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排序方式: 共有1054条查询结果,搜索用时 15 毫秒
1.
吴兰 《红外与毫米波学报》2002,21(2):87-90
用垂直入射的中红外光束调制非掺杂SiGe/Si量子阱中光致子带间吸收,氩离子激光器作为子带间跃迁的光泵浦源在阱中产生载流子,红外调制光谱用步进式傅立叶变换光谱仪记录,实验中观察到明显的层间干涉效应与子带间跃迁有关的色散效应,理论和实验分析认为样品折射率变化造成的位相调制可以补偿吸收所造成的幅度调制。 相似文献
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The authors explore the extent to which eating disorders, specifically anorexia nervosa (AN) and bulimia nervosa (BN), represent culture-bound syndromes and discuss implications for conceptualizing the role genes play in their etiology. The examination is divided into 3 sections: a quantitative meta-analysis of changes in incidence rates since the formal recognition of AN and BN, a qualitative summary of historical evidence of eating disorders before their formal recognition, and an evaluation of the presence of these disorders in non-Western cultures. Findings suggest that BN is a culture-bound syndrome and AN is not. Thus, heritability estimates for BN may show greater variability cross-culturally than heritability estimates for AN, and the genetic bases of these disorders may be associated with differential pathoplasticity. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
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Devenport Jill A.; Patterson Megan R.; Devenport Lynn D. 《Canadian Metallurgical Quarterly》2005,119(3):352
The variability of most environments taxes foraging decisions by increasing the uncertainty of the information available. One solution to the problem is to use dynamic averaging, as do some granivores and carnivores. Arguably, the same strategy could be useful for grazing herbivores, even though their food renews and is more homogeneously distributed. Horses (Equus callabus) were given choices between variable patches after short or long delays. When patch information was current, horses returned to the patch that was recently best, whereas those without current information matched choices to the long-term average values of the patches. These results demonstrate that a grazing species uses dynamic averaging and indicate that, like granivores and carnivores, they can use temporal weighting to optimize foraging decisions. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
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Bracken,Pteridium aquilinum, exerts a strong dominance over associated plants throughout much of its worldwide range. Associated plants are often severely inhibited or even excluded from dense stands of the fern. This study investigated the various aspects of herb suppression in bracken stands and assessed the contribution of the various forms of interference between plants to the establishment and maintenance of bracken dominance. It was shown that competition for soil moisture, light, and nutrients could not account for the lack of herbs in bracken stands. Further, uniformity of soil pH, texture, water-holding capacity, and organic matter content ruled out variability in physical factors as a cause. Baiting and trapping experiments showed that the higher concentration of animal activity inside the bracken stands contributed significantly to the pattern of herb suppression, but only against select species. The maintenance of this pattern in the animal-free Santa Cruz Island stands indicates the importance of another factor, allelopathy. It was found that phytotoxins leached from the dead, standing bracken fronds with the first few rains of the wet season were largely responsible for herb suppression. These toxins were isolated in raindrip and from soil inside the fern stands. Removal of the fronds from the stand before the rains could leach them resulted in reinvasion by the herbs after several seasons, and, conversely, placing fronds over the herbs in the grassland brought about herb inhibition. A number of known allelopathic chemicals were tentatively identified from bracken leachates. The importance of the interaction of allelopathy with other factors of plant interference is illustrated by bracken. 相似文献
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Leslie矩阵模型在带年龄结构的种群动力学中具有重要地位,研究成果表明当Leslie矩阵为素矩阵时,目标种群将趋向一稳定的年龄分布。该文给出了一个无需通过烦琐的矩阵幂运算来判定Leslie矩阵是素矩阵的充分必要条件,它为Leslie模型的实际应用提供了很大方便。 相似文献
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前视探地雷达三维合成孔径成像及多视处理 总被引:1,自引:0,他引:1
提出了一种变波速条件下前视探地雷达三维合成孔径成像及多视处理方法。该方法利用非平稳卷积滤波处理在频率域重构目标图像频谱,经傅里叶反变换后实现前视探地雷达三维合成孔径成像;利用估计的目标入射角实现折射校正及前视探地雷达多视处理。对实测数据进行处理,结果证明了该方法的有效性。 相似文献
9.
我国区域科技创新能力评价研究 总被引:1,自引:0,他引:1
在对区域科技创新理论进行回顾的基础上,从科技投入和科技产出两个方面构建科技创新能力的综合评价指标体系。运用灰色关联分析法,对所选的八个区域科技创新能力进行综合评价。通过对比分析,提出各省科技创新能力及地区间的差异及解决对策。 相似文献
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从灰关联分析中最核心的灰关联度构造及相应的挖掘算法出发,结合正态分布的普适性,提出了一种体现数据分布特点的正态灰数,并给出了相应的灰度及灰关联度计算方法。在此基础上,构建了一种多粒度无监督的快速灰聚类方法,无需先验知识即可完成自动聚类。通过实验验证了本文方法的有效性,为大数据下灰关联分析的进一步发展提供了新思路。 相似文献