBP2) secreted by metastatic cells recruits endothelia by modulating IGF1-mediated activation of the IGF type-I receptor on endothelial cells; whereas c-Mer tyrosine kinase (MERTK) receptor cleaved from metastatic cells promotes endothelial recruitment by competitively antagonizing the binding of its ligand GAS6 to endothelial MERTK receptors. Co-injection of endothelial cells with breast cancer cells non-cell-autonomously rescues their miR-126-induced metastatic defect, revealing a novel and important role for endothelial interactions in metastatic initiation. Through loss-of-function and epistasis experiments, we delineate an miRNA regulatory network's individual components as novel and cell-extrinsic regulators of endothelial recruitment, angiogenesis and metastatic colonization. We also identify the IGFBP2/IGF1/IGF1R and GAS6/MERTK signalling pathways as regulators of cancer-mediated endothelial recruitment. Our work further reveals endothelial recruitment and endothelial interactions in the tumour microenvironment to be critical features of metastatic breast cancer. 相似文献
56.
This research proposes a prediction model of multistage financial distress (MSFD) after considering contextual and methodological issues regarding sampling, feature and model selection criteria. Financial distress is defined as a three‐stage process showing different nature and intensity of financial problems. It is argued that applied definition of distress is independent of legal framework and its predictability would provide more practical solutions. The final sample is selected after industry adjustments and oversampling the data. A wrapper subset data mining approach is applied to extract the most relevant features from financial statement and stock market indicators. An ensemble approach using a combination of DTNB (decision table and naïve base hybrid model), LMT (logistic model tree) and A2DE (alternative N dependence estimator) Bayesian models is used to develop the final prediction model. The performance of all the models is evaluated using a 10‐fold cross‐validation method. Results showed that the proposed model predicted MSFD with 84.06% accuracy. This accuracy increased to 89.57% when a 33.33% cut‐off value was considered. Hence the proposed model is accurate and reliable to identify the true nature and intensity of financial problems regardless of the contextual legal framework. 相似文献
57.
Mg–3Al–1Zn–CNTs composites, with different weight fractions(0.25–1.0 wt%) of carbon nanotubes(CNTs) were successfully fabricated via a powder metallurgy method. The processing parameters were adopted in such a way to have uniform dispersion of short length CNTs without any damage, as well as re fi ned and dissolved β phases structures throughout the composite matrix. The composite exhibited impressive increase in microhardness(about+23%) and tensile failure strain value(about+98%) without signi fi cant compromise in tensile strength, compared to the un-reinforced Mg–3Al–1Zn alloy. The synthesized composites can be used in automotive and aerospace industries due to their low density and high speci fic strength. 相似文献
58.
Introduction The steel tube in a concrete filled tubular column (CFT) acts as both longitudinal and lateral reinforcement, and is thus subjected to biaxial stresses of longitudinal compression and hoop tension. At the same time, con- crete is stressed tri… 相似文献
59.
Amir Zada Nauman Ali Fazle Subhan Natasha Anwar Muhammad Ishaq Ali Shah Muhammad Ateeq Zahid Hussain Khair Zaman Momin Khan 《自然科学进展(英文版)》2019,29(2):138-144
The photocatalytic activities of g-C_3N_4 can be significantly improved by increasing life time of the photogenerated charges. Here, in this work we introduced TiO_2 as proper energy platform to accept the photogenerated electrons from g-C_3N_4 during photocatalysis. The nanophotocatalysts formed from the combination of a suitable amount of TiO_2 nanoparticles and g-C_3N_4 nanosheets showed 8.75 and 4.22% enhancement in photocatalytic activities for CO_2 reduction and 2-chlorophenol(2-CP) degradation under visible light illumination as compared to bare g-C_3N_4. Based on the surface photovoltage spectra, photoluminescence spectra and examination of formed hydroxyl radicals, it was confirmed that these enhanced photoactivities were attributed to the much-improved charge separation via the electron transfer from g-C_3N_4 to TiO_2. From trapping experiments,it was found that hydroxyl radicals were the major species involved in the photocatalytic degradation of 2-CP.This study is helpful to synthesize efficient photocatalysts to cope with energy and environmental issues. 相似文献
60.
Sohail Iqbal Malik 《Systemic Practice and Action Research》2018,31(6):637-656
Programming is a rewarding and yet demanding field in the ICT labormarket, but it is considered a challenging and difficult area of learning for significant numbers of novice programmers. As a result, high attrition rates from introductory programming (IP) courses are reported despite extensive research which attempts to address the issue. In this research, an action research methodology was used with three cycles to investigate and improve the teaching and learning process of the IP course. There were nine activities performed during the span of these three cycles. All three entities of the didactic triangle (student, instructor, and content) together with learning context were incorporated into the research design to understand the problem better and execute the proposed solution. The result shows that three cycles of the action research methodology helps in understanding and improving the students learning outcomes in the IP course. Moreover, the attrition rate was also reduced in the IP course. 相似文献