首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   386篇
  免费   0篇
系统科学   5篇
教育与普及   2篇
现状及发展   91篇
研究方法   50篇
综合类   227篇
自然研究   11篇
  2018年   2篇
  2017年   2篇
  2014年   3篇
  2013年   7篇
  2012年   14篇
  2011年   23篇
  2010年   7篇
  2009年   2篇
  2008年   16篇
  2007年   17篇
  2006年   18篇
  2005年   14篇
  2004年   16篇
  2003年   12篇
  2002年   17篇
  2001年   17篇
  2000年   7篇
  1999年   8篇
  1998年   2篇
  1996年   2篇
  1992年   7篇
  1991年   9篇
  1990年   6篇
  1989年   5篇
  1987年   6篇
  1986年   4篇
  1985年   11篇
  1984年   5篇
  1983年   2篇
  1982年   7篇
  1981年   5篇
  1980年   3篇
  1979年   9篇
  1978年   8篇
  1977年   3篇
  1976年   4篇
  1975年   3篇
  1974年   8篇
  1972年   2篇
  1971年   6篇
  1970年   11篇
  1969年   8篇
  1968年   14篇
  1967年   8篇
  1966年   4篇
  1965年   9篇
  1960年   2篇
  1959年   1篇
  1957年   1篇
  1954年   1篇
排序方式: 共有386条查询结果,搜索用时 15 毫秒
321.
Density forecasts for weather variables are useful for the many industries exposed to weather risk. Weather ensemble predictions are generated from atmospheric models and consist of multiple future scenarios for a weather variable. The distribution of the scenarios can be used as a density forecast, which is needed for pricing weather derivatives. We consider one to 10‐day‐ahead density forecasts provided by temperature ensemble predictions. More specifically, we evaluate forecasts of the mean and quantiles of the density. The mean of the ensemble scenarios is the most accurate forecast for the mean of the density. We use quantile regression to debias the quantiles of the distribution of the ensemble scenarios. The resultant quantile forecasts compare favourably with those from a GARCH model. These results indicate the strong potential for the use of ensemble prediction in temperature density forecasting. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
322.
Mutations of mitochondrial DNA (mtDNA) are an important cause of genetic disease. We describe a family with an unusual homoplasmic mutation that resulted in six neonatal deaths and one surviving child with Leigh syndrome. The mother is clinically normal, but a severe biochemical and molecular genetic defect was present in both a fatally affected child and the mother. This family highlights the role of homoplasmic mt-tRNA mutations in genetic disease.  相似文献   
323.
Issues arising during a participatory action research project with a group of midwives are explored, in particular, those related to group process, membership, and roles. A Midwives Action Research Group established an Early Mothering Group for women in hospital to talk to each other and form supportive social networks. The time-honored ritual of sharing morning tea and cake allowed both midwives and mothers to experience the therapeutic potential and power of women's ordinary talk.  相似文献   
324.
Polo-like kinase-1 (PLK1) is an essential mitotic kinase regulating multiple aspects of the cell division process. Activation of PLK1 requires phosphorylation of a conserved threonine residue (Thr 210) in the T-loop of the PLK1 kinase domain, but the kinase responsible for this has not yet been affirmatively identified. Here we show that in human cells PLK1 activation occurs several hours before entry into mitosis, and requires aurora A (AURKA, also known as STK6)-dependent phosphorylation of Thr 210. We find that aurora A can directly phosphorylate PLK1 on Thr 210, and that activity of aurora A towards PLK1 is greatly enhanced by Bora (also known as C13orf34 and FLJ22624), a known cofactor for aurora A (ref. 7). We show that Bora/aurora-A-dependent phosphorylation is a prerequisite for PLK1 to promote mitotic entry after a checkpoint-dependent arrest. Importantly, expression of a PLK1-T210D phospho-mimicking mutant partially overcomes the requirement for aurora A in checkpoint recovery. Taken together, these data demonstrate that the initial activation of PLK1 is a primary function of aurora A.  相似文献   
325.
The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the mysterious 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles-the excitations resulting from the breaking of Cooper pairs-should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi(2)Sr(2)CaCu(2)O(8+delta) in r-space and k-space simultaneously. We find that although the low-energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy-independent way. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.  相似文献   
326.
Sulphur isotope evidence for an oxic Archaean atmosphere   总被引:1,自引:0,他引:1  
Ohmoto H  Watanabe Y  Ikemi H  Poulson SR  Taylor BE 《Nature》2006,442(7105):908-911
The presence of mass-independently fractionated sulphur isotopes (MIF-S) in many sedimentary rocks older than approximately 2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr ago. This is because the only mechanism known to produce MIF-S has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen-poor atmosphere. Here we report the absence of MIF-S throughout approximately 100-m sections of 2.76-Gyr-old lake sediments and 2.92-Gyr-old marine shales in the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archaean era; (2) the atmosphere has remained oxic since approximately 3.8 Gyr ago, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere; or (3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, and thus is not linked to atmospheric chemistry.  相似文献   
327.
Chromosome 11, although average in size, is one of the most gene- and disease-rich chromosomes in the human genome. Initial gene annotation indicates an average gene density of 11.6 genes per megabase, including 1,524 protein-coding genes, some of which were identified using novel methods, and 765 pseudogenes. One-quarter of the protein-coding genes shows overlap with other genes. Of the 856 olfactory receptor genes in the human genome, more than 40% are located in 28 single- and multi-gene clusters along this chromosome. Out of the 171 disorders currently attributed to the chromosome, 86 remain for which the underlying molecular basis is not yet known, including several mendelian traits, cancer and susceptibility loci. The high-quality data presented here--nearly 134.5 million base pairs representing 99.8% coverage of the euchromatic sequence--provide scientists with a solid foundation for understanding the genetic basis of these disorders and other biological phenomena.  相似文献   
328.
329.
H F Taylor  T A Smith 《Nature》1967,215(5109):1513-1514
  相似文献   
330.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号